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Applications and limitations of U-Pb thermochronology to middle and lower crustal thermal histories

Authors:

Abstract

Volume diffusion of Pb occurs over micron length scales in apatite and rutile at temperatures relevant to the evolution of the middle and lower crust. Continuous thermal history information can be resolved from inversion of intracrystalline U-Pb date profiles preserved within individual grains. Recent developments in microbeam analysis permit rapid measurement of these age profiles at sub-micron spatial resolution, thus heralding a new era for U-Pb thermochronology. Here, we review the theoretical, experimental and empirical basis for U-Pb thermochronology and show that rutile, in particular, presents an exceptional opportunity to obtain high-resolution thermal history information from the deep crust. We present a Bayesian procedure that is well suited to the inversion of U-Pb date profile datasets and balances computational efficiency with a full search of thermal history coordinate space. Complications relevant to accurate application of U-Pb thermochronology are discussed i) theoretically and ii) empirically, using a rutile U-Pb dataset from the lower crust of the Grenville orogeny. Purely diffusive date profiles are shown to be the exception to uniform, or step-like, young profiles, suggesting that processes other than thermally-activated volume diffusion may control U-Pb systematics in rutile residing in the lower crust. However, the data obtained from apparent diffusive profiles systematically match cooling histories inferred from other thermochronometers. This result emphasises the importance of integrating microtextural observations, and trace-element concentrations, with U-Pb age data in order to discriminate between diffusive and non-diffusive Pb transport mechanisms in accessory phases and thus minimize the risk of generating spurious thermal histories.
Invited Review Article for Chemical Geology 1"
2"
Applications and Limitations of U-Pb Thermochronology to
3"
Middle and Lower Crustal Thermal Histories 4"
5"
Smye, A.J.1*, Marsh, J.H.2, Vermeesch, P.3 Garber, J.M.1 and Stockli, D.F.4 6"
7"
1Department of Geosciences, Pennsylvania State University, University Park, PA 16801, 8"
USA
9"
2Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian 10"
University, Sudbury, ON P3E 2C6, Canada
11"
3Department of Earth Sciences, University College London, London, WC1E 6BT, UK
12"
4Department of Geological Sciences, The University of Texas at Austin, Austin, TX 13"
78712, USA
14"
15"
*Corresponding author; e-mail: smye@psu.edu
16"
Abstract 17"
Volume diffusion of Pb occurs over micron length scales in apatite and rutile at 18"
temperatures relevant to the evolution of the middle and lower crust. Continuous thermal 19"
history information can be resolved from inversion of intracrystalline U-Pb date profiles
20"
preserved within individual grains. Recent developments in microbeam analysis permit
21"
rapid measurement of these age profiles at sub-micron spatial resolution, thus heralding a
22"
new era for U-Pb thermochronology. Here, we review the theoretical, experimental and 23"
empirical basis for U-Pb thermochronology and show that rutile, in particular, presents an 24"
exceptional opportunity to obtain high-resolution thermal history information from the 25"
deep crust. We present a Bayesian procedure that is well suited to the inversion of U-Pb 26"
date profile datasets and balances computational efficiency with a full search of thermal 27"
history coordinate space. Complications relevant to accurate application of U-Pb 28"
thermochronology are discussed i) theoretically and ii) empirically, using a rutile U-Pb 29"
dataset from the lower crust of the Grenville orogeny. Purely diffusive date profiles are 30"
shown to be the exception to uniform, or step-like, young profiles, suggesting that 31"
processes other than thermally-activated volume diffusion may control U-Pb systematics 32"
in rutile residing in the lower crust. However, the data obtained from apparent diffusive 33"
profiles systematically match cooling histories inferred from other thermochronometers.
34"
This result emphasises the importance of integrating microtextural observations, and
35"
trace-element concentrations, with U-Pb age data in order to discriminate between
36"
diffusive and non-diffusive Pb transport mechanisms in accessory phases and thus 37"
minimize the risk of generating spurious thermal histories.
38"
1. Introduction 39"
Geodynamic processes impart characteristic thermal signatures to the lithosphere that are 40"
recorded by the distribution of daughter nuclides in minerals with radiogenic parent 41"
elements. The noble gas decay systems 40Ar/39Ar and (U-Th)/He harness thermal history
42"
information from temperatures 500 °C and have been routinely applied to tectonic and
43"
geomorphological investigations of the middle and upper crust (Farley, 2002; McDougall
44"
and Harrison, 1999). Conversely, volume diffusion of Pb in apatite, rutile and, 45"
potentially, titanite is effective at temperatures characteristic of the deep crust (>400 °C). 46"
U-Pb thermochronology can thus be used to constrain cooling from high temperature, 47"
and, by inference, exhumation rates of deep seated metamorphic and plutonic rocks in 48"
active and ancient orogenic belts (e.g. Cochrane et al., 2014; Flowers et al., 2006; 49"
Kooijman et al., 2010; Kylander-Clark et al., 2008; Mezger et al., 1989; Mezger et al., 50"
1991; Möller et al., 2000) as well as long-duration cooling of cratonic lower crust to 51"
investigate continent stabilization (e.g. Blackburn et al., 2011; Blackburn et al., 2012; 52"
Davis, 1997; Davis et al., 2003; Schmitz and Bowring, 2003; Schoene and Bowring, 53"
2007). Traditionally, U-Pb thermochronology has been applied using whole-grain isotope 54"
dilution analysis in which the measured U-Pb date is assigned to a nominal, volume-55"
averaged closure temperature (Dodson, 1973). Whilst this approach has been successfully
56"
applied to constrain thermal histories of crustal rocks, interpolation between discrete
57"
temperature-time (
!"#
) data points derived from whole grain analyses i) yields low-
58"
resolution thermal history information and ii) assumes that the effective diffusion radius 59"
is the entire grain. In contrast, near-continuous thermal history information can be
60"
obtained through numerical inversion of within-grain U-Pb date profiles (Harrison et al., 61"
2005). Until recently, measurement of U-Pb date profiles was only possible by secondary 62"
ion mass spectrometry (Grove and Harrison, 1999; Harrison et al., 2005); however, 63"
technological developments have enabled routine measurement of radiogenic Pb and 64"
trace-element concentrations at sub-micron spatial resolution by laser ablation
65"
inductively-coupled plasma mass spectrometry (e.g. Cottle et al., 2009; Smye and
66"
Stockli, 2014; Stearns et al., 2016; Steely et al., 2014). The ease, rapidity, precision, and
67"
spatial resolution of LA-ICP-MS herald a new era for deep lithosphere 68"
thermochronometry. 69"
70"
Proliferation of high spatial resolution U-Pb measurements raises the challenge of 71"
accurately interpreting intracrystalline U-Pb date distributions as forming in response to a 72"
host of diffusive or non-diffusive processes. Various intragrain transport processes, 73"
including recrystallization, short-circuit diffusion, secondary growth and volume 74"
diffusion, can each affect the topology of a U-Pb date profile. Furthermore, the effect of 75"
neighboring mineral phases and the presence/absence of grain-boundary fluids may have 76"
significant effects on the boundary conditions for volume diffusion of Pb through 77"
accessory phases. In contrast, such effects have been shown to influence the 78"
incorporation of extraneous 40Ar (e.g. Kelley, 2002; Smye et al., 2013) and the efficacy
79"
of recrystallization (e.g. Villa and Hanchar, 2017) in K-bearing minerals. Developing an
80"
understanding of the kinetic controls on Pb transport over sub-micron length scales in
81"
accessory minerals is critical to accurately identifying U-Pb datasets that are suitable for 82"
U-Pb thermochronology, and avoiding generation of spurious or non-unique thermal
83"
histories. Complementary analysis of trace-element abundances collected from the same 84"
analytical volume as U-Pb dates has the potential to shed light on these processes (e.g. 85"
Kylander-Clark, 2017; Kylander-Clark et al., 2013). Motivated by recent methodological 86"
advances, this paper reviews and demonstrates the basis for U-Pb thermochronology by 87"
evaluating the kinetic processes that control the topology of U-Pb date distributions.
88"
89"
2. (U-Th)/Pb thermochronometry
90"
2.1 Theory 91"
The physics describing volume diffusion-controlled thermochronology are well 92"
established"(Dodson, 1986; Dodson, 1973; Fechtig and Kalbitzer, 1966); here, we provide 93"
an overview of fundamental concepts applied to the U-Pb system in apatite, rutile and 94"
titanite. Length scales (
$
) of Pb diffusion through monazite and zircon are predicted to be 95"
limited at temperatures <900 °C (
$%&%
1
%
µm for monazite and 3 µm for zircon at 900 °C, 96"
over 10 Myr); such short length scales of diffusive transport limits their use as 97"
thermochronometers to regions of the lithosphere cooling from (ultra-)high temperature 98"
conditions (Cherniak and Watson, 2001b; Cherniak et al., 2004). Therefore, we do not 99"
consider zircon and monazite further, but the concepts discussed below are relevant to 100"
monazite and zircon U-Pb thermochronology. 101"
102"
The concentration of radiogenic Pb, '(
), at radial position,
*
, within mineral
+
residing at 103"
temperature
!
, for duration
#
, is given by: 104"
,-.
/
,0 1 2)345 6( (1) 105"
where, 2) is the diffusivity of Pb described by an Arrhenius law (2)1 27
)89:;<
/=>?@), 106"
where 27
) is the diffusivity at infinite
!
,%AB
) is the activation energy and
C
is the universal 107"
gas constant), 3 is the Laplacian operator and 6(
%
represents radiogenic production of Pb, 108"
controlled by the spatially-dependent concentration of 238U, 235U and 232Th. From 109"
inspection of Equation 1, the concentration of radiogenic Pb at any point in time and 110"
space within a mineral grain reflects a competition between diffusive loss and radiogenic 111"
production. The rate of diffusive loss exceeds the rate of production at high temperatures, 112"
and vice-versa at low temperatures. Between these two end-member behaviours, there 113"
exists a region of
!9#@
space in which the rate of diffusive loss is comparable to the rate 114"
of radiogenic production; the absolute magnitude of this “partial retention zone” (PRZ) 115"
depends on 2),
D!=D#
, and
$
. 116"
117"
Figure 1 shows the relationship between PRZ and U-Pb date profile for single grains of 118"
apatite and rutile undergoing cooling during exhumation from the deep crust. Titanite is 119"
not considered here due to uncertainties over Pb diffusion parameters that are discussed 120"
in section 2.3. We assume here that Pb diffusive loss only occurs at the outermost grain 121"
boundary, and that each mineral crystallizes immediately prior to the onset of exhumation
122"
at 50 Ma. In this example, progressive exhumation advects heat to shallow crustal levels
123"
where conductive heat loss to the surface occurs. These competing effects increase
D!=DE
124"
(gray geotherms, Fig. 1a) and dictate that exhuming rocks will experience a 125"
monotonically increasing
D!=D#
as long as exhumation continues. Figure 1a shows the
126"
thermal and vertical motion histories for three rocks initially located at 22.5, 30 and 37.5
127"
km, respectively, that are exhumed along a continental geotherm (initially 680 °C at 40
128"
km) at 1 km/Myr. The shallow sample (yellow markers, Fig. 1a) exhumes through 129"
temperatures <400 °C that are cold enough to inhibit significant diffusive loss of Pb from 130"
both apatite and rutile (Fig. 1b); in this case, single grains of both minerals would 131"
preserve crystallisation ages at all but their outermost portions. The two more deeply 132"
seated samples are exhumed from depths at which initial temperatures are >500 °C; in 133"
both cases
$%
>5 µm in apatite and rutile. However, significantly younger U-Pb dates are 134"
recorded by apatite grain interiors, whereas U-Pb dates in rutile grain interiors preserve 135"
the timing of crystallisation due to slow Pb diffusion. Diffusive rounding and degree of 136"
interior younging of the U-Pb date profile is controlled by the duration the rock resides 137"
within each mineral’s PRZ. We define PRZs for apatite and rutile as the values of
!9EF#@
138"
between which 10 and 90 % of radiogenic Pb is retained (gray bands, Fig. 1a). Using 139"
experimental diffusivity data, we calculate that the rutile PRZ spans ~13 km and 140"
temperatures between ~560 and 650 °C, whereas the apatite PRZ spans ~12 km and 141"
temperatures between 430 and 520 °C. Using this formulation, the depth and temperature 142"
interval of the PRZ vary with exhumation rate; slower exhumation decreases the depth 143"
range but increases the absolute depth of each PRZ. These calculations demonstrate the 144"
sensitivity of age profile topologies to different forms of
!9#@
and also show that
145"
combined apatite and rutile thermochronology may independently constrain thermal
146"
history information over a temperature interval of ~250 °C. In constrast to traditional
147"
bulk-grain thermochronology, inversion of U-Pb date profiles for
!9#@
can be done 148"
without any external constraints, avoiding potential biasing of thermal history.
149"
150"
Traditionally, U-Pb thermochronology has been applied to deep crustal rocks using the
151"
bulk closure temperature approach, in which a volume-average mineral age is correlated 152"
with a nominal closure temperature (Tc) that represents the temperature at which the grain 153"
effectively closes to Pb loss during cooling. Based on Dodson’s Tc concept"(Dodson, 154"
1973), this approach carries with it several stringent requirements, including knowledge 155"
of mineral-specific diffusion parameters, a zero-Pb concentration boundary condition, 156"
and constant, monotonic cooling. Whilst informative, the closure temperature approach 157"
yields thermal histories of limited resolution. This is apparent from inspection of the 158"
closure temperature (
!G
) equation 159"
H
I1;<
>JK L>?
M
NOP=QN;<R?=R0 (2) 160"
where
S
is a geometry factor and the other variables are as introduced previously. The 161"
equation shows that
!G
is proportional to the inverse of the natural logarithm of a large 162"
product term, effectively dampening the sensitivity of
!G
to variations in
D!=D#
. Solving 163"
instead for time-dependent variations in
D!=D#
–as opposed to bulk
!G
–provides a more 164"
sensitive means to calculate lithospheric thermal histories. 165"
166"
Figure 2 shows calculated U-Pb date profiles for a rutile grain (150 µm spherical radius) 167"
that has undergone a variety of different thermal histories, including slow cooling (black
168"
lines), reheating (purple), residence at elevated temperatures (orange), and two-stage
169"
growth at low temperature (red). The spatial integral of each 206Pb concentration profile
170"
(' T UT
7
B, where
V%
is the grain radius and
W
is 206Pb concentration) is identical in each 171"
case, yielding whole-grain ages of 40 Ma with a homogenous distribution of 238U. These
172"
calculations illustrate the inability of bulk grain analysis to differentiate between various
173"
radiogenic Pb distributions that record thermal information of geodynamic interest.
174"
175"
2.2 Previous applications of U-Pb thermochronology to continental lithosphere 176"
There is a large body of literature connecting U-Pb dates and distributions to the thermal 177"
structure of continental lithosphere. Motivated by the ubiquity of discordant zircon U-Pb 178"
dates, Wetherill (1956) devised a graphical method based on U-Pb concordia to assess the 179"
extent of diffusive loss of Pb following crystallization. Zircon grains that have undergone 180"
varying degrees of Pb loss during a post-crystallization thermal event should define a 181"
linear array in concordia space (i.e., discordia) with the upper and lower intercepts 182"
recording the timing of crystallization and the timing of reheating, respectively. In this 183"
model of episodic Pb loss, the position of an analysis along discordia is controlled by the 184"
length scale of Pb diffusion; smaller diffusive domains retain ages closer to the timing of 185"
reheating than larger domains. We note that Wetherill’s secondary Pb loss model is 186"
predicated on the assumption that a discordant array of U-Pb dates formed during 187"
reheating. Tilton (1960) followed by developing an analytical model for the continuous 188"
loss of radiogenic Pb, analogous to slow cooling. In contrast to Wetherill’s model, the 189"
continuous loss of Pb during cooling results in a curvilinear discordia. Tilton’s model is 190"
predicated on the assumption that the rate of Pb diffusion is not temperature-dependent.
191"
Whilst both of these works were motivated by discordant zircon U-Pb datasets—a
192"
mineral now known to only lose Pb by diffusion under extreme temperatures"(e.g."
193"
Cherniak and Watson, 2001a) or when metamict (e.g."Geisler et al., 2007)—their 194"
graphical and numerical approaches are relevant to minerals in which Pb is diffusively
195"
mobilized, including apatite, rutile and (potentially) titanite.
196"
197"
Mezger et al. (1989) observed that rutile U-Pb dates from the Archean Pikwitonei 198"
granulite terrane and the Proterozoic Adirondack terrane correlated with grain 199"
dimensions. In conjunction with existing thermochronometric data, they used observed 200"
age versus grain-size correlations to estimate that the rutile U-Pb system closed to 201"
diffusive Pb loss at ~420 °C. Schmitz and Bowring (2003) collected whole-grain U-Pb 202"
dates from lower crustal xenoliths to constrain the thermal evolution of cratonic 203"
lithosphere beneath South Africa. Specifically, they demonstrated that rutile is a 204"
particularly effective thermochronometer at lower and middle crustal temperatures. 205"
Schoene and Bowring (2007) used the topology of U-Pb date versus grain-size curves in 206"
conjunction with a numerical model of Pb diffusion to show that the Barberton 207"
Greenstone Belt underwent slow, non-linear cooling during the Archean and not later 208"
reheating. More recently, Blackburn et al. (2011) used a numerical solution to Eq. 1 to 209"
demonstrate that the combined effects of variable production rate and diffusion result in 210"
data topologies on a concordia diagram that permit distinction between slow cooling and 211"
reheating thermal histories. This method was subsequently applied to rutile and titanite 212"
grains from lower crustal xenoliths to estimate long-term cooling rates of the North 213"
American craton (Blackburn et al., 2012).
214"
215"
Each of the above studies focused on the use of whole-grain U-Pb thermochronology, but
216"
other studies have focused on in-situ measurement of intracrystalline U-Pb date profiles. 217"
Grove and Harrison (1999) measured Th-Pb date gradients in the outermost 1 µm of
218"
Himalayan monazite crystals using ion probe depth profiling. Sampled at 500 Å, the age
219"
profiles were interpreted as representing diffusive closure profiles that formed during
220"
rapid Pliocene cooling in the hanging wall of the Main Central Thrust. This study was the 221"
first to demonstrate the utility of directly inverting (U/Th)-Pb closure profiles for near-222"
continuous thermal history information. A number of subsequent studies have showed 223"
that U-Pb closure profiles can be coarsely sampled using in-situ laser ablation traverses 224"
across individual mineral grains (e.g. Vry and Baker, 2006; Warren et al., 2012; Zack et 225"
al., 2011b). Kooijman et al. (2010) used such an approach to measure U-Pb closure 226"
profiles in slowly cooled rutile from the Pikwitonei granulite terrane. Inversion of the 227"
profiles using an updated closure temperature model showed that cooling of the terrane 228"
slowed over time, from initial rates of ~2 °C/Myr to 0.4 °C/Myr. Using a combination of 229"
whole-grain and laser ablation spot traverses, Cochrane et al. (2014) showed that apatite 230"
U-Pb systematics are a sensitive recorder of transient variations in cooling rate between 231"
~370 and 570 °C. Smye and Stockli (2014) applied laser ablation depth-profiling to 232"
measure diffusive U-Pb date profiles in the outermost 30 µm of lower crustal rutile from 233"
the Ivrea Zone. Numerical inversion of the profiles resulted in identification of a 234"
reheating event, previously unrecognised by 40Ar/39Ar and K-Ar whole-grain 235"
thermochronology (Siegesmund et al., 2008, and refs therein). Finally, Kohn and Corrie 236"
(2011) and Stearns et al. (2016) applied laser-ablation depth profiling to collect U-Pb
237"
dates and trace-element concentrations in the rims of individual titanite grains from the
238"
Greater Himalayan Sequence and Pamir gneisses, respectively. In both studies, the
239"
titanite grains experienced temperatures above 700 °C, theoretically sufficient to drive Pb 240"
diffusion over micron length scales; however, Zr and Pb concentration profiles do not
241"
conform to the topology predicted by diffusive loss from grain boundaries, even though
242"
some of them mimic typical diffusion profiles. These observations suggest that growth
243"
and/or recrystallization controlled the distribution of Zr and radiogenic Pb. 244"
245"
2.3 Pb diffusion kinetics 246"
Application of U-Pb thermochronology requires a priori knowledge of the diffusivity of 247"
Pb through the target mineral lattice. Here, we review experimental and empirical 248"
constraints on Pb diffusion rates through apatite, rutile and titanite, noting that an 249"
extensive body of literature exists concerning the energetics of Pb diffusion through 250"
accessory phases (e.g. Cherniak, 2010; Van Orman and Crispin, 2010 and refs therein). 251"
Specifically, we focus on the comparison between laboratory- and field-based estimates 252"
of Pb diffusivity. 253"
254"
2.3.1 Experimental Pb diffusivities 255"
Diffusion of Pb through apatite was first experimentally measured by Watson et al. 256"
(1985) at temperatures between 900 and 1250 °C and, subsequently, at lower 257"
temperatures, between 600 and 900 °C, by Cherniak et al. (1991). Arrhenian parameters 258"
from both studies are in broad agreement and predict closure of apatite grains to Pb loss 259"
between ~450 and ~550 °C for 100-1000 µm diffusion radii cooling at 1 °C/Myr. Lead
260"
diffusion through natural and synthetic rutile was experimentally measured by Cherniak
261"
(2000) at temperatures between 700 and 1100 °C using Rutherford Backscattering
262"
Spectrometry (RBS). Despite different trace-element compositions, results for diffusion 263"
through natural and synthetic rutile are similar. The resultant diffusion law yields closure
264"
temperatures between ~590 and ~720 °C for the same cooling parameters considered
265"
above for apatite. Pb diffusion in natural titanite was measured by Cherniak (1993) also
266"
using RBS; these parameters result in orientation-independent closure temperatures 267"
between ~570 and ~660 °C for the thermal history and diffusion domain sizes used for 268"
apatite above. Therefore, experimentally derived Pb diffusivities define an order of 269"
relative closure to Pb loss,
!*X#+YZ
[%!#+#V\+#Z%[%!V]V#+#Z
. 270"
271"
2.3.1 Empirical constraints on Pb diffusivities 272"
Despite the influence that experimentally-derived Pb diffusion rates have had on the 273"
interpretation of thermochronometric datasets from middle and lower-crustal terranes, a 274"
significant number of empirical U-Pb studies show that rutile U-Pb dates are younger 275"
than co-genetic titanite dates, contradicting the experimentally-based closure order (e.g. 276"
Bibikova et al., 2001; Christoffel et al., 1999; Connelly et al., 2000; Corfu and Easton, 277"
2001; Cox et al., 1998; Flowers et al., 2005; Flowers et al., 2006; Kylander-Clark et al., 278"
2008; Mezger et al., 1989; Möller et al., 2000; Norcross et al., 2000; Schärer et al., 1986; 279"
Schmitz and Bowring, 2003; Wit et al., 2001). Various explanations for this disagreement 280"
between experimental and empirical estimates of Pb diffusivities have been presented, 281"
including: i) fast diffusion of Pb through rutile facilitated by a reduced diffusion domain 282"
size by ilmenite and zircon exsolution (Lee, 1995; Zack and Kooijman, 2017), or by the
283"
presence of hydrogen within defective natural rutile crystals (Schmitz and Bowring,
284"
2003); ii) slower diffusion of Pb through titanite than predicted by experiments (Gao et
285"
al., 2012; Kohn, 2017; Marsh and Smye, 2017; Schärer et al., 1994; Spencer et al., 2013; 286"
Zhang and Schärer, 1996); and iii) mechanisms other than volume diffusion as the
287"
dominant process controlling Pb mobility through titanite. Regarding the latter point, a
288"
growing body of evidence suggests that recrystallization or coupled substitutions are the
289"
dominant mechanisms controlling U-Pb and trace element systematics in titanite (Garber 290"
et al., 2017; Marsh and Smye, 2017; Stearns et al., 2016; Stearns et al., 2015). 291"
292"
To further assess compatibility between experimental and empirical estimates of Pb 293"
diffusitivies in U-Pb thermochronometers, Figure 3 shows a comparison between 294"
empirical and laboratory-based Pb diffusivities for apatite (Fig. 3a), rutile (Fig. 3b), and 295"
titanite (Fig. 3c). Estimates of
^_`
are calculated from the empirical data using a forward 296"
modelling procedure in which best fit values of
aV
and
^b
are determined by minimizing 297"
the misfit between computed and published U-Pb date profiles or age-grain size curves 298"
for a specified thermal history. We defined the misfit as c41 % dR
)e df
)g4
K
)hi , 299"
where
\
is the total number of data points, dR
) is the measured age, df
) is the computed age 300"
and g is the data point uncertainty. This analysis is appropriate for estimating permissible 301"
values of
^_`
through apatite and rutile due to the significant number of U-Pb datasets in 302"
which the U-Pb systematics have been shown to be dependent on grain dimension and in 303"
which the thermal history is independently constrained by other thermochronometers. 304"
However, due to its elevated
!G
, there is a scarcity of studies that directly constrain the 305"
intracrystalline U-Pb date distribution profile for titanite; accordingly, estimates of
^_`
in
306"
titanite were calculated with a different approach. Using estimates of the duration spent
307"
(
#
) at peak conditions, grain size (
V
) and fraction of radiogenic Pb retained, we used
308"
values of the combined parameter
^#=Vj
, which is relevant for different degrees of Pb 309"
loss from a purely spherical mineral grain (Crank, 1979, his eq. 6.19), to solve for
^_`
.
310"
For reference, values of
^#=Vj
<0.03 are required for the central region of a crystal to
311"
preserve its original U-Pb date; values of
^#=Vj
>0.40 are required for >95% Pb loss from
312"
the mineral core. 313"
314"
Several thermochronometric studies place relatively precise limits on
^_`%
in apatite for 315"
crustal temperatures."DeWitt et al. (1984) measured whole-grain U-Pb apatite dates from 316"
Proterozoic crystalline basement of the Halloran Hills, southeastern California. Rocks 317"
that yielded 1710 Ma zircon dates also produced concordant, ~140 Ma apatite dates, 318"
interpreted to suggest that the apatite U-Pb dates record resetting during Jurassic 319"
metamorphic reheating. As cogenetic hornblende K-Ar ages are also reset, peak 320"
temperatures during the Jurassic event must have exceeded ~500 °C (Harrison, 1982). 321"
Assuming >90 % loss of radiogenic Pb (
^#=Vj
> 0.40), the reported grain diameter of 200 322"
µm and durations of reheating from 10 to 50 Myr results in minimum values for
^_`
in 323"
the range 1-5×1023 m2/s (DeW84 box, Fig. 3a). Cliff and Cohen (1980) showed that 324"
apatite from a metatonalite of the Hercynian basement complex in the Eastern Alps was 325"
reset during Alpine Barrovian metamorphism at 20-30 Ma. Recent geochronological 326"
work shows that peak metamorphic temperatures between 550 and 650 °C persisted for 327"
<10 Myr following the Alpine collision at ~35 Ma (Schneider et al., 2015; Smye et al., 328"
2011). For grain radii between 200 and 500 µm, values of
^_`
greater than 3×1022 and 5
329"
×1023 m2/s, respectively, are required to promote >90 % Pb loss (C&C80 box, Fig. 3a).
330"
Permissible combinations of
aV
and
^b
were also derived from three U-Pb apatite whole-
331"
grain TIMS datasets from localities with well-constrained cooling histories. Gulson 332"
(1984) constructed a 207Pb-206Pb apatite isochron from whole-grain mineral separates
333"
collected from the slowly-cooled Broken Hill orebody, New South Wales, Australia.
334"
Diffusivities were calculated using the 40Ar/39Ar-based thermal history for the Broken
335"
Hill block proposed by"Harrison and McDougall (1981). Best-fit diffusivities form a 336"
poorly defined (~4 log units range in
^_`
) envelope that overlaps with the experimental 337"
regression (G84 envelope, Fig. 3a). The large uncertainty associated with this estimate 338"
reflects uncertainty in the cooling rate (2-4 °C/Myr) and range of grain diameters 339"
considered (100 µm to 1 mm). Estimates of
^_`
%were also derived from the apatite dataset 340"
of von Blackenburg (1992), who measured whole-grain U-Pb ages from apatite in a 341"
granodiorite and tonalite sample pertaining to the Bergell pluton, Central Alps. This 342"
dataset is of particular value as it permits assessment of
^_`
%in apatite (vB92 envelope, 343"
Fig. 3a)%from a thermal history characterised by fast cooling, > 80 °C/Myr (Samperton et 344"
al., 2015; Villa and von Blankenburg, 1991), in contrast to the Broken Hill calculation. 345"
Finally,"Krogstad and Walker (1994) showed that the cores of large (1-2 cm diameter) 346"
apatite crystals yield concordant U-Pb ages that are 15-40 Ma younger than the age of 347"
crystallization of the Tin Mountain pegmatite body in the Black Hills, South Dakota. 348"
Precise U-Pb monazite, Rb-Sr muscovite, and K-Ar mica analyses independently 349"
constrain cooling rates to 2-3 °C/Myr (Redden et al., 1990; Riley, 1970), enabling 350"
determination of a tightly constrained (< 2 log units) envelope of permissible values of 351"
apatite
^_`
(K&W94 envelope, Fig. 3a).
352"
353"
Each of these calculations resulted in estimates of
^_`
in apatite that overlap the
354"
experimentally-derived values of Cherniak et al. (1991). With the caveat that we 355"
considered only five whole-grain U-Pb apatite datasets, this analysis implies that i) the
356"
experimental diffusion parameters accurately estimate
^_`
in natural apatite regardless of
357"
cooling rate and ii) the effective diffusion domain for Pb in apatite is comparable to, or
358"
defined by, grain dimensions. 359"
360"
For analysis of
^_`
in rutile, we considered three U-Pb datasets that clearly demonstrate a 361"
length scale dependence of U-Pb date on either grain size or distance from the crystal 362"
rim. Mezger et al. (1989) showed that U-Pb rutile dates from the Proterozoic Adirondack 363"
terrane correlate with grain size. Combining pre-existing zircon, garnet and monazite U-364"
Pb dates with amphibole and biotite 40Ar/39Ar dates, the authors estimated a time-365"
integrated cooling rate for the Adirondack Highlands of 1.5 °C/Myr between 1030 and 366"
800 Ma; using this cooling rate they assigned values of
!G
of 420 °C for grain radii 367"
between 90 and 210 µm and 380 °C for grain radii between 70 and 90 µm. However, 368"
subsequent reinterpretation of these data in light of upward-revision of values of Tc for 369"
monazite and titanite results in values of Tc between 500 °C and 540 °C (Vry and Baker, 370"
2006). We solved for permissible combinations of
aV
and
^b
that best fit Mezger’s 371"
whole-grain rutile U-Pb ages from the Adirondack Highlands; the resultant
%^_`
envelope 372"
(M89, Fig. 3b) overlaps experimental estimates of
^_`
between ~700 and 800 °C. Vry 373"
and Baker (2006) used LA-MC-ICP-MS to collect in-situ Pb-Pb ages over the outer 300 374"
µm of mounted rutile crystals from granulite facies rocks of the Reynolds Range,
375"
Australia. Using the established cooling rate of 2-3.5 °C/Myr, we estimated
^_`%
by
376"
minimizing the misfit between Vry’s Pb-Pb dates and those calculated over a 300 µm
377"
depth increment for rutile grains with diameters between 500 µm and 2 cm. The resultant 378"
best-fit envelope (V06, Fig. 3b) is relatively imprecise, spanning ~3 log units in
^_`
,
379"
which reflects the range of cooling rates considered. Finally, Kooijman et al. (2010) also
380"
used in-situ LA-ICP-MS to collect 207Pb/206Pb age profiles across individual grains of
381"
metamorphic rutile from granulite facies metapelites of the Archean Pikwitonei terrane, 382"
Manitoba, Canada. Traverses of 35 µm spots across 15 grains with diameters between 383"
120 and 280 µm yielded concordant ages decreasing by ~200 Ma from core to rim. This 384"
work built upon the previous work of Mezger et al. (1989) who established that rutile U-385"
Pb systematics in the Pikwitonei granulites exhibited a strong grain-size dependence. 386"
Following Mezger et al. (1989) and Kooijman et al. (2010), we calculated
^_`
using time-387"
integrated cooling rates between 0.5 and 1.5 °C/Myr by assessing the misfit between 388"
modelled and observed 207Pb/206Pb age profiles. Due to the well-defined nature of the 389"
closure profiles, our analysis resulted in a precise best-fit
^_`
envelope (K10, Fig. 3b) that 390"
spans 1-2 log units. 391"
392"
Each of the field-based rutile U-Pb datasets yield estimates of
^_`
that are both internally 393"
consistent and in excellent agreement with the experimental results of Cherniak (2000) 394"
between 650 and 750 °C. Our analysis of these three rutile U-Pb datasets demonstrates 395"
that laboratory rates of Pb diffusion can be extrapolated down-temperature to accurately 396"
interpret rutile U-Pb ages under conditions relevant for the middle and lower crust. This 397"
further highlights the potential for rutile to be used as a high-temperature U-Pb
398"
thermochronometer.
399"
400"
Empirical estimates of
^_`
in titanite are complicated by the fact that titanite can react 401"
and grow over an expansive P-T range, encompassing conditions well beneath its
!G
(e.g.
402"
Frost et al., 2001; Kohn, 2017). Furthermore, there is a scarcity of studies that document
403"
a length-scale dependency of U-Pb dates in titanite crystals. Here, we expand the
404"
compilation of estimates of
^_`
in titanite from Kohn (2017) using the combined 405"
parameter
^#=Vj
, as introduced above. Verts et al. (1996) dated whole titanite grains 406"
along a traverse through a contact aureole surrounding the Red Mountain pluton, Laramie 407"
Anorthosite Complex, Wyoming. Titanite grains in samples that experienced T >700 °C 408"
were shown to be completely reset to the age of pluton emplacement, whereas samples 409"
that experience peak T <700 °C define an array of ages spread between emplacement and 410"
a pre-emplacement regional metamorphic event. Samples within ~0.6 km of the pluton 411"
are estimated to have experienced peak temperatures between 700 and 1030 °C for 104-412"
105 years; combined with observed grain diameters of 100-400 µm, we estimate that
^_`
413"
exceeded ~2×1020 m2/s (V96, Fig. 3c). This estimate is valid for T between 700 and 1030 414"
°C, but the authors acknowledged that “…young U-Pb sphene ages in samples at greater 415"
distances must be produced by metamorphic growth of sphene.” Scott and St-Onge 416"
(1995) obtained whole-grain U-Pb ages from metamorphic titanite in a mafic tonalite 417"
gneiss from the Ungava/Trans-Hudson Orogen, Canada. They showed that in one sample, 418"
titanite grains ranging from 100 to 1000 µm in diameter yielded identical dates. Peak 419"
conditions for the metamorphic event were precisely constrained by multimineral 420"
thermobarometry to between 660 and 700 °C for <70 Myr. Under these conditions,
421"
diffusion rates <3×1025 m2/s (S&SO95, Fig. 3c) are required for a 50 µm radius titanite
422"
grain to retain radiogenic Pb (i.e.
^#=Vj
<0.03). Garber et al. (2017) showed that the cores
423"
of Precambrian titanite crystals from the Western Gneiss Region, Norway, escaped 424"
resetting despite being subjected to peak temperatures of 750-800 °C for 20-40 Myr
425"
during Caledonian metamorphism. For a titanite crystal of 200 µm diameter to preserve
426"
Precambrian core ages requires that
^_`
<4×1025 m2/s (G17, Fig. 3c). Marsh and Smye
427"
(2017) used LA-ICP-MS to collect U-Pb spot age profiles across large (<0.5 mm radius) 428"
titanite grains from the Grenville orogen. Despite peak metamorphic temperatures of 750-429"
800 °C that persisted for <50 Myr, the authors did not observe any systematic core-to-rim 430"
age variability. Retention of Pb under these conditions requires that
^_`
<2×1025 m2/s 431"
(M&S17, Fig. 3c). Finally, Holder et al. in review, used LA-ICP-MS to directly measure 432"
Pb and trace element concentration profiles in large (0.5-1 cm diameter), ultrahigh 433"
temperature titanite from southern Madagascar that conform to diffusion theory. They 434"
showed that the observed length scales of Pb diffusion through titanite from two samples 435"
that experienced peak temperatures of 750-800 °C and 900-1000 °C are consistent with 436"
values of
^_`
from ~3×1021 to ~1×1022 m2/s and from ~2×1025 to ~6×1027 m2/s, 437"
respectively (H1 and H2, Fig. 3c). 438"
439"
Our analysis demonstrates that, between 700 and 1000 °C, Pb diffusion in natural titanite 440"
occurs at rates that are 2-4 log units slower than predicted by experiments Cherniak 441"
(1993), similar to experimental rates of Sr diffusion (Cherniak, 1995)(Fig. 3c), as 442"
previously suggested by Garber et al. (2017), Kohn, (2017), Kohn and Corrie (2011), 443"
Marsh and Smye (2017) and Stearns et al. (2016; 2015). This shows that titanite U-Pb
444"
dates derived from crustal rocks are more likely to record processes other than thermally-
445"
enhanced volume diffusion, such as deformation, fluid flow, and recrystallization.
446"
Empirical studies have shown that all, some, or none of these behaviours may be 447"
significant in titanite during thermal events; though in some cases metamorphism
448"
foments (albeit slow) Pb diffusion and fluid-driven recrystallization (e.g. Garber et al.,
449"
2017), other studies have shown that titanite may entirely escape recrystallization during
450"
>700 oC heating and fluid flow, such that trace-element growth zoning (including Pb) is 451"
preserved (e.g. Stearns et al., 2016). Likewise, though titanite recrystallization may be 452"
associated with U-Pb age resetting, recent atomic-scale work even suggests that Pb is not 453"
necessarily mobilized from the titanite lattice during intracrystalline deformation 454"
(Kirkland et al., 2018). Further, there are a range of complex chemical substitutions in the 455"
titanite lattice (e.g. Prowatke and Klemme, 2006), such that Pb mobility may be partially 456"
coupled to the diffusive behaviour of other elements. Given the broad spectrum of titanite 457"
petrological behaviors, any attempt to tie titanite U-Pb data to a thermal history i) 458"
requires extensive geochemical characterization to exclude the influence of non-diffusive 459"
processes, ii) must account for growth or recrystallization zoning profiles that potentially 460"
mimic diffusion profiles (Stearns et al., 2016), and iii) must account for highly imprecise 461"
Pb diffusion parameters (this study). For these reasons, we suggest that titanite is better 462"
suited to “petrochronology”, i.e., records of interactions between minerals, fluids, and 463"
melts, rather than “thermochronology”, i.e., thermally activated intracrystalline diffusive 464"
records. 465"
466"
2.4 Measurement of U-Pb date profiles
467"
Traditionally, U-Pb thermochronology has been performed using whole-grain age versus
468"
grain size correlations (e.g. Blackburn et al., 2011; Schoene and Bowring, 2007). The
469"
length-scale dependency of thermally activated volume diffusion means that a single 470"
thermal history is expected to generate a predictable age v. grain size trend which can
471"
then be inverted for cooling rate. The strength of such an approach is that the U-Pb
472"
isotopic composition of individual grains can be measured precisely with state-of-the-art
473"
ID-TIMS techniques. This means that both 206Pb-238U and 207Pb-235U dates can be used in 474"
the derivation of thermal histories, in contrast to the typical 1-5% 206Pb-238U date 475"
uncertainty associated with ICP-MS analyses. However, whole-grain U-Pb 476"
thermochronology assumes that the entire analysed grain is equal to the effective 477"
diffusion radius. 478"
479"
Figure 1 shows that the steepest age gradient within a U-Pb date profile occurs proximal 480"
to the grain rim (at least for the case of homogenous U growth zoning). Given that 481"
accessory mineral grain sizes are typically on the order of 100 µm, distinction between 482"
thermal histories and effective Pb diffusion radii requires direct sampling of the profile at 483"
spatial resolutions better than a few microns. Slowly cooled accessory minerals with U-484"
Pb date profiles in excess of ~100 µm can be sampled in-situ with laser-ablation spot 485"
traverses (e.g. Cochrane et al., 2014; Kooijman et al., 2010). The benefit of this approach 486"
is that spot traverses can be collected in-situ, thus preserving the micro-textural context of 487"
each grain; furthermore, individual grains can be characterised for major- and minor-488"
element zoning prior to laser ablation, which is important for distinguishing between 489"
competing formation mechanisms. However, spot measurements integrate Pb
490"
concentration profiles with a resolution determined by spot diameter; typical spot
491"
diameters are 20-100 µm, meaning that unless the Pb diffusion length scale is >> 100 µm,
492"
resultant thermal history information will be imprecise or even unresolvable. 493"
Furthermore, the error function form of a diffusion profile means that material analysed
494"
within a single laser spot will be spatially weighted to reflect the zone with the highest
495"
concentrations (i.e. grain cores); this restricts the precision of derivative thermal history
496"
information. Furthermore, non-central sectioning of individual grains can lead to aliasing 497"
of the diffusion profile and overestimating the time-integrated magnitude of diffusion. 498"
499"
In contrast, depth profiling affords sampling of a mineral age or concentration profile at 500"
sub-micron intervals. This approach is based on the ability to resolve discrete variations 501"
in mineral chemistry or age as a function of depth into the crystal’s interior. First 502"
proposed by Zeitler and Williams (1988) and Zeitler et al. (1989), depth profiling of U-Pb 503"
accessory phases has traditionally been undertaken using secondary-ion mass 504"
spectrometry (SIMS) (e.g. Abbott et al., 2012; Breeding et al., 2004; Kelly et al., 2014; 505"
Lee et al., 1997; McFarlane and Harrison, 2006; Trail et al., 2007). However, the 506"
moderate sputtering rate of SIMS depth profiling (~0.075 µm per mass scan)(Breeding et 507"
al., 2004) limits pit depths to less than a few microns. In contrast, the aggressive pit 508"
excavation associated with LA-ICP-MS analysis has resulted in the emergence of two 509"
distinct depth-profiling methodologies. Continuously pulsed ablation (e.g. Kohn and 510"
Corrie, 2011; Paton et al., 2010; Smye and Stockli, 2014; Tollstrup et al., 2012) has the 511"
benefit of rapid data acquisition and the ability to sample intracrystalline gradients over 512"
tens of microns—typical for U-Pb date zonation in slowly cooled rutile and apatite. Both
513"
aerosol mixing of the analyte and time-dependent elemental fractionation restrict the
514"
spatial resolution and analytical precision of the approach. Various smoothing devices
515"
and downhole fractionation correction schemes are regularly employed to minimize such 516"
effects. In contrast, single-pulse ablation and derivative methodologies (e.g. Cottle et al.,
517"
2009; Cottle et al., 2012; Stearns et al., 2016; Steely et al., 2014; Viete et al., 2015) avoid
518"
these complications by integrating total counts collected in discrete laser pulses. By
519"
reducing ablation volume, sample mixing is minimized. Cottle et al. (2009) demonstrated 520"
an ablation rate of 0.1 µm per pulse which approaches the typical analytical volumes 521"
associated with SIMS depth profiling. Finally, the recent advent of Laser Ablation Split 522"
Stream (LASS) analysis heralds a new era for depth profiling in which complementary 523"
U-Pb date and trace-element information are collected from the same sub-micron 524"
analytical volume. This approach has great potential to resolve distinct diffusive and non-525"
diffusive mechanisms for elemental zonations by assessing how length scales of 526"
elemental zonation conform to the relative order predicted by experimental diffusivities 527"
(e.g. Stearns et al., 2016; Viete et al., 2015). 528"
529"
2.5 Inversion of U-Pb date profiles 530"
Given the complex relationship between age data and thermal history, extracting thermal 531"
histories information from measured U-Pb profiles is suited to treatment as an inverse 532"
problem. Various algorithms have been previously applied, including different Bayesian 533"
approaches (e.g. Gallagher, 1995; Gallagher, 2012; Willett, 1997) and basic Monte Carlo 534"
methods (e.g. Grove and Harrison, 1999; Ketcham et al., 2000; Smye and Stockli, 2014). 535"
The latter techniques are straightforward to implement but are prohibitively inefficient in
536"
searching large ranges of thermal histories and may not yield any solutions in large and
537"
precise datasets (Vermeesch and Tian, 2014). Here, we describe a new Bayesian
538"
approach that is well suited to the inversion of U-Pb date profile datasets in balancing 539"
computational efficiency with searching thermal history coordinate space. It comprises of
540"
the following steps:
541"
542"
1. Generate a random
#e!
history. Draw a small number (e.g. 5) of values for each of 543"
these two parameters from a preset range, and interpolate between these ‘anchor 544"
points’ with a piecewise cubic hermite polynomial function. Monotonic thermal 545"
histories can be enforced by ensuring that the anchor points are arranged in 546"
increasing order before the interpolation. 547"
548"
2. Predict the expected U-Pb depth profile. Given one or more sets of kinetic 549"
parameters and a specified (spherical, elliptical, cylindrical, tetragonal or 550"
hexagonal) geometry, simulate the combined radiogenic ingrowth and volume 551"
diffusion of U and Pb for the specified tT history using a Crank-Nicolson finite 552"
difference approach. 553"
554"
3. Compare the expected U-Pb depth profile(s) with the measured one(s). Let
%k
be the 555"
number of depth profiles (
k
1) and let ni be the number of U-Pb date 556"
measurements in the
+
th profile (for 1
+
k
). Further let
#+l
be the
l
th U-Pb date 557"
estimate of the
+
th
%
profile (for 1
l
\+
) and
m
[
#+l
] its standard error. Finally, let
D+l
558"
be the depth at which
#+l%
was measured. The goodness-of-fit of the predicted depth
559"
profile to the measured values can then be quantified by the following log-
560"
likelihood function:
561"
nn 1d)o e d U)o
g d)o
4
K/
ohi
p
)hi
564"
where
#
q
D+l
r is the predicted age at depth
D+l
, obtained from the piecewise
562"
polynomial interpolation that was discussed in step 1.
563"
565"
4. Modify the
#e!
path obtained in step 1, rerun steps 2 and 3, and reject or accept the 566"
new
#e!
path depending on the new log-likelihood value. Repeat until the 567"
algorithm has converged to a representative set of ‘likely’
#e!
solutions. It is 568"
customary to ignore the first ~20% of the solutions to account for the ‘burn-in’ time 569"
required to locate the solution space. 570"
571"
The mechanics of this iterative Markov Chain Monte Carlo (MCMC) process are handled 572"
by Foreman-Mackey et al. (2013)‘s implementation of the Goodman and Weare (2010) 573"
ensemble sampler. This general-purpose algorithm (which is also known as the ‘MCMC 574"
Hammer’) has several benefits over traditional MCMC methods. Most importantly, it 575"
simplifies the modification step of the
!"#
paths and is able to search the possible solution 576"
space in parallel by evaluating an ensemble of ‘walkers’. This results in an increased 577"
convergence rate that enables rapid global exploration of thermal histories. The above 578"
algorithm is implemented in a MATLAB function named UPbeat, which includes an 579"
intuitive graphical user interface. The software and its source code are available from 580"
http://UPbeat.london-geochron.com. In its present form, UPbeat does not readily
581"
accommodate external
!e#
constraints such as temperatures at specific times, or specific
582"
rates of cooling/heating. In our view, it is better to use external constraints to validate the
583"
inverse model results, rather than bias them (Vermeesch and Tian, 2014). A final 584"
important quality of our software is its ability to handle large datasets comprised of
585"
multiple depth profiles from the same sample; the use of multiple profiles increases the
586"
algorithm’s power to resolve the thermal history, which allows the user to increase the
587"
number of anchor points or to consider non-monotonic cooling histories without 588"
sacrificing precision. However, we stress that this approach is predicated on identifying 589"
U-Pb date profiles that are diffusive in nature, with an effective diffusive radius 590"
equivalent to the grain size (i.e. each profile conforms to an error function and has the 591"
same age at the outermost depth interval). 592"
593"
To demonstrate the MCMC inversion approach applied to U-Pb thermochronology, we 594"
present results of an example inversion of a rutile U-Pb date profile measured using the 595"
depth-profiling methodology outlined in Smye and Stockli (2014) (Fig. 4). The example 596"
shown is from an unpublished rutile U-Pb depth profile dataset collected from a suite of 597"
Permian lower crustal granulites from the Pyrenees that were exhumed during Cretaceous 598"
(~100 Ma) hyper-extension of the crust and mantle lithosphere in southwestern France 599"
and northern Spain (Hart et al., 2017, and references therein). An additional example is 600"
contained in Fig. 14, showing a joint inversion for two sets of three rutile U-Pb depth 601"
profiles from the Grenville orogeny. These figures clearly exhibit the power of our model 602"
to rapidly discard non-permissible thermal histories, and to converge on a best-fit thermal 603"
history through the rutile PRZ.
604"
605"
3. Additional controls on U-Pb date profiles
606"
U-Pb thermochronology is dependent on diffusive transport of Pb, which should yield 607"
core-rim age profiles similar to that shown in Figure 5a. However, there are numerous
608"
alternative Pb transport mechanisms other than grain-scale volume diffusion that can
609"
influence the topology of U-Pb date profiles; these processes are equally relevant to
610"
within-grain differences in a range of trace elements, including either or both U and Pb. 611"
The section is focused on how such processes can be distinguished using intracrystalline 612"
U-Pb or trace-element profiles. 613"
614"
3.1 Secondary growth 615"
Overgrowths reflect a hiatus in crystal growth and are most often characterized by a sharp 616"
change in
DW=D*
, where
W
is radiogenic Pb or trace-element concentration. Growth of 617"
rims at temperatures sufficient to drive diffusive Pb transport will result in a smoothed 618"
core-rim boundary; conversely, low-temperature rim growth will result in a step-like 619"
discontinuity. On a Tera-Wasserburg concordia plot, a simple core-rim overgrowth 620"
relationship (disregarding common Pb) will result in discordia with upper and lower 621"
intercepts at the U-Pb isotopic compositions of the core and rim, respectively (Fig. 5b). 622"
The extent to which analyses spread along the discordia is governed by the width of the 623"
core-rim interface relative to the spatial resolution of the analytical technique (Fig. 5b). 624"
Rim overgrowths may be a common feature of zircon grains (e.g. Cottle et al., 2009), 625"
suggesting that pre-existing crystal facets are kinetically favourable sites for new growth
626"
compared to newly nucleated crystals. However, rutile and apatite overgrowths are less
627"
commonly observed in U-Pb datasets, plausibly because their reactive nature means that
628"
they are unlikely to survive multiple metamorphic cycles. 629"
630"
3.2 Recrystallisation
631"
Recrystallisation involves re-growth and re-ordering of disordered portions of the crystal
632"
lattice due to i) lattice strain"from thermodynamic incompatibility of trace-element 633"
species incorporated at different
_"!
conditions (e.g. Stünitz et al., 2003), ii) differential 634"
stresses exerted by the surrounding matrix (e.g. Twiss, 1977; Urai et al., 1986), or iii) 635"
periods of undersaturation/saturation with respect to grain boundary fluid phases (e.g. 636"
Villa, 1998; Williams et al., 2011a). Unlike secondary growth, there may be negligible 637"
addition of new material to the crystal grain. Instead, structural reordering of the mineral 638"
lattice promotes loss of incompatible elements, including radiogenic Pb. The 639"
susceptibility of the U-Pb thermochronometers to recrystallization is controlled by ionic 640"
bond strength (Dahl, 1996; Dahl, 1997; Villa, 1998). Whilst there is little evidence for 641"
apatite recrystallization at temperatures relevant to its PRZ (Chamberlain and Bowring, 642"
2001), recrystallization of apatite has been documented under amphibolite- to granulite-643"
facies conditions during monazite-forming reactions (e.g. Bingen et al., 1996) and at low-644"
temperatures (<150 °C) in the presence of Cl- and F-bearing fluids (Boudreau et al., 645"
1986; Romer, 1996). Not withstanding the propensity for rutile to exsolve Fe-oxides and 646"
zircon, there is some evidence to suggest that rutile is commonly affected by pervasive, 647"
grain-scale recrystallization (e.g. Mücke and Chaudhuri, 1991; Rösel et al., 2014). A 648"
coherent body of evidence shows that resetting of the U-Pb and trace-element systematics
649"
of monazite (Crowley and Ghent, 1999; Poitrasson et al., 1996; Poitrasson et al., 2000;
650"
Seydoux-Guillaume et al., 2002; Williams et al., 2011b) and titanite (Hawkins and
651"
Bowring, 1999; Spencer et al., 2013; Stearns et al., 2016; Stearns et al., 2015; Zhang and 652"
Schärer, 1996) is controlled by recrystallization. Such minerals often exhibit step-like
653"
boundaries across which trace-element concentrations and U-Pb dates differ markedly
654"
(e.g. Fig. 5c), consistent with recrystallization proceeding by a reaction front mechanism.
655"
In contrast to diffusion, there is no elegant length-scale dependency that can predict 656"
elemental (re)distributions associated with recrystallization; rather, partial 657"
recrystallization typically results in patchy, fracture-controlled, or twin-plane controlled 658"
within-grain U-Pb date and trace-element differences (e.g. Garber et al., 2017; Putnis, 659"
2009; Spencer et al., 2013). The degree of chemical change associated with 660"
recrystallization is controlled by changes in the solubility and transport of components in 661"
grain boundary media (e.g. Putnis, 2009); prolonged recrystallization thus has the 662"
potential to preserve a record of time-dependent variations in
_"!
conditions or fluid 663"
chemistry, (e.g."Stearns et al., 2016). These factors mean that recrystallization can result 664"
in a variety of topologies on U-Pb concordia plots. 665"
666"
3.3 Common Pb 667"
Incorporation of common Pb—the portion of non-radiogenic Pb within a U-bearing 668"
mineral—results in discordant age data. U and Pb are fractionated during mineral growth 669"
because the two ions have different charges (U4+ vs. Pb2+) and ionic radii (U4+=1.00 Å; 670"
Pb2+=1.29 Å, in VIII- coordination) (Shannon, 1976). However, isovalence between Ca2+ 671"
and Pb2+ means that titanite and apatite commonly incorporate common Pb during
672"
(re)crystallization, whereas common Pb concentrations in rutile are typically subordinate
673"
to titanite and apatite (e.g. Chew et al., 2011; Frost et al., 2001; Zack et al., 2011b).
674"
Provided that the common Pb component has a single isotopic composition, the 675"
incorporation of variable quantities of common Pb defines discordia in Tera-Wasserburg
676"
coordinate space with upper and lower intercepts defined by the isotopic composition of
677"
the non-radiogenic and radiogenic components, respectively (Figs. 5a-d). If uncorrected,
678"
common Pb can yield apparent inversely zoned U-Pb date profiles in which rim ages 679"
exceed interior ages (e.g. if rim analyses contain more common Pb than core analyses). 680"
Inherited Pb—a specific type of common Pb—is incorporated when crystal growth 681"
occurs at the site of a radiogenic precursor phase. It is an uncommon feature of rutile and 682"
apatite U-Pb systematics, but several studies have documented inherited Pb components 683"
in titanite (Romer and Rötzler, 2003; Zhang and Schärer, 1996). Inherited Pb will form 684"
discordia with upper and lower intercepts defined by the age of the inherited and 685"
radiogenic components, respectively—similar to secondary growth. An accurate common 686"
Pb correction is required in order to discriminate between the various Pb transport 687"
mechanisms discussed here. For example, comparison of uncorrected U-Pb analyses 688"
displayed on the Tera-Wasserburg diagrams in Figs. 5a and 5b shows that mixing with 689"
common Pb can obscure the characteristic data topologies associated with volume 690"
diffusion and secondary growth. 691"
692"
3.4 Inclusions and exsolution 693"
Mineral inclusions sampled during analysis cause mixing between U-Pb compositions of 694"
the host and inclusion phases. Here, we restrict our treatment of inclusions to those
695"
mineral phases older than the host. Similar to secondary growth and inherited Pb,
696"
incorporation of included phases during an in situ measurement will result in discordia
697"
with end-member U-Pb isotopic compositions defined by the included and host phases. 698"
Optically visible inclusions should obviously be avoided during analysis, but optically
699"
minute micro-inclusions are commonplace in titanite, rutile and apatite,"(e.g. Schmitz and
700"
Bowring, 2001). In theory, closed-system exsolution of zircon and ilmenite from rutile
701"
should not alter the bulk U and Pb budget of a crystal; the process is predicted, however, 702"
to redistribute both U and Pb between host rutile and lamellae phases, according to 703"
relative solubilities. In the case of zircon lamellae, in which U4+ readily substitutes for 704"
Zr4+, the age of the zircon needles will reflect the timing of exsolution. Mixed analyses of 705"
host and lamellae will spread between the age of the host and exsolution. Partitioning 706"
experiments also suggest that U is weakly fractionated from Pb during ilmenite 707"
exsolution"(Klemme et al., 2006; Klemme et al., 2005). 708"
709"
3.5 Short-circuit diffusion 710"
Mineral lattices are imperfect and commonly contain extended defects, including 711"
dislocations, micropores, microfractures and subgrain boundaries. These defects have the 712"
potential to act as fast diffusion pathways, the effects of which have been studied in depth 713"
by the materials science community (Joesten, 1991; Le Claire and Rabinovitch, 1984; 714"
Ruoff and Balluffi, 1963). The large difference in ionic radii between Pb2+ and parent U4+ 715"
(0.29 Å) means that radiogenic Pb does not energetically favour the crystallographic site 716"
occupied by parent U. Consequently, daughter atoms of Pb are predicted to partition into 717"
structural defects and subsequently undergo rapid diffusive transport relative to the rate
718"
of lattice volume diffusion. An important prediction of short circuit diffusion theory is the
719"
presence of flat or, more generally, intracrystalline concentration profiles that are
720"
controlled by the density of structural defects and relative diffusivities between defect 721"
and host (Lee, 1995) rather than by the size of the grain. Investigations of short-circuit
722"
diffusion in thermochronometers have been focused on the 40Ar/39Ar (e.g. Lo et al., 2000;
723"
Lovera et al., 2002) and (U-Th)/He systems"(e.g. Shuster et al., 2004). Short-circuit
724"
behaviour of Pb has been observed in zircon following the formation of microfractures 725"
arising from lattice expansion during metamictization"(Geisler et al., 2007). Networks of 726"
ilmenite and zircon exsolution lamellae in rutile could plausibly form fast diffusion 727"
pathways for radiogenic Pb where the bulk grain
!G
is controlled by the density of 728"
exsolution plates. 729"
730"
3.6 Parent zonation 731"
Within-grain differences in U and Th concentrations lead to spatially dependent Pb 732"
production rates and gradients in radiogenic Pb concentrations that drive intracrystalline 733"
diffusion. Therefore, parent zonation influences the shape of U-Pb date profiles; because 734"
U and Th diffusion rates in minerals are nearly always more sluggish than Pb, diffusion 735"
of radiogenic Pb from regions of high U and Th will lead to artificially old ages in 736"
neighboring domains. An example suite of U concentration profiles from lower crustal 737"
rutile and apatite is presented in Fig. 6; specifically, these profiles are from rutile from 738"
the Ivrea Zone, and apatite and rutile from Corsica (Seymour et al., 2016). There is no 739"
systematic U zonation between these profiles: grains from the same sample exhibit a 740"
variety of topologies from inwardly to outwardly decreasing U concentrations and from
741"
smoothly varying profiles to those with sharp discontinuities. Profiles with sharp
742"
discontinuities in U concentration are consistent with secondary growth (e.g. yellow
743"
curve, Fig. 6b), whereas smoothly varying profiles are consistent with U incorporation 744"
during protracted growth (e.g. purple curve, Fig. 6a). To demonstrate the effect of U
745"
zonation on the shape of U-Pb date profiles, Fig. 7 presents calculated core-to-rim
746"
profiles for four U zonation types: uniform, secondary growth of a high-U rim, growth
747"
zoning controlled by Rayleigh fractionation, and oscillatory zoning. The profiles are 748"
calculated using experimental Pb diffusion parameters for rutile (Cherniak, 2000) and 749"
cooling from 700 °C for 1 Ga at 0.3°C /Myr. The overgrowth scenario (Fig. 7c-d) shows 750"
that U-rich rims drive diffusion of radiogenic Pb toward the grain center, and restrict the 751"
loss of Pb across the grain boundary (*=*b%1 1 in Fig.7) at elevated temperatures; this 752"
case assumes that the high-U rim formed soon (< 1 Ma) after the core. Growth zoning of 753"
U in which the core region is enriched (Fig. 7e-f) drives rimward diffusion of radiogenic 754"
Pb from adjacent high U domains through low U portions of the crystal, resulting in a 755"
concave U-Pb date profile with the oldest preserved date positioned away from the grain 756"
core. Oscillatory zoning in which U concentrations vary over micron length scales (Fig. 757"
7g-h) produces an age profile characterised by discontinuities that are progressively 758"
dampened toward the grain rim. These calculations show that near-rim effects of U 759"
zonation are likely to be removed as a result of the large chemical potential gradient 760"
across the grain boundary (assuming a zero Pb matrix). Furthermore, it is important to 761"
note that unless intragrain U concentration differences are >10 ppm, the effect of U 762"
zonation on age topology will only be resolvable in old samples (>~107 years) with 763"
significant ingrown radiogenic Pb. With the exception of the yellow curve, the magnitude
764"
of U zonation in the rutile profiles shown in Fig. 6a is typically <1 ppm over the 30 µm
765"
profile depth, whereas U zonation magnitudes in apatite (Fig. 6b) are between 4 and 20
766"
ppm. Regardless, these considerations establish that the accuracy of a U-Pb date profile 767"
inversion will be enhanced by incorporating the specific within-grain U zonation.
768"
769"
3.7 Flux-limited boundary conditions
770"
Chemical equilibrium between the surface of a mineral grain and the rock matrix can be 771"
impeded by a number of kinetic factors, including slow, or inefficient, grain-boundary 772"
mass transport, slow intracrystalline diffusion and slow rates of dissolution of a source 773"
mineral phase and/or precipitation onto the surface of the target mineral. Each of these 774"
processes serve to limit the rate at which thermodynamic equilibrium is established 775"
between rock matrix and crystal surface (Dohmen and Chakraborty, 2004). Of particular 776"
relevance here are the cases when either the capacity of the grain boundary reservoir is 777"
limited by slow transport rates (i.e. absence of a fluid phase) or, when the rate of interface 778"
reaction is slow relative to the rate of intracrystalline diffusion. Both cases are expected 779"
to result in mineral concentration profiles with elevated rim concentrations and less 780"
curvature compared to the classic case in which intracrystalline diffusion is the rate-781"
limiting transport process. Whilst the specific chemical parameters that control the 782"
behaviour of Pb in grain boundary fluids and across mineral-fluid interfaces under deep 783"
crustal conditions remain incompletely understood, the observation that accessory 784"
minerals, such as rutile, can exhibit disparate trace element concentration profiles with 785"
different rim concentrations in crystals from the same hand sample is strong evidence that 786"
flux-limited boundary conditions are potentially of great importance to the formation of
787"
trace element and U-Pb date profiles in accessory minerals (e.g. Kohn et al., 2016). It
788"
should also be noted that for the case in which a mineral grain has experienced
789"
temperatures above its PRZ, but with a flux-limited boundary condition, a flat internal U-790"
Pb date concentration profile will likely be present (e.g. Fig. 5d).
791"
792"
4. Case study: lower crustal rutile from the Grenville Province
793"
To demonstrate some applications and limitations of U-Pb thermochronology, we present 794"
a new rutile U-Pb and trace-element dataset from the exhumed lower crust of the 795"
Grenville orogen, eastern Canada. 796"
797"
4.1 Geological Background 798"
The Grenville orogen is a major Mesoproterozoic orogenic belt spanning from southern 799"
Ontario to Labrador and exposes deep structural levels of a large, hot collisional orogen, 800"
similar in size and structure to the modern Himalayan-Tibetan system (Beaumont et al., 801"
2006). The samples investigated in this case study (GB119C and GB132A) are both 802"
rutile-bearing mafic granulites that were collected from meter-scale mafic pods from the 803"
lower allocthonous domains of the Central Gneiss Belt (CGB; Fig. 8). Details of the 804"
samples, regional geology and geochronology are provided in the Supplementary 805"
Material; here, we summarize key information relevant to the interpretation of the rutile 806"
U-Pb dataset. 807"
808"
Phase equilibrium modelling, supported by multi-equilibria thermobartometry and single-
809"
phase solution thermometry in rutile and titanite, define a clockwise
_"!
path for the
810"
samples, evolving from rutile growth at temperatures above 700 °C at ~1.5 GPa to peak 811"
granulite facies conditions of >800 °C at 1-1.5 GPa (Grant, 1989; Marsh and Kelly,
812"
2017). Zircon U-Pb geochronology constrains the timing of the early high-pressure
813"
metamorphism to 1090-1110 Ma (Ketchum and Krogh, 1998; Marsh and Culshaw, 2014)
814"
and the subsequent granulite-facies overprint to 1040-1080 Ma (Tuccillo et al., 815"
1992;Slagstad et al., 2004). Hornblende 40Ar-39Ar ages throughout the lower 816"
allochthonous domains of the Grenville typically fall between 930 and 1000 Ma, 817"
clustering around 970 Ma, whereas mica and K-feldspar 40Ar-39Ar ages cluster around ca. 818"
900 Ma (Cosca et al., 1992; Cosca et al., 1991). Compilation of these data indicate an 819"
extended period of high temperature (~750–850 °C) metamorphism from ~1110–1040 820"
Ma, followed by relatively slow cooling (<3 °C/Myr) to ~500 °C by ca. 970 Ma and ~300 821"
°C by 900 Ma (Cosca et al., 1991). Thus, rutile from samples GB119C and GB132A 822"
formed at temperatures in excess of 700 °C, were subsequently exposed to temperatures 823"
in excess of 800 °C, and apparently remained above 700 °C for up to 80 Myr during the 824"
Ottowan phase of the Grenville orogeny that marked the transition from warm subduction 825"
to burial in lower orogenic crust. 826"
827"
4.2 Methods 828"
4.2.1 LA-ICP-MS spot analyses 829"
We collected U-Pb spot dates and depth profiles from samples GB119C and GB132A 830"
using LA-ICP-MS and LASS analysis, respectively. Spot dates were collected from
831"
polished thin sections at Laurentian University using an iCap-TQ ICP-MS coupled to a
832"
Photon Machines Analyte G2 laser ablation system. Optimal signal strengths were
833"
attained using a 65 µm spot diameter, a fluence of 2 J/cm-2 and a repetition rate of 10 Hz. 834"
Oxide interferences were minimized by tuning gas flows such that UO/U < 0.5%. For U–
835"
Pb isotopic abundance measurements, correction for instrumental drift and laser-induced
836"
elemental fractionation was addressed via analysis of rutile standard R10 (Luvizotto et
837"
al., 2009), using a standard-sample-bracketing routine. Rutile R19 was used to assess age 838"
accuracy; 206Pb/238U ratios for R19 were consistently within 2σ uncertainty of the ID-839"
TIMS values reported by Zack et al. (2011a). 840"
841"
4.2.2 LA-ICP-MS depth-profile analysis 842"
Trace-element concentrations and U-Pb date depth-profiles were collected from separated 843"
whole grains of rutile mounted (unpolished) on tape at the University of Texas following 844"
the methodology of Smye and Stockli (2014). Rutile R19 was used to assess age 845"
accuracy; 206Pb/238U ratios for R19 were consistently within 2σ uncertainty of the ID-846"
TIMS values reported by Zack et al. (2011a). 847"
848"
4.3 Results 849"
U-Pb isotope data for all LA-ICP-MS analyses are presented in the supplementary 850"
material. 851"
852"
4.3.1. LA-ICP-MS spot analyses
853"
Matrix (n=8) and inclusion (n=3) rutile grains from sample GB119C yielded U-Pb spot
854"
analyses that define an array in Tera-Wasserburg concordia space; because some spots
855"
plot off concordia and others define U-Pb dates equivalent to or significantly younger 856"
than zircon U-Pb dates, the analyses are interpreted to indicate both Pb loss and mixing
857"
with common Pb (Fig. 9). Common-Pb corrected analyses are concordant within
858"
analytical uncertainty and yield a spectrum of dates between ~1050 and 800 Ma (Fig.
859"
S1). Figure 10a shows 207Pb-corrected 238U-206Pb ages plotted as a function of distance 860"
from the grain rim for samples GB119C. From visual inspection of the figure, it is clear 861"
that there is no systematic correlation between age and within-grain position, as would be 862"
the case for volume diffusion in which the effective diffusion radius was equivalent to the 863"
grain size. The relationship between U-Pb age and textural setting is demonstrated in Fig. 864"
10c; note that the matrix grains yield a significant date spread (846-959 Ma) and that the 865"
rutile grain included within garnet yields a significantly older age (972 Ma) . The 866"
remaining rutile inclusions in garnet yield ages of 904 and 1400 Ma, respectively; the 867"
oldest age is consistent with incorporation of inherited radiogenic Pb from a precursor 868"
phase. 869"
870"
The U-Pb systematics of sample GB132A are similar to GB119C. U-Pb analyses of 871"
matrix grains (n=12) define an array that is consistent with both Pb loss and mixing with 872"
common Pb (Fig. 9b); common-Pb corrected analyses fall along concordia between ~800 873"
and ~1040 Ma (Fig. S1b). Rutile grains large enough to permit measurement of multiple 874"
spot ages do not yield rims with younger ages than grain cores (Fig. 10b) 875"
876"
4.3.2. LA-ICP-MS depth-profile analysis
877"
We collected 45 and 53 depth profiles from individual rutile crystals from samples
878"
GB119C and GB132A, respectively. The full U-Pb depth profile dataset is presented in 879"
the supplementary material (Table S2), in addition to compilation plots of the different
880"
profile topologies collected from GB119C (Fig. S2) and GB132A (Fig. S3). Both
881"
samples exhibit U-Pb date profiles with three characteristic topologies: i) rounded
882"
profiles with younger rim than core ages (GB119C n=13/45; GB132A n=4/53), including 883"
some age profiles that increase over ~20 µm from ~900 Ma at grain rims to homogeneous 884"
~1100 Ma cores (Fig. 11a; Figs. S2 and S3), ii) profiles with ages that vary linearly with 885"
depth (GB119C n=20/45; GB132A n=32/53; Fig. 11b; Figs. S2 and S3), and iii) profiles 886"
with sharp (typically <5 µm) spatial discontinuities in U-Pb dates (GB119C n=12/45; 887"
GB132A n=17/53; Fig. 11c; Figs. S2 and S3). HFSE concentrations are generally flat and 888"
do not correlate with U or Pb; Zr in particular has concentrations between 1100 and 1500 889"
ppm and defines flat profiles even in grains in which the U-Pb profile decreases toward 890"
the grain rim. 891"
892"
4.4 Integrating spot and depth-profile U-Pb datasets 893"
Having discussed the various kinetic processes that can affect intracrystalline U-Pb date 894"
distributions, here we integrate these two datasets with petrographic observations to 895"
identify conditions that are favourable for the formation of diffusive date profiles 896"
required for U-Pb thermochronology. 897"
898"
Zircon and rutile crystallized at ~1100 Ma as part of the dominant HP metamorphic
899"
assemblage; therefore, the occurrence of U-Pb ages between 800 and 1100 Ma shows that
900"
rutile grains in both samples must have undergone significant Pb loss since
901"
crystallisation. However, the absence of a systematic relationship between U-Pb spot date 902"
and position (Fig. 10) combined with the observation that the majority of U-Pb depth
903"
profiles (n=81/98) exhibit non-diffusive topologies is consistent with the following
904"
explanations: i) U-Pb systematics were affected by partial recrystallization of rutile grains
905"
following metamorphic growth (section 3.2), ii) Fickian-type volume diffusion of Pb 906"
through rutile did not operate over whole-grain length scales, or iii) diffusive loss of Pb 907"
was flux-limited by grain boundary kinetic factors, but only in certain textural settings 908"
(section 3.7). 909"
910"
The presence of homogenous HFSE concentration profiles (Fig. 11) and the absence of 911"
significant chemical variations in matrix rutile (note homogeneous rutile BSE maps in 912"
Fig. 10) suggests that partial recrystallization – expected to yield patchy element 913"
distributions (Fig. 5c) – did not significantly affect the studied grains. Textural evidence 914"
for recrystallization of the Grenville rutile grains is limited to the presence of ilmenite 915"
exsolution lamellae that form micron-scale networks of variable density throughout both 916"
included and matrix grains. Although the role of exsolution on U-Pb systematics of rutile 917"
is unclear, experimental constraints on partitioning of Pb between rutile and ilmenite 918"
suggest that radiogenic Pb would not partition strongly into ilmenite on exsolution 919"
(2(0:)Jf
st 1 u e ubv%(Foley et al., 2000; Klemme et al., 2006; Klemme et al., 2005)). 920"
Rather, as discussed in section 3.5, it is conceivable that the grain boundaries between 921"
exsolution plates and host rutile crystals operate as fast diffusion pathways that would
922"
result in Pb diffusive length scales smaller than the rutile grain, as observed in both
923"
GB119C and GB132A, and a lower value of Pb
!G
. In this process, the loss of radiogenic
924"
Pb from a rutile grain would be controlled by the spacing between adjacent exsolution 925"
plates. Unfortunately, we were unable to establish a relationship between ilmenite
926"
lamellae density and U-Pb date due to the large laser spot sizes we used relative to the
927"
length scale of the lamellae networks. However, consistent experimental and empirical
928"
constraints on rutile
^_`
(Fig. 3b) suggest a Pb diffusive length scale of ~250–400 µm for 929"
the metamorphic conditions experienced by the studied rocks (~800 °C for ~10–20 Myr), 930"
which should have been sufficient to homogenize nearly all grains in both samples. 931"
Therefore, though fast diffusion pathways may have locally modified individual date 932"
profiles, it is clear another process must be responsible for the retention of higher 933"
radiogenic Pb concentrations than predicted by volume diffusion. 934"
935"
Figure 12 is a rank-order plot of U-Pb dates collected from the outermost depth increment 936"
of the depth profiles. Rim ages spread from ~800 to 1100 Ma, the timing of zircon 937"
growth and, by inference, rutile growth, in each sample. Volume diffusion calculations 938"
predict that the U-Pb age of the outermost depth increment of a cooling crystal is 939"
independent of grain size and records the timing at which the grain passes through the 940"
base of the PRZ, closing to Pb diffusion (Dodson, 1986). The fact that both samples show 941"
a ~300 Myr spread in rim ages indicates that U-Pb systematics in the Grenville rutile 942"
dataset cannot be explained by intracrystalline volume diffusion. Such a spread in rim 943"
ages is, however, predicted by flux-limited Pb transport, where the local capacity of the 944"
grain boundary reservoir to accommodate Pb controls the extent to which Pb is lost from
945"
the host rutile grain (Dohmen and Chakraborty, 2004). Under these conditions,
946"
intracrystalline Pb diffusion can occur efficiently over the length scale of the rutile
947"
crystal, but the net loss of radiogenic Pb is independent of intracrystalline diffusion rate. 948"
One prediction of flux-limited Pb transport is that an inverse correlation will exist
949"
between rutile U-Pb age and proximity to a mineral phase that can structurally
950"
accommodate Pb. Figure 13 shows a box plot of common-Pb corrected U-Pb spot ages
951"
grouped according to the mineralogy of the nearest grain boundary phase. We identify no 952"
systematic correlation between any of the rock-forming mineral phases; in particular, the 953"
lack of a correlation with proximity to plagioclase is surprising because plagioclase has 954"
been shown to be an important Pb sink (e.g. Chamberlain and Bowring, 2001). In the 955"
absence of texturally-controlled U-Pb rim ages, we suggest that dry grain boundaries 956"
could impede the rate of grain boundary transport of Pb and, ultimately, restrict the 957"
capacity of the grain boundary to host rutile-derived radiogenic Pb. An equally plausible 958"
explanation is that proximity to an Pb-bearing accessory phase could control the chemical 959"
potential gradient across rutile grain boundaries. Regardless, these observations highlight 960"
the importance of developing a more in-depth understanding of the physical and chemical 961"
controls on the behaviour of Pb along grain boundaries under deep crustal conditions. 962"
963"
The small number of depth profiles with monotonically increasing 238U-206Pb dates from 964"
rim to core share similar length scales of curvature and exhibit identical ages (within 965"
analytical uncertainty) over the outermost ~2 µm depth increment. This suggests that the 966"
boundary conditions for each of these grains during cooling were similar. Furthermore, 967"
each of the profiles conforms to the expectation of linearity when inverted through an
968"
inverse error function. These factors support the interpretation that profile formation was
969"
controlled by intracrystalline volume diffusion of Pb through rutile under conditions in
970"
which the effective diffusion domain was defined by grain dimensions. The occurrence of 971"
these profiles in the same samples as “non-diffusive” profiles may indicate heterogeneous
972"
or small-volume fluid flow along the grain boundary network that affected a limited
973"
number of grains.
974"
975"
Finally, it is important to note that the short length scales of diffusive Pb transport (~20 976"
µm) were not resolvable by spot analysis of grain cross-sections. Whilst spot analysis 977"
enables ages and trace element concentrations to be directly related to textural features, 978"
the coarse sampling resolution of the technique limits the resolution of derivative thermal 979"
history information. Conversely, depth profiling enables high resolution thermal history 980"
information to be extracted from single crystals, but does not preserve microtextural 981"
relations. The Grenville case study presented here shows that both techniques are 982"
required in order to accurately identify diffusive date profiles that can be used to generate 983"
non-spurious thermal history information. 984"
985"
4.5 Tectonic implications 986"
To ascertain the tectonic significance of the diffusive U-Pb date profiles, joint inversion 987"
of selected U-Pb date profiles from each sample was undertaken using the method 988"
outlined in section 2.5. The inversion computation was performed for monotonic cooling 989"
histories with a total of 10,000 iterations. Initial temperature was set at 825 °C, in
990"
agreement with thermobarometric constraints on peak temperatures. Grain-specific U
991"
profiles were used in conjunction with the diffusion parameters of Cherniak (2000).
992"
Resultant thermal histories for both samples are presented in Fig. 14; best-fit profiles are 993"
characterised by an early period of fast cooling from peak temperatures at rates of ~10
994"
°C/Myr followed by slow cooling to <500 °C at <1 °C/Myr. These
!"#
trajectories are
995"
consistent with existing zircon U-Pb growth ages and hornblende 40Ar-39Ar cooling ages
996"
for the allochthonous domain host gneisses (Fig. 15), passing through granitic melt 997"
crystallization conditions (~650 °C) between ~1080-1050 Ma and through 500 °C at 998"
~1000 Ma; existing biotite 40Ar-39Ar dates around 900 Ma require a further stage of 999"
cooling from the rutile PRZ that is not resolvable with the rutile dataset, or suggests that 1000"
the biotite Ar/Ar dates are not cooling ages. Even recognizing the array of U-Pb date
1001"
profiles, the rutile data therefore demonstrate how the methods described here are capable 1002"
of providing accurate and near-continuous cooling history information from carefully 1003"
selected individual crystals. 1004"
1005"
The non-linear cooling history presented here—as opposed to a slow, monotonic cooling 1006"
over ~200 Myr previously assumed for the western CGB (Fig. 10)—has important 1007"
implications for understanding tectonic processes in deep orogenic crust. An early phase
1008"
of rapid cooling from HT eclogite/HP granulite conditions suggests that the metabasite
1009"
pods were detached from lower crustal depths and exhumed to shallower crustal levels 1010"
over <~100 Myr. Previous workers have suggested that exhumation of deep-seated mafic
1011"
bodies within the Grenville orogeny is aided by a low-viscosity, low-density carapace of 1012"
granitic and metasedimentary migmatites (Marsh and Culshaw, 2014). A similar process
1013"
has been envisaged for other collisional orogens"(e.g. Brown and Dallmeyer, 1996;
1014"
Gordon et al., 2008; Little et al., 2011; Schulmann et al., 2008; Whitney et al., 2009) and 1015"
is consistent with the results of geodynamic models (Beaumont et al., 2006; Jamieson et
1016"
al., 2007), where post-subduction collision of rigid crustal blocks drives extrusion of the
1017"
basal portions of lower allocthonous domains to shallow crustal levels. The well-
1018"
documented extensional kinematics within the Shawanaga and overlying Parry Sound 1019"
shear zones may have also contributed to rapid cooling from peak temperature
1020"
conditions, prior to long-term residence at shallow levels of post-orogenic crust 1021"
(Jamieson and Beaumont, 2011; Ketchum and Davidson, 2000; Wodicka et al., 1996). 1022"
1023"
5. Remaining Questions 1024"
This review and demonstration of U-Pb thermochronology serves to highlight several 1025"
areas for future research. 1026"
1027"
1. What controls Pb mobility along grain boundaries in metamorphic rocks? A growing 1028"
body of evidence shows the importance of flux-limited boundary conditions for the U-1029"
Pb systematics of accessory minerals in deep crustal metamorphic rocks. Experimental
1030"
work constraining the solubility and diffusivity of Pb in grain boundary fluids of
1031"
variable chemistry would be helpful. Furthermore, systematic characterisation of 1032"
which phases act as sinks and sources for radiogenic Pb derived in apatite and rutile
1033"
would enable targeted U-Pb thermochronology. 1034"
1035"
2. What controls the mobility of Pb in titanite? Empirical and experimental studies are
1036"
required to reconcile the disagreement between existing experimental diffusion 1037"
parameters and empirically derived estimates of Pb diffusivity. A potentially fruitful
1038"
topic of study is the comparison between length scales of Pb and trace-element
1039"
zonation in high-grade titanite from metamorphic terranes.
1040"
1041"
3. How does exsolution affect Pb transport through rutile? Numerous workers have
1042"
acknowledged the potential importance of exsolution lamellae in forming a short-
1043"
circuit diffusion network in rutile, potentially capable of reducing whole grain
!G
1044"
(Ewing et al., 2013; Lee, 1995; Zack and Kooijman, 2017). This mechanism would 1045"
explain the absence of grain-scale diffusive profiles in rutile grains that can be shown
1046"
to have lost radiogenic Pb. Confirmation of this hypothesis will require measurement 1047"
of Pb concentration profiles normal to ilmenite/zircon lamellae-rutile interfaces. 1048"
1049"
4. Monazite and zircon U-Pb thermochronology. Microanalytical U-Pb analysis by SIMS
1050"
or LA-ICP-MS can resolve U-Pb dates over sub-micron length scales. Such distances 1051"
are comparable to those expected for diffusion of Pb in monazite and zircon in regions 1052"
of the crust that have experienced temperatures above >~900 °C. Previous work has
1053"
shown the utility of monazite Th-Pb (e.g. Grove and Harrison, 1999), and zircon U-Pb
1054"
thermochronology"(e.g."Wheeler et al., 2015), but the full potential of these minerals 1055"
as high-temperature thermochronometers remains to be exploited. Furthermore, lattice
1056"
distortion or metamictization in zircon allows Pb diffusion at lower temperatures than 1057"
in undistorted crystals (Wheeler et al., 2013), extending the zircon PRZ and the
1058"
temperature range over which thermal history information could plausibly be
1059"
recovered. 1060"
1061"
5. Combined U-Pb thermochronology and trace-element speedometry. Diffusive trace-
1062"
element zonation in accessory phases provides an additional record of thermal history.
1063"
In contrast to U-Pb thermochronology, trace-element speedometry is unable to 1064"
constrain the absolute timing of a thermal event; rather, the curvature of the
1065"
concentration profile constrains the magnitude of time-integrated diffusion (<
^w#
>)
1066"
that has occurred since crystal formation. Provided that boundary conditions can be 1067"
constrained, and given that all diffusion profiles must be internally consistent, trace-1068"
element speedometry could be combined with U-Pb thermochronology to yield high-
1069"
resolution thermal histories. The HFSEs in rutile (Cherniak et al., 2007; Kohn et al., 1070"
2016; Marschall et al., 2013), Sr in apatite (Ague and Baxter, 2007) and Li in zircon 1071"
(Trail et al., 2016) hold particular promise in this regard. 1072"
1073"
6. Summary 1074"
Within-grain distributions of U-Pb dates and trace-element concentrations can now be 1075"
routinely and rapidly measured over sub-micron length scales, heralding a new era for U-
1076"
Pb thermochronology. Uranium-lead depth profiling of rutile and apatite provides an
1077"
extraordinary opportunity to obtain continuous thermal history information from rocks of 1078"
the middle to lower crust—a temperature range that is pertinent to a number of important
1079"
geodynamic processes. Routine application of U-Pb titanite thermochronology is 1080"
presently limited by uncertainty regarding the diffusion systematics of Pb in titanite.
1081"
Caution must be exercised to ensure that measured radiogenic Pb concentration profiles
1082"
are diffusive in nature; such profiles are rare in rocks of the deep crust due to the effects 1083"
of flux-limited boundary conditions and energetically favourable non-diffusive processes
1084"
such as recrystallization and short-circuit diffusion. Microtextural observations are
1085"
required to accurately discriminate between diffusive and non-diffusive U-Pb profiles.
1086"
Accordingly, U-Pb and trace element depth profiles should be integrated with spot 1087"
analyses to identify profiles suitable for inversion for thermal history information.
1088"
1089"
Acknowledgements 1090"
AS acknowledges support from the Sauermann family through receipt of the Slingerland 1091"
Early Career Award. JM acknowledges support from the MERC and Harquail School of
1092"
Earth Sciences at Laurentian University. The manuscript was reviewed by two 1093"
anonymous referees whose comments served to significantly improve the quality of the 1094"
paper. We are grateful for the invitation to submit this work, additional comments and 1095"
deft handling of manuscript by editor, Klaus Mezger. Finally, we thank Robert Holder for
1096"
providing us with empirical estimates of Pb diffusivities through titanite. 1097"
1098"
Figure captions
1099"
Figure 1. U-Pb thermochronology. Panel A illustrates the effect of erosion on the
1100"
temperature-depth evolution of three rock samples initially located at 22.5 (yellow 1101"
particle), 30 (orange) and 37.5 km depth (red). Gray lines are geotherms, plotted at 2 Myr
1102"
intervals. Shaded regions delineate the zones of partial retention for Pb in apatite and 1103"
rutile. Calculations performed using an erosion/exhumation rate of 1 km/Myr. Panel B
1104"
shows calculated 238U-206Pb date profiles for single grains of apatite and rutile in each of
1105"
the three rocks shown in panel A after 50 Myr of erosion. Both apatite and rutile date 1106"
profiles were calculated using experimentally determined Pb diffusion parameters
1107"
(Cherniak, 2000; Cherniak et al., 1991) and a cylindrical geometry (200×250×100 µm).
1108"
1109"
Figure 2. Closure profiles and whole grain ages. Panel A shows four different thermal 1110"
histories: progressive cooling (black line), lengthy residence at high temperatures
1111"
(orange), reheating (purple) and low-grade metamorphism (red). Panel B shows
1112"
computed 238U-206Pb date profiles for a rutile grain (100 µm equivalent spherical radius) 1113"
after following each of the thermal histories displayed in A. The red line (low grade 1114"
metamorphism) represents the profile shape typical of a secondary growth event
1115"
occurring at low temperatures. Calculations were performed using a homogenous 1116"
distribution of U, and the Pb diffusion parameters of Cherniak (2000). Note that the 1117"
volume integral of each U-Pb date profile yields a whole-grain date of 40 Ma, 1118"
independent of thermal history.
1119"
1120"
Figure 3. Comparison between experimental and empirical rates of Pb diffusion in 1121"
U-Pb thermochronometers. Panel A: Pb diffusion in apatite; empirical estimates from
1122"
Cliff and Cohen (1980) (C&C80), DeWitt et al. (1984) (DeW84), Gulson (1984) (G84),
1123"
von Blackenburg (1992) (vB92) and Krogstad and Walker (1994) (K&W94). 1124"
Experimental data are from Watson et al. (1985) (white square markers) and Cherniak et
1125"
al. (1991) (white circles). Panel B: Pb diffusion in rutile; empirical estimates from 1126"
Mezger et al. (1989) (M89), Vry and Baker (2006) (V06) and Kooijman et al. (2010)
1127"
(K10). Experimental data (white circles) are from Cherniak (2000). Panel C: Pb diffusion
1128"
in titanite; empirical estimates from Verts et al. (1996) (V96), Scott and St-Onge (1995) 1129"
(S&SO95), Garber et al. (2017) (G17), Kohn (2017, and refs therein) (shaded boxes
1130"
labelled K17),"Marsh and Smye, (2017) (M&S17) and Holder et al in review (H1 and 2).
1131"
Experimental data (white circles) are from Cherniak (1993); Sr diffusivities are from
1132"
Cherniak (1995) shown for comparison. Arrowheads denote whether estimates represent 1133"
maximum or minimum values. See text for discussion.
1134"
1135"
Figure 4. U-Pb date profile inversion. U-Pb data are shown for a lower crustal rutile 1136"
from the Pyrenees. Panel A shows common-Pb corrected 238U-206Pb date profile plotted 1137"
against the best fit (maximum log likelihood) model 238U-206Pb date profile (black line).
1138"
Panel B shows the evolution of the log likelihood value as a function of iteration number; 1139"
note the pre- and post-burn-in stages, where burn-in refers to a group of initial, 1140"
explorative iterations. Panel C shows post-burn-in candidate thermal histories shaded 1141"
according to log likelihood.
1142"
1143"
Figure 5. Controls on U-Pb date profile topology. Panel A shows a schematic sketch of 1144"
a U-Pb date profile collected by LA-ICP-MS across a half-width of an accessory mineral
1145"
grain, a common Pb-corrected plot of U-Pb spot date against position within the grain
1146"
and an associated Tera-Wasserburg concordia diagram containing both corrected (bold 1147"
ellipses) and uncorrected (faded ellipses) U-Pb analyses. For this case, the distribution of
1148"
radiogenic Pb is controlled by volume diffusion from grain cores into the grain boundary 1149"
medium. Panel B: as for A, but for a scenario in which a mineral grain undergoes a
1150"
period of secondary growth. Panel C: as for A, but for partial recrystallization of an
1151"
accessory mineral grain. Panel D: as for A, but for the case in which the grain boundary 1152"
cannot host radiogenic Pb (flux-limited boundary condition). Note the importance of an
1153"
accurate common Pb correction; uncorrected data topologies for each of these processes
1154"
are non-unique. See text for discussion.
1155"
1156"
Figure 6. U zonation in rutile and apatite. Panel A shows a series of U concentration
1157"
depth profiles from lower crustal rutile of the Ivrea Zone. Data are from Smye and
1158"
Stockli (2014). Panel B shows a series of U profiles from lower crustal apatite of Corsica; 1159"
data are from Seymour et al. (2016). For both panels, colors correspond to different 1160"
grains.
1161"
1162"
Figure 7. Effect of rutile U zonation on U-Pb date profile topology. Panels show U 1163"
concentration and resultant U-Pb date profiles for commonly encountered types of U 1164"
zoning in rutile: uniform U concentration (panels A, B), secondary growth (panels C, D),
1165"
Rayleigh distillation (panels E, F) and oscillatory zonation (G, H). U-Pb age profiles 1166"
were calculated using rutile Pb diffusion parameters (Cherniak, 2000) and a thermal 1167"
history in which cooling occurred from 700 °C over 1 Ga at 0.3°C /Myr. See text for
1168"
discussion.
1169"
1170"
Figure 8. Tectonic map of the Grenville orogeny. Note the locations of samples
1171"
GB119C and GB132A. Map is modified after Marsh and Culshaw (2014); see 1172"
Supplementary Material for detailed discussion of Grenville geology and explanation of
1173"
the various structural units.
1174"
1175"
Figure 9. Tera-Wasserburg concordia plots for laser ablation U-Pb spot data. Panel
1176"
A: analyses from sample GB119C; panel B: analyses for sample GB132A. Note the
1177"
dispersion of analyses along concordia for both samples that is consistent with Pb loss
1178"
during cooling from high temperatures. Analyses are uncorrected for common Pb. 1179"
1180"
Figure 10. Relationship between U-Pb spot date and within-grain position. Panels A 1181"
and B show 207Pb-corrected spot dates plotted against distance from grain rims for 1182"
samples GB119C and GB132A, respectively. Spot analyses from the same crystal have
1183"
the same color; m and i refer to matrix and included rutile grains, respectively. Analytical 1184"
errors are 2s. Panel C shows the microtextural environment of a subset of the spot dates 1185"
for sample GB119C; dates are common-Pb corrected. 1186"
1187"
Figure 11. Example U-Pb date and trace element concentration depth profiles. 1188"
Panel A: rounded U-Pb date profile and associate U, Zr and Nb concentration profiles; 1189"
panel B: linear U-Pb date profile; panel C: step-like U-Pb date profile. Note the similarity
1190"
between the shapes of the trace element profiles, independent of the type of U-Pb profile.
1191"
Errors are 2s. 1192"
1193"
Figure 12. Rutile rim U-Pb dates. Panel A: 238U-206Pb dates from the outermost depth 1194"
increment (~1 µm) of each depth profile collected from individual rutile crystals from
1195"
sample GB119C; panel B: as for A, but for sample GB132A. Black horizontal line
1196"
corresponds to the age of zircon crystallization; red circles correspond to the U-Pb date 1197"
profiles used for joint inversion (Fig. 14). Errors are 2s.
1198"
1199"
Figure 13. Relationship between adjacent mineral phase and U-Pb date. Panels A
1200"
and B show box plots of 207Pb-corrected U-Pb spot dates grouped according to the 1201"
mineralogy of the nearest grain boundary phase for samples GB119C and GB132A,
1202"
respectively. Each box represents, from bottom to top, the second and third quartile (25
1203"
and 75% of the population), and the bar inside the box represents the median; whiskers 1204"
represent the 10th and the 90th percentiles. Numbers beneath the boxes represent the 1205"
number of analyses considered and outliers, when they occur, are represented by small
1206"
black circles. Note the absence of a systematic relationship between date and mineralogy 1207"
for both samples. 1208"
1209"
Figure 14. Joint inversion of Grenville rutile U-Pb date profiles. Panel A shows the fit
1210"
between the U-Pb date profiles (sample GB119C) and forward modeled profile for the 1211"
maximum log likelihood thermal history (black line in panel B). Panel B shows the 1212"
candidate thermal histories color shaded for log likelihood; black line is the solution with
1213"
the maximum log likelihood value. Panels C and D are as A and B, for sample GB132A.
1214"
1215"
Figure 15. Grenville thermal history. Black lines are thermal histories derived from
1216"
inversion of rutile U-Pb data profiles (this study); grayscale arrow represents thermal 1217"
history derived from interpolation between zircon U-Pb (Ketchum and Krogh, 1998;
1218"
Marsh and Culshaw, 2014), hornblende 40Ar-39Ar and biotite 40Ar-39Ar whole grain dates
1219"
(Cosca et al., 1992; Cosca et al., 1991). 1220"
1221"
1222"
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... Apatite from different lithologies may be distinguished as its mineral chemistry is strongly governed by the composition of the host rock. Apatite will retain source information unless secondary processes cause elements to be redistributed; this can be thermally-activated volume diffusion or element mobility driven by aqueous or silicate fluids (Bingen et al., 1996;Chamberlain and Bowring, 2001;Cherniak, 2000;Cochrane et al., 2014;Smye et al., 2018). Apatite mineral chemistry can be used to understand the growth or modification mechanism (e.g. ...
... This slight scatter may suggest that some grains and grain rims have been affected by LREE and Y remobilisation. At the grain scale, partial recrystallisation during metamorphism would result in patchy element distribution (Smye et al., 2018). This patchy distribution is not typically seen in the grains across the samples (Fig. 8). ...
... However, in that case apparent ages are older in the core. This latter observation implies Pb mobility driven by thermally activated volume diffusion, whereby the mobility of Pb from high-U domains into adjacent low-U domains produced apparently old ages (Cherniak, 2005;Smye et al., 2018). The ages for the apatite in the leuconorite and many olivinewebsterite samples therefore represent cooling from temperatures where Pb was able to remobilise. ...
Article
The trace element signature and U-Pb systematics of apatite are useful in identifying petrogenic processes and tracking crust through the ∼375–600 °C thermal interval. Furthermore, as apatite is more abundant in mafic rocks than other more conventional U-Pb chronometers (e.g. zircon), it may be better suited to constrain magmatic ore systems. This work focuses on a case study of apatite from the Nova-Bollinger Ni-Cu magmatic sulfide deposit in the Fraser Zone of the Albany Fraser Orogen, Western Australia. LA-ICP-MS analysis is used to characterise the U-Pb systematics and trace element chemistry of apatite across four rock types at Nova-Bollinger. These analyses are combined with whole-rock geochemistry and petrography to resolve the poorly understood thermal evolution of Nova-Bollinger after peak metamorphic conditions. Apatite U-Pb data indicate an initial 15 °C/my cooling rate to 1284 ± 12 Ma from peak metamorphic conditions of ∼850 °C at 1304 ± 22 Ma. After 1284 Ma the cooling rate dramatically slowed to 1.2 °C/my until at least 1225 ± 17 Ma. Most apatite retains an igneous trace element signature where thermal diffusion has not influenced trace element concentrations, suggesting that any exposure to temperatures above ∼670 °C was short-lived or trace element diffusion was slow. Furthermore, trace element chemistry discriminates apatite that crystallised from an evolved intercumulus disequilibrium liquid, and apatite that crystallised in equilibrium with the mineral assemblage of more fractionated rocks. The new results from this study highlight the change from a fast to slow cooling regime in this litho-tectonic zone, likely driven by ebbing regional magma emplacement. This new information enables better understanding of the tectono-magmatic evolution of the Proterozoic Nova-Bollinger deposit and the broader Fraser Zone.
... U-Pb isotopic analysis was performed using a RESOlution ArF excimer laser ablation system coupled to an Agilent 7900x ICP-MS mass spectrometer at Adelaide University. U-Pb apatite has closure temperatures estimated at 375-625°C, and is at the higher-T part of this range for fast cooling rates (Krogstad and Walker, 1994;Schoene and Bowring, 2007;Blackburn et al., 2011;Chew and Spikings, 2015;Kirkland et al., 2018;Smye et al., 2018). U-Pb titanite has similar closure temperatures estimated at 550-600°C (Smye et al., 2018). ...
... U-Pb apatite has closure temperatures estimated at 375-625°C, and is at the higher-T part of this range for fast cooling rates (Krogstad and Walker, 1994;Schoene and Bowring, 2007;Blackburn et al., 2011;Chew and Spikings, 2015;Kirkland et al., 2018;Smye et al., 2018). U-Pb titanite has similar closure temperatures estimated at 550-600°C (Smye et al., 2018). (Table S5). ...
... U-Pb analyses were carried out on identical instrumentation and setup as that for apatite and titanite. U-Pb monazite has relatively high closure temperatures spanning 530-700°C (Smye et al., 2018). (Table S5). ...
Article
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Collision, high-angle contraction, crustal thickening and heating at 555–516 Ma, primed the Damara Belt ready for crustal collapse, which was triggered by a transition to ENE–WSW contraction along the length of the belt in response to orogenic events in east Gondwana at 516–505 Ma. Along-orogen shortening reworked and thickened the high-grade core of the belt, increasing gravitational instability, and establishing an NW–SE extension direction across the belt that was conducive to reactivation of pre-existing structures and eventual collapse. This extension direction persisted, and the switch to vertical σ1 and collapse was signalled by decompression melting at ∼502 Ma and subsequent rapid cooling. Collapse was focused on the high-grade core of the belt that was exhumed as a ∼170 km wide, semi-coherent massif-type metamorphic core complex with steep extensional shear zones and faults in the marginal flanks. During exhumation the core complex was reactivated by oblique-slip extensional shear zones that responded to external transient stress fields. Reactivation by middle- to lower-amphibolite facies dextral-normal shear zones at ∼500–495 Ma and ∼495–490 Ma, involved E–W to ENE–WSW shortening consistent with accretionary events in the west Gondwana margin during the Pampean Orogeny. Reactivation by greenschist facies sinistral-normal shear zones at ∼485 Ma, involved N–S shortening consistent with accretionary events in the south Gondwana margin during the Famatinian Orogen. Early stages of exhumation involved decompression melting, flattening folds and ductile ultramylonite zones within carbonate that formed by NW–SE extension. Late stage exhumation in the brittle field from ∼480 Ma onwards, involved a stress-switch to radial extension directions dominated by NE–SW. This stage involved flat-lying breccia, inclined faults, vertical fractures, and oxidizing fluids partitioned into the top of the lower-levels of the massif. Ongoing exhumation of the core complex drove localized NW–SE shortening within the flanking margins and hanging-wall, and produced low-strain reverse structures that straddle the ductile to brittle transition. The pressure difference between exhumed massif (4.8–5.5 kbar) and hanging-wall margins (3.9–4.2 kbar), indicate that ∼3.2–4.6 km of crust was stripped from above the core complex.
... The closure temperatures for Pb diffusion in zircon [15] and monazite [16] are both in excess of 900 °C, significantly higher than the wet granite solidus. Hence, they have had very limited application in U-Pb thermochronology studies of crustal rocks, although potentially regions of the crust that have experienced temperatures above ≳900 °C can be explored by U-Pb monazite or zircon thermochronology by generating U-Pb date profiles at sub-micron resolution from crystal rims [17,18]. ...
... Numerous variables contribute to bulk and in situ U-Pb dates (e.g., t-T history, grain size, boundary condition, fluid interaction history), and thus mathematical inversion is the optimal approach to seek solutions from potentially hundreds of thousands of iterations, depending on CPU power and code efficiency. Input parameters usually include (i) U-Pb dates, either as bulk dates obtained by ID-TIMS [6,77,78], or in situ dates obtained using LA-(MC)-ICPMS from traverses across polished mineral interior surfaces [39,77] or depth-profiles [18,29], (ii) a length parameter, which may be the cylindrical radius perpendicular to the c-axis (appropriate for bulk dates), or traverse distance or depth (for in situ dates), and iii) an appropriate set of Pb-in-apatite diffusion parameters (which are usually taken from Cherniak et al., 1991 [23]) and diffusion geometry. Additional input should include information about the intra-grain distribution of parent isotopes [39], which is relevant for both bulk and in situ dates, and potentially a kinetic parameter(s) that describes how diffusivities vary with other characteristics such as crystal composition (although these currently remain unconstrained). ...
... The MCMC approach is an iterative sampling procedure where better fitting models are sought by perturbing the existing best-fit model. Examples of this approach are the stand-alone computer program QTQt [79], and the code "Upbeat" [18] that is implemented in MATLAB. QTQt and Upbeat permit the input of in situ U-Pb dates, and thus they are amenable to modelling LA-(MC)-ICPMS dates. ...
Article
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The temperature sensitivity of the U-Pb apatite system (350-570 °C) makes it a powerful tool to study thermal histories in the deeper crust. Recent studies have exploited diffusive Pb loss from apatite crystals to generate t-T paths between ~350-570 °C, by comparing apatite U-Pb ID-TIMS (isotope dilution-thermal ionisation mass spectrometry) dates with grain size or by LA-MC-ICP-MS (laser ablation-multicollector-inductively coupled plasma-mass spectrometry) age depth profiling/traverses of apatite crystals, and assuming the effective diffusion domain is the entire crystal. The key assumptions of apatite U-Pb thermochronology are discussed including (i) that Pb has been lost by Fickian diffusion, (ii) can experimental apatite Pb diffusion parameters be extrapolated down temperature to geological settings and (iii) are apatite grain boundaries open (i.e., is Pb lost to an infinite reservoir). Particular emphasis is placed on detecting fluid-mediated remobilisation of Pb, which invalidates assumption (i). The highly diverse and rock-type specific nature of apatite trace-element chemistry is very useful in this regard-metasomatic and low-grade metamorphic apatite can be easily distinguished from sub-categories of igneous rocks and high-grade metamor-phic apatite. This enables reprecipitated domains to be identified geochemically and linked with petrographic observations. Other challenges in apatite U-Pb thermochronology are also discussed. An appropriate choice of initial Pb composition is critical, while U zoning remains an issue for inverse modelling of single crystal ID-TIMS dates, and LA-ICP-MS age traverses need to be integrated with U zoning information. A recommended apatite U-Pb thermochronology protocol for LA-MC-ICP-MS age depth profiling/traverses of apatite crystals and linked to petrographic and trace element information is presented.
... Rutile forms predominantly in high-grade metamorphic rocks and typically incorporates one to tens of parts per million of U and is characterized by a Pb closure temperature between ca. 500 and 650°C making it sensitive to thermal processes in the middle to lower crust (Kooijman et al., 2010;Smye et al., 2018;Vry & Baker, 2006). Compared to zircon and apatite, most rutile does not incorporate Th and exhibits extremely low Th/U ratios and thus allowing for a 208 Pb-based common Pb correction for U-Pb ages (Zack et al., 2011). ...
... Apatite typically incorporates 0.1 to tens of parts per million of U and Th in its crystal lattice and exhibits a Pb thermal sensitivity window between 375 and 550°C (Cochrane et al., 2014;Harlov, 2015;Kirkland et al., 2018;Smye et al., 2018). Apatite U-Pb can record both crystallization ages in volcanic and upper crustal intrusive rocks, and cooling histories of middle to lower crustal igneous and amphibolite to eclogite facies metamorphic rocks (e.g. ...
Article
Full-text available
The Pyrenees Mountains are a classic example of a doubly‐verging collisional orogenic system with flanking retro‐ and pro‐foreland basin systems. Previous bedrock and detrital geo‐/thermochronologic studies have observed magmatic and exhumational related ages that reflect a complex thermo‐tectonic evolution of the European and Iberian plate margins related to break‐up and assembly of the Gondwana, Pangea, and Pyrenean‐Alpine orogenic cycles. This study integrates detrital zircon, rutile, and apatite U‐Pb dating and, detrital zircon and apatite (U‐Th)/He dating from modern river sands from the northern and southern Pyrenees, with PECUBE thermokinematic modeling of bedrock cooling ages to simulate detrital age distributions in order to evaluate: (1) regional patterns in long‐term crustal processes associated with pre‐Pyrenean crustal shortening, crustal thinning, and magmatism along the Iberian and European plate margin; (2) timing of regional cooling and inferred erosion related to Pyrenean orogenesis; and (3) the exhumation processes associated with post‐orogenic decay and erosion. Modern river multi‐mineral detrital geo‐/thermochronometry results are consistent with previous bedrock thermal history models and records punctuated Variscan and Pyrenean cooling events in the pro‐wedge that contrasts with protracted Permian to Pliocene thermal history preserved in the retro‐wedge. Detrital age distributions from PECUBE modeling predict the Pyrenean age component in both detrital apatite and zircon (U‐Th)/He age distributions, indicating the modeled exhumation patterns in the Axial Zone and Northern Pyrenean Zone can predict observed Pyrenean thermochronology ages. The presence of strong Pyrenean age peaks among the modern river sand and modeled detrital cooling age distributions suggest retro‐wedge deformation and exhumation remained active during the main phase of pro‐wedge activity and experienced significant orogenic decay. Isolated Miocene apatite He ages from the North Pyrenees modern river record post‐orogenic cooling, due to tectonic mode switch to extension and (or) climate driven enhanced exhumation.
... In contrast, studies by Corfu and Muir (1989), Corfu and Ayres (1991), Verts et al. (1996) and others suggested that variations in titanite U-Pb ratios may reflect multiple or protracted metamorphic and/or metasomatic events. A growing number of studies of natural titanite over the past decade suggest that volume diffusion rates of Pb are greatly overestimated (Gao et al., 2012;Garber et al., 2017;Hartnady et al., 2019;Kohn & Corrie, 2011;Kohn, 2017;Smye et al., 2018;Spencer et al., 2013;Stearns et al., 2016;Walters & Kohn, 2017). Revised estimates for Pb diffusion coefficients range from 1 Â 10 À20 to 1 Â 10 À26 (m 2 s À1 ) Kohn, 2017), equivalent to the experimentally derived diffusion rate for Sr in titanite (Cherniak, 1995). ...
... Lead-loss, U-mobility, and/or inheritance of unsupported radiogenic Pb can result in apparent ages that do not reflect the timing of metamorphism (e.g., Bonamici & Blum, 2020;Castelli & Rubatto, 2002;Garber et al., 2017;Kirkland et al., 2016Kirkland et al., , 2018Romer & Rötzler, 2001, 2011Romer & Siegesmund, 2003;Smye et al., 2018). As a result, U-Pb dates may decouple from major and trace element compositional zoning. ...
Article
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Titanite is a potentially powerful U‐Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, which may lead to inaccurate and geologically ambiguous U‐Pb dates if a proper correction is not or cannot be applied. Here we present laser ablation inductively coupled mass spectrometry (LA‐ICP‐MS)‐derived titanite U‐Pb dates and trace element concentrations for two banded calc‐silicate gneisses from south‐central Maine, USA (SSP18‐1A & ‐1B). Single spot common Pb‐corrected dates range from 400 to 280 Ma with ±12–20 Ma propagated 2SE. Titanite grains in sample SSP18‐1B exhibit regular core‐to‐rim variations in texture, composition, and date. We identify four titanite populations: 1) 397 ± 5 Ma (95% CL) low Y + HREE cores and mottled grains, 2) 370 ± 7 Ma high Y + REE mantles and cores, 3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and 4) 295 ± 6 Ma LREE‐depleted rims. We interpret the increase in titanite Y + HREE between ca. 397 and ca. 370 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr‐in‐titanite temperatures (803 ± 36°C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 370 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (>80 vol%), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite date the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well‐defined date‐composition‐texture relationships observed for titanite from SSP18‐1B, titanite grains from sample SSP18‐1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed ages spanning 380–330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U‐Pb data along with chemical and textural data to screen for inherited radiogenic Pb.
... In contrast, studies by Corfu & Muir (1989), Corfu & Ayers (1991), Verts, Chamberlain, & Frost, (1995), and others suggested that variations in titanite U-Pb ratios may reflect multiple or protracted metamorphic and/or metasomatic events. A growing number of studies of natural titanite over the past decade suggest that volume diffusion rates of Pb are greatly overestimated (Gao et al., 2012;Garber et al., 2017;Hartnady et al., 2019;Kohn & Corrie, 2011;Kohn, 2017;Smye et al., 2018;Spencer et al., 2013;Stearns et al., 2016;Walters & Kohn, 2017). Revised estimates for Pb diffusion coefficients range from 1 ´ 10 -20 to 1 ´ 10 -26 (m 2 s -1 ) (Holder, Hacker, Seward, & Kylander-Clark, 2019;Kohn, 2017), equivalent to the experimentally derived diffusion rate for Sr in titanite (Cherniak, 1995). ...
... Lead-loss, U-loss, and/or inheritance of unsupported radiogenic Pb can result in apparent ages that do not reflect the timing of metamorphism (e.g., Bonamici & Blum, 2020;Castelli & Rubatto, 2002;Garber et al., 2017;Kirkland et al., 2016;2018;Smye et al., 2018). As a result, U-Pb dates may decouple from major and trace element compositional zoning. ...
Preprint
Full-text available
Titanite is a potentially powerful U-Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, the correction for which may introduce inaccuracies and result in geologically ambiguous U-Pb dates. Here we present laser ablation inductively coupled mass spectrometry (LA-ICP-MS)-derived titanite U-Pb dates and trace element concentrations for two banded calc-silicate gneisses from south-central Maine, USA (SSP18-1A &-1B). Single spot common Pb-corrected dates range from 400 to 280 Ma with 12-20 Ma propagated 2SE. Titanite in sample SSP18-1B exhibit regular core-to-rim variations in texture, composition, and date. We identify four titanite populations: 1) 399 ± 5 Ma (95 % CL) low Y + HREE cores and mottled grains, 2) 372 ± 7 Ma high Y + REE mantles and cores, 3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and 4) 295 ± 6 Ma LREE-depleted rims. We interpret the increase in titanite Y + HREE between ca. 400 and ca. 372 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr-in-titanite temperatures (803 ± 36 °C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 372 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (> 80 vol %), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite date the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well-defined date-composition-texture relationships observed for titanite from SSP18-1B, titanite grains from sample SSP18-1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed ages spanning 380-330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U-Pb data along with chemical and textural data to screen for inherited radiogenic Pb. 2
... Another problem is that the algorithm of parameter uncertainties in the diffusion coefficients is rarely described in experimental studies, which introduces known but unquantified uncertainties. Available programs for diffusion modeling (e.g., Costa et al., 2008;Dunai, 2005;Faryad and Ježek, 2019;Girona and Costa, 2013;Jollands, 2020;Mutch et al., 2021;Robl et al., 2007;Smye et al., 2018) do not always estimate the aforementioned uncertainties. Therefore, a program that uses an explicit curve-fitting method to determine parameters in the diffusion coefficient and propagates their uncertainties into timescales will make diffusion studies more internally consistent and reproducible. ...
Article
Full-text available
Chemical diffusion in minerals has shown great potential to quantify timescales of geological processes. The presence of chemical gradients, along with favorable temperature and time conditions, lead to the formation of measurable diffusion profiles. Temporal information can be extracted from measured diffusion profiles using either analytical or numerical solutions of Fick's second law. Currently, there is a lack of widely adopted programs for diffusion studies. In addition, the uncertainties associated with timescales derived from diffusion chronometry are critical for geological studies, but are not always robustly evaluated. In many cases, only uncertainties in curve fitting parameters and temperature are considered, whereas other uncertainties, such as those associated with the experimentally determined diffusion coefficients themselves, are rarely propagated into the calculated timescales. Ignoring these uncertainties reduces the reproducibility and intercomparability of results. In response to these challenges, we present Diffuser, a user-friendly program to standardize diffusion chronometry with transparent and robust propagation of uncertainties. Using analytical and numerical methods, our program provides an automatic, visual, and efficient curve fit to extract chronological information from diffusion profiles. The method is complemented by an algorithm to propagate all uncertainties (i.e., measurement, temperature, curve fitting, and diffusion coefficient) to derived timescales. Three examples are provided to highlight how the program can recover timescales with internal consistency, efficient computing and easy-to-use features. Our freely available and user-friendly program will hopefully increase the accessibility and consistency of diffusion modeling and thereby to facilitate more high-quality diffusion studies.
... Apatite U-Pb has a temperature sensitivity of ∼450-550 °C (Watson et al., 1985;Cherniak et al., 1991;Smye et al., 2018), dynamically recrystallizes during deformation (e.g., Nakano et al., 2001;Saka et al., 2008;Ribeiro et al., 2020), and is susceptible to dissolutionprecipitation (Harlov, 2015). Apatite trace and rare earth element (TREE) geochemistry is a sensitive recorder of petrogenic processes during (re)crystallization (Belousova et al., 2002;Bruand et al., 2017). ...
Article
Full-text available
The timing and processes of ductile deformation and metasomatism can be documented using apatite petrochronology. We integrated microstructural, U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Electron backscatter diffraction (EBSD) analyses documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb maps indicate that dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite rare earth element (REE) and U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs record higher-temperature processes than U-Pb isotopic systems. Apatite from (ultra)mylonitic portions of the shear zone showed evidence of metasomatism, and the youngest dates constrain the age of metasomatism. Collectively, these results demonstrate that crystal plastic microstructures and fluid interactions can markedly change apatite isotopic signatures, making single-grain apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events, which are often not captured by higher-temperature chronometers.
... Experimental titanite data suggest that the Pb closure temperature for 100 μm-diameter titanite subject to a ~ 10 °C/ Myr cooling rate is ~ 600 °C (Cherniak 1993). However, the length scales of Pb diffusion in natural rocks indicate a closure temperature > 750 °C, compatible with slower Pb diffusion (Kohn and Corrie 2011;Spencer et al. 2013;Stearns et al. 2016;Kohn 2017;Marsh and Smye 2017;Smye et al. 2018;Hartnady et al. 2019;. The naturally constrained intracrystalline diffusivity of Pb in titanite is similar to the experimentally measured diffusion of Sr in titanite, which is reasonable given their similar charge and ionic radii (Cherniak 1995;LaTourrette et al. 1996). ...
Article
Full-text available
Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.
... Because Pb loss can be significant in rutile at eclogite-facies conditions (Cherniak, 2000;Smye et al., 2018), we modeled Pb diffusion to assess whether the rutile U-Pb dates are cooling or crystallization ages. Zircon fission-track dates from the As Sifah and Hulw Windows yield extremely reproducible, 66-70 Ma dates, suggesting that all samples experienced ≥30°C/My cooling from eclogite-facies conditions (Saddiqi et al., 2006), which we adopt as a minimum cooling rate. ...
Article
Full-text available
Studies of the high‐pressure (HP) As Sifah eclogites in the NE Saih Hatat Window, Oman, have used different combinations of radiometric dating results (Ar/Ar, Sm‐Nd vs. U‐Pb, Rb‐Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction—including its relationship to the Samail Ophiolite. To determine the timing of continental subduction, we coupled petrochronological analyses of major (garnet) and accessory phases (zircon and rutile) from the highest grade metamorphic rocks (As Sifah eclogites) in the Saih Hatat. Early Permian (283.8 ± 0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions and metamorphic assemblages consistently suggest a single metamorphic event with garnet growth from ∼81 to 77 Ma—though with slight, consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous interpretations for continental HP metamorphism in a single NE‐dipping subduction zone beneath the obducted Samail Ophiolite; there is no robust evidence for an ∼110 Ma event or for SW‐dipping subduction beneath the Arabian plate. Combined with other constraints, our data suggest that the As Sifah unit was subducted at rates similar to other small continental HP bodies, followed by two stages of cooling and exhumation—likely associated with the dragging to mantle depths by a mafic root, an initial phase of rapid exhumation from mantle depths, and a lengthy residence (≤40 My) in the lower to middle crust.
Article
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Common Pb, the portion of non-radiogenic Pb within a U bearing mineral, needs to be accurately accounted for in order to subtract its effect on U-Pb isotopic ratios so that meaningful ages can be calculated. The propensity to accommodate common Pb during crystallization, or later, is different across the range of U bearing minerals used for geochronology. Titanite frequently accommodates significant amounts of common Pb. However, the most appropriate method to correct for this requires knowledge on the mechanism and timing of common Pb incorporation; information that is commonly difficult to extract. In this study, the spatial and compositional distribution of trace elements (including Pb) in metamorphic titanites from a Greenland amphibolite is investigated on the grain- to nano-scale. Titanites have an isotopically similar signature for both common and radiogenic-Pb in all grains but significantly different quantities of the non-radiogenic component. Microstructural and compositional examination of these grains reveals undeformed, but high common Pb (F207%) titanites have homogeneous element distributions on the atomic scale suggesting common Pb is incorporated into titanite during its growth and not during later processes. In contrast, deformed titanite comprising low-angle boundaries, formed by subgrain rotation recrystallization, comprise networks of dislocations that are enriched in Mg, Al, K and Fe. Smaller cations may migrate due to elastic strain in the vicinity of the dislocation network, yet the larger K cations more likely reflect the mobility of externally-derived K along the orientation interface. The absence of Pb enrichment along the boundary indicates that either Pb was too large to fit into migrating lattice dislocations or static low-angle boundaries and/or that there was no external Pb available to diffuse along the grain boundary. As the common Pb composition is distinctly different to regional Pb models, the metamorphic titanite grew in a homogeneous Pb reservoir dominated by the break-down of precursor U-bearing phases. The different quantity of common Pb in the titanite grains indicates a mineral-driven element partitioning in an isotopically homogeneous metamorphic reservoir, consistent with low U, low total REE and flat LREE signatures in high F207% analyses. These results have implications for the selection of appropriate common Pb corrections in titanite and other accessory phases.
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