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Phase heritage during replacement reactions in Ti-bearing minerals

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Mark Pearce
Mark Pearce
Mark Pearce
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Angela J. Escolme at University of Tasmania
Angela J. Escolme
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Abstract and Figures

Replacement reactions occur during metamorphism and metasomatism in response to changes in pressure, temperature and bulk rock and fluid compositions. To interpret the changes in conditions, it is necessary to understand what phases have previously been present in the rocks. During fluid-mediated replacement, the crystallography of the replacement phases is often controlled by the parent reactant phase. However, excessive fluid fluxing can also lead to extreme element mobility. Titanium is not mobile under a wide range of fluid compositions and so titanium-bearing phases present an opportunity to interpret conditions from the most extreme alteration. We map orientation relationships between titanium-bearing phases from ore deposits using EBSD and use symmetry arguments and existing relationships to show that completely consumed phases can be inferred in ore deposits. An ilmenite single crystal from Junction gold deposit is replaced by titanite, rutile and dolomite. The rutile has the following well-documented orientation relationship to the ilmenite [0001]ilmenite // < 100 > rutile and < 101¯0101ˉ010{\bar{1}}0 > ilmenite // [001]rutile The anatase is a single crystal and shows a potential orientation relationship [0001]ilmenite = (0001)ilmenite // {211}anatase and < 101¯0101ˉ010{\bar{1}}0 > ilmenite // < 01¯101ˉ10{\bar{1}}1 > anatase The single crystal orientation and lack of symmetrical equivalent variants suggest nucleation dominates the anatase production. Dolomite grew epitaxially on the ilmenite despite only sharing oxygen atoms suggesting the surface structure is important in dolomite nucleation. Titanite partially replaced ilmenite during metasomatism at Plutonic gold deposit. The titanite orientation is weakly related to the ilmenite orientation by the following relationship: [0001]ilmenite // < 100 > titanite and {101¯0101ˉ010{\bar{1}}0}ilmenite // (001)titanite The prevalence of subgrain boundaries in the titanite suggests multiple nucleation points on an already deformed ilmenite needle leading to the formation of substructure in the absence of deformation. Existing known topotaxial replacement relationship can be used to infer completely replaced phases using the misorientation distributions of the replacement polycrystals. Orientation modelling for a cubic phase replaced by rutile in a sample from Productora tourmaline breccia complex shows misorientation distributions consistent with < 001 > Rutile // < 110 > cubic and < 100 > Rutile // < 111 > cubic Combining this with volume constraints and assuming Ti is immobile, the composition of the cubic phase is constrained as titanomagnetite with 85% ulvospinel. Complex microstructures with domanial preferred orientations can also be used to document the microstructure of replaced phases. An aggregate of rutile grains with two parts that share a common < 100 > axis is interpreted as having replaced a twinned ilmenite grain. Modelling shows that the misorientation distribution for the aggregate is consistent with the above relationship replacing ilmenite with a {101¯2101ˉ210{\bar{1}}2} twin.
EBSD data showing replacement of an ilmenite single crystal by TiO2 and dolomite. Pole figures are show with data rotated into the ilmenite reference orientation not in sample co-ordinates. a EBSD phase map with the ilmenite (blue) needle being replaced by both anatase (green) and rutile (red). The volume lost due to Fe mobility is filled with dolomite (yellow). b Orientation data showing that the ilmenite, coloured relative to the mean ilmenite orientation, and the anatase, coloured using an inverse pole figure colour scheme showing which crystal direction is parallel to the sample Y-axis, are both single crystals. White arrows indicate euhedral crystal faces. c Relative misorientation map showing dolomite orientations within 20° of the ilmenite orientation (both minerals are space group R). Dolomite misoriented by > 20° is coloured yellow. Much of the dolomite filling the porosity created during ilmenite breakdown is misoriented < 20° from the ilmenite. The reference orientation used for the dolomite is the mean ilmenite orientation that has Euler angles 80.4, 56.3, and 291.4. d Pole figures for ilmenite orientations coloured using the colour scheme in (b). e Rutile orientations coloured as in (b). f Anatase orientations coloured using the same colour scheme in (b). g Dolomite orientations coloured to show the grains that are misoriented < 20° from the mean ilmenite orientation (as in c). Other orientations are grey
EBSD data showing replacement of an ilmenite single crystal by TiO2 and dolomite. Pole figures are show with data rotated into the ilmenite reference orientation not in sample co-ordinates. a EBSD phase map with the ilmenite (blue) needle being replaced by both anatase (green) and rutile (red). The volume lost due to Fe mobility is filled with dolomite (yellow). b Orientation data showing that the ilmenite, coloured relative to the mean ilmenite orientation, and the anatase, coloured using an inverse pole figure colour scheme showing which crystal direction is parallel to the sample Y-axis, are both single crystals. White arrows indicate euhedral crystal faces. c Relative misorientation map showing dolomite orientations within 20° of the ilmenite orientation (both minerals are space group R). Dolomite misoriented by > 20° is coloured yellow. Much of the dolomite filling the porosity created during ilmenite breakdown is misoriented < 20° from the ilmenite. The reference orientation used for the dolomite is the mean ilmenite orientation that has Euler angles 80.4, 56.3, and 291.4. d Pole figures for ilmenite orientations coloured using the colour scheme in (b). e Rutile orientations coloured as in (b). f Anatase orientations coloured using the same colour scheme in (b). g Dolomite orientations coloured to show the grains that are misoriented < 20° from the mean ilmenite orientation (as in c). Other orientations are grey
… 
EBSD data showing titanite replacing ilmenite in Plutonic Gold Mine, Western Australia. a Relative misorientation map showing misorientation in titanite (blue to pink) and ilmenite (yellow to red) relative to the crystallographic orientation of ilmenite at Xi, and the theoretical titanite based on the observed orientation relationship. Thin black lines are boundaries with misorientation angles > 2° and thick black lines are boundaries with misorientation angles > 10°. b Orientation data for ilmenite presented as pole figures with the same colour scheme as (a). c Orientation data for titanite presented as pole figures with the same colour scheme as (a)
EBSD data showing titanite replacing ilmenite in Plutonic Gold Mine, Western Australia. a Relative misorientation map showing misorientation in titanite (blue to pink) and ilmenite (yellow to red) relative to the crystallographic orientation of ilmenite at Xi, and the theoretical titanite based on the observed orientation relationship. Thin black lines are boundaries with misorientation angles > 2° and thick black lines are boundaries with misorientation angles > 10°. b Orientation data for ilmenite presented as pole figures with the same colour scheme as (a). c Orientation data for titanite presented as pole figures with the same colour scheme as (a)
… 
EBSD data of a euhedral rutile aggregate from Productora, Chile. a Phase map showing the distribution of white mica in a truncated triangular shape that contains the rutile. b EBSD orientation map showing the variation in orientation between rutile grains and within the larger aggregates. c Raw data as stereograms coloured using the same colour scheme as Fig. 3 showing clustering in the data. d Contoured versions of the data from (c) shows a systematic pattern in the clustering with four major clusters in < 100 > , six in < 001 > and three in < 110 > with other minor clusters. Contours are coloured according the multiples of uniform distribution (all with the same scale)
EBSD data of a euhedral rutile aggregate from Productora, Chile. a Phase map showing the distribution of white mica in a truncated triangular shape that contains the rutile. b EBSD orientation map showing the variation in orientation between rutile grains and within the larger aggregates. c Raw data as stereograms coloured using the same colour scheme as Fig. 3 showing clustering in the data. d Contoured versions of the data from (c) shows a systematic pattern in the clustering with four major clusters in < 100 > , six in < 001 > and three in < 110 > with other minor clusters. Contours are coloured according the multiples of uniform distribution (all with the same scale)
… 
Misorientation distribution plots. a Simulations of expected misorientations distributions for the two posited orientation relationships between a cubic precursor and rutile. For clarity overlapping bars are shifted by half their width along the abscissa. b Experimental misorientation distribution histogram for the rutile in samples at one pair per boundary. c Misorientation space showing the modelled orientations (large dots) and the observed orientations (small dots) in misorientation angle-axis space
Misorientation distribution plots. a Simulations of expected misorientations distributions for the two posited orientation relationships between a cubic precursor and rutile. For clarity overlapping bars are shifted by half their width along the abscissa. b Experimental misorientation distribution histogram for the rutile in samples at one pair per boundary. c Misorientation space showing the modelled orientations (large dots) and the observed orientations (small dots) in misorientation angle-axis space
… 
EBSD data of an anhedral rutile aggregate from Productora, Chile. a EBSD orientation map showing orientation variation within the aggregate. Two domains have been defined based on orientation differences and are highlighted using white dashed lines. b Pole figures coloured using the same colour scheme as for (a) showing the orientations of rutile within subset 1. c Pole figures coloured using the same colour scheme as for (a) showing the orientations of rutile within subset 2. For (b) and (c) the common < 100 > direction is highlighted with a red circle
+1
EBSD data of an anhedral rutile aggregate from Productora, Chile. a EBSD orientation map showing orientation variation within the aggregate. Two domains have been defined based on orientation differences and are highlighted using white dashed lines. b Pole figures coloured using the same colour scheme as for (a) showing the orientations of rutile within subset 1. c Pole figures coloured using the same colour scheme as for (a) showing the orientations of rutile within subset 2. For (b) and (c) the common < 100 > direction is highlighted with a red circle
… 
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1 3
Contributions to Mineralogy and Petrology (2021) 176:21
https://doi.org/10.1007/s00410-021-01775-8
ORIGINAL PAPER
Phase heritage duringreplacement reactions inTi‑bearing minerals
MarkPearce1 · AngelaEscolme2
Received: 10 July 2020 / Accepted: 25 January 2021 / Published online: 4 March 2021
© The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature 2021
Abstract
Replacement reactions occur during metamorphism and metasomatism in response to changes in pressure, temperature and
bulk rock and fluid compositions. To interpret the changes in conditions, it is necessary to understand what phases have
previously been present in the rocks. During fluid-mediated replacement, the crystallography of the replacement phases is
often controlled by the parent reactant phase. However, excessive fluid fluxing can also lead to extreme element mobility.
Titanium is not mobile under a wide range of fluid compositions and so titanium-bearing phases present an opportunity to
interpret conditions from the most extreme alteration. We map orientation relationships between titanium-bearing phases
from ore deposits using EBSD and use symmetry arguments and existing relationships to show that completely consumed
phases can be inferred in ore deposits.
An ilmenite single crystal from Junction gold deposit is replaced by titanite, rutile and dolomite. The rutile has the following
well-documented orientation relationship to the ilmenite
[0001]ilmenite // < 100 > rutile and <
10
̄
10
> ilmenite // [001]rutile
The anatase is a single crystal and shows a potential orientation relationship
[0001]ilmenite = (0001)ilmenite // {211}anatase and <
10
̄
10
> ilmenite // <
0
̄
11
> anatase
The single crystal orientation and lack of symmetrical equivalent variants suggest nucleation dominates the anatase pro-
duction. Dolomite grew epitaxially on the ilmenite despite only sharing oxygen atoms suggesting the surface structure is
important in dolomite nucleation.
Titanite partially replaced ilmenite during metasomatism at Plutonic gold deposit. The titanite orientation is weakly related
to the ilmenite orientation by the following relationship:
[0001]ilmenite // < 100 > titanite and {
10
̄
10
}ilmenite // (001)titanite
The prevalence of subgrain boundaries in the titanite suggests multiple nucleation points on an already deformed ilmenite
needle leading to the formation of substructure in the absence of deformation.
Existing known topotaxial replacement relationship can be used to infer completely replaced phases using the misorientation
distributions of the replacement polycrystals. Orientation modelling for a cubic phase replaced by rutile in a sample from
Productora tourmaline breccia complex shows misorientation distributions consistent with
< 001 > Rutile // < 110 > cubic and < 100 > Rutile // < 111 > cubic
Combining this with volume constraints and assuming Ti is immobile, the composition of the cubic phase is constrained as
titanomagnetite with 85% ulvospinel. Complex microstructures with domanial preferred orientations can also be used to docu-
ment the microstructure of replaced phases. An aggregate of rutile grains with two parts that share a common < 100 > axis
is interpreted as having replaced a twinned ilmenite grain. Modelling shows that the misorientation distribution for the
aggregate is consistent with the above relationship replacing ilmenite with a {
10
̄
12
} twin.
Keywords Rutile· Ilmenite· Titanite· EBSD· Replacement· Pseudomorph
Introduction
Geological environments that undergo protracted meta-
morphic or metasomatic histories frequently contain mul-
tiple stages of mineral replacement. In partially reacted
Communicated by Steven Reddy.
* Mark Pearce
mark.pearce@csiro.au
Extended author information available on the last page of the article
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... brittle conditions) and such veins should also be present in other mineral phases, such as primary plagioclase or clinopyroxene, which is, however, not the case; (ii) fluids entrapped in the inclusions seem to be in equilibrium with ilmenite, as FI-bearing zones have the same major element composition of FI-free zones (e.g. Figs. 8 and 9), whereas the occurrence of titanite and rutile clearly speaks for a later destabilization of ilmenite, as also observed in the literature (Pearce and Escolme, 2021;Pochon et al., 2017). In addition, the occurrence of small, randomly oriented baddeleyite inclusions in ilmenite (Fig. 7i) suggests a synchronous crystallization of the two phases (Beckman and Möller, 2018). ...
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... The concept of phase heritage analysis has been applied in TEM studies (e.g., Nord 1992Nord , 1994Kerschhofer et al. 2000;Lussier et al. 2017), and more recently, using EBSD. EBSD-based phase heritage applications have provided evidence for the former existence of high-pressure and/or -temperature phases for zirconia (Humbert et al. 2010;Timms et al. 2017aTimms et al. , 2017bWhite et al. 2018), zircon (e.g., Cavosie et al. 2016Cavosie et al. , 2018aCavosie et al. , 2018bErickson et al. 2017), monazite (Erickson et al. 2019), and Ti-bearing phases (Pearce and Escolme 2021). ...
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  • Mar 2019
  • ORE GEOL REV
In our study we explore the applicability of rutile as a pathfinder for orogenic gold deposits, which are an important source of this metal worldwide. We analysed rutile associated with orogenic Au deposits from three different Precambrian terranes, the Capricorn Orogen, the Barberton Greenstone Belt and the Ashanti Belt, all of which formed under greenschist conditions and share similarities in the style of mineralisation. Microtextural evidence from scanning electron microscopy and electron back-scatter diffraction indicates that rutile formed during the main deformation and alteration stage in these rocks, and is therefore related to mineralisation. We used electron microprobe and laser ablation ICP-MS to investigate the trace element compositions of rutile and we compared our results to other gold deposits. We find that hydrothermal rutile from gold deposits contains certain trace element characteristics, in particular high Sb concentrations (up to ∼1500 ppm in Au deposits of the Capricorn Orogen), that are distinct from rutile from non-mineralised rocks of various petrogenetic origin. Other elements, such as W and Sn, are found to be more enriched in rutile from other rock types, namely felsic magmatic rocks and hydrothermal veins, and are therefore not diagnostic of Au mineralisation in this type of deposits. We also find that the presence of sub-µm-scale inclusions – in particular Zr-(Si, Th)-bearing phases, sulfide minerals and native Au – can severely affect analyses of this type of rutile and compromise the applicability of Zr-in-rutile geothermometry.
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Cubic zirconia in >2370 °C impact melt records Earth's hottest crust
Article
  • Nov 2017
  • EARTH PLANET SC LETT
Bolide impacts influence primordial evolution of planetary bodies because they can cause instantaneous melting and vaporization of both crust and impactors. Temperatures reached by impact-generated silicate melts are unknown because meteorite impacts are ephemeral, and established mineral and rock thermometers have limited temperature ranges. Consequently, impact melt temperatures in global bombardment models of the early Earth and Moon are poorly constrained, and may not accurately predict the survival, stabilization, geochemical evolution and cooling of early crustal materials. Here we show geological evidence for the transformation of zircon to cubic zirconia plus silica in impact melt from the 28 km diameter Mistastin Lake crater, Canada, which requires super-heating in excess of 2370 °C. This new temperature determination is the highest recorded from any crustal rock. Our phase heritage approach extends the thermometry range for impact melts by several hundred degrees, more closely bridging the gap between nature and theory. Profusion of >2370 °C superheated impact melt during high intensity bombardment of Hadean Earth likely facilitated consumption of early-formed crustal rocks and minerals, widespread volatilization of various species, including hydrates, and formation of dry, rigid, refractory crust.
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Crystallographic orientation relationships
Chapter
  • Mar 2017
Crystallographic orientation relationships (CORs) of next-neighbour crystals represent a special case of crystallographic preferred orientation (CPO), where relative crystallographic orientations of neighbour-crystals follow defined rules of misorientation systematics (COR rules). The presence/absence and nature of crystallographic orientation relationships between next-neighbour crystals can be used to infer petrogenetic information from polycrystalline materials provided that the processes of COR formation are understood and parameters that control the kinetics of COR formation can be identified. After giving an overview on COR terminology, this chapter highlights non-genetic criteria for COR characterization, including a discussion of analytical methods that are used to constrain these criteria. The development of electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM) has provided new information on CORs, which is complementary to data obtained from transmission electron microscopy (TEM) analysis. Based on these non-genetic criteria, different types of CORs are characterized. Subsequently, physical parameters that can potentially influence COR formation are discussed. Furthermore, different scenarios and mechanisms leading to COR formation are outlined together with examples fromexperiments and fromnatural mineral and rock systems. The different boundary conditions of COR formation in various petrogenetic scenarios and the potential mechanisms that have to be taken into account when studying COR genesis are addressed. This chapter highlights the necessity of a multi-stage investigative approach in COR studies. First, the presence/absence and nature of CORs needs to be analysed based on non-genetic criteria. In a second step the formation mechanism of the CORs under consideration must be constrained, before in a third step, petrogenetic information can potentially be inferred. Moving from the second to the third step requires understanding of the parameters controlling COR development, which is by no means complete and leaves open tasks for future COR research.
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