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  • Jongkyu ParkJongkyu Park
  • Hoseong LimHoseong Lim
  • Bora MyeongBora Myeong
  • [...]
  • Marco BrennaMarco Brenna
Volcanoes with ocean island basalt (OIB)-like characteristics occur in all settings. Their origin is particularly enigmatic when they occur in close spatial and temporal association with subduction-related volcanism (e.g., back-arc basin basalts; BABB). A set of cognate enclaves and a late-intruding dike from Dokdo Island (Republic of Korea) were used to probe an intraplate mafic alkaline plumbing system developed in a back-arc basin. Detailed petrography, mineral chemistry, and whole-rock major- and trace-element geochemistry were combined with geothermobaro-hygrometric and whole-rock geochemical models. The cognate samples are defined as basalt–trachybasalt and gabbro, while a late-intruding dike has basaltic trachyandesite composition. In general, plagioclase crystals in the samples are classified as labradorite–bytownite, with cores richer in anorthite component compared to rims. Clinopyroxene crystals are sector-zoned diopside characterized by [2IVSi4+ + VIMg2+]basal ↔ [2IVAl3+ + VITi4+]prism. The prism sector-based thermobarometric models suggest that the cognate enclaves may represent multi-stage magmatic conditions from near-source (> 1250 °C and 1.5 GPa) through underplating (~ 1220 °C at ~ 640 MPa) to intermediate (~ 1180 °C at ~ 400 MPa) stages. Shallow crustal stagnant conditions (~ 1100 °C at ~ 120 MPa) are estimated from the late-intruding dike. Clinopyroxene- and plagioclase-based hygrometric models give a constant H2O abundance of ~ 1.5–2 wt% during the Dokdo basaltic magma evolution and ascent. According to our trace-element models, deep near-source parental melt may result from ~ 35% partial melting of hornblendite + spinel-peridotite and the associated mixing with clinopyroxene mush-derived 0.5% partial melt by a ratio of 0.6:0.4. From the parent, crystallization of up to 50% with the fractionating proportions of 0.7 plagioclase + 0.3 clinopyroxene can produce the whole compositional range of the Dokdo mafic alkaline magma. Rare or absent olivine in the early formed enclave may suggest that clinopyroxene and olivine are, respectively, crystallized at high and low pressure due to depressed olivine liquidus via melt compositional effect. We suggest that, during back-arc basin opening, BABB-related melt percolation into the extensional cracks in the lithospheric mantle and accompanying metasomatism provide hornblenditic source for the subsequent OIB magmatism.
 
Article
We explore crystal growth and magma recharge during the formation of intermediate lavas using bulk rock compositions and zoning patterns and textural variation in plagioclase feldspars from Hasandağ volcano in south-central Turkey. Hasandağ intermediate lavas formed primarily through fractionation of the observed mineral phases, and also show abundant evidence for magma mixing and thermochemical disequilibrium. Sparse basaltic andesites originated through mixing of observed mafic and felsic lavas rather than fractionation. Plagioclase phenocrysts from all lava types have uniform core (An~39) and variable rim (An39–64) compositions, suggesting plagioclase feldspars from a common magma reservoir are present in all erupted lavas. An-rich crystal rims are regularly enriched in Mg, Fe and Ti, indicating mafic recharge and magma mixing. Mixing timescales determined by diffusion modeling of Mg profiles across core-rim boundaries suggest recharge to eruption occurs on the order of up to ~ 45 days. Thermobarometric calculations constrain shallow crustal storage of dacite to 1.5–2 kbar (~ 5 km) and 800–890 °C, with deeper reservoirs for more mafic magmas (up to 35–40 km for basalts). Abundant disequilibrium textures (sieve-textured zones, oscillatory zoning, resorbed and patchy-zoned cores) indicate mobilization of a homogeneous crystal-rich dacitic reservoir by injection of mafic magma. We suggest that incomplete and dynamic physical mixing at shallow crustal levels results in distinct crystal morphologies, some of which record punctuated ascent and storage, while others are erupted rapidly after the influx of new magma.
 
Article
The solubility of N 2 in basaltic (MORB) and haplogranitic melts was studied at oxidizing conditions (oxygen fugacity about two log units above the Ni–NiO buffer). Under these conditions, N 2 is expected to be the only significant nitrogen species present in the melt. Experiments were carried out from 0.1 to 2 GPa and 1000–1450 ˚C using either an externally heated TZM pressure vessel, an internally heated gas pressure vessel or a piston cylinder apparatus. Nitrogen contents in run product glasses were quantified by SIMS (secondary ion mass spectrometry). To discriminate against atmospheric contamination, ¹⁵ N-enriched AgN 3 was used as the nitrogen source in the experiments. According to infrared and Raman spectra, run product glasses contain N 2 as their only dissolved nitrogen species. Due to interactions with the matrix, the N 2 molecule becomes slightly infrared active. Nitrogen solubility in the melts increases linearly with pressure over the entire range studied; the effect of temperature on solubility is small. The data may, therefore, be described by simple Henry constants K haplogranite = (1461 ± 26) ppm N 2 /GPa and K MORB = (449 ± 21) ppm N 2 /GPa, recalculated to ppm by weight (μg/g) of isotopically normal samples. These equations describe the solubility of nitrogen during MORB generation and during melting in the crust, as we show by thermodynamic analysis that N 2 is the only significant nitrogen species in these environments. Nitrogen solubility in the haplogranitic melt is about three times larger than for the MORB melt, as is expected from ionic porosity considerations. If expressed on a molar basis, nitrogen solubility is significantly lower than argon solubility, in accordance with the larger size of the N 2 molecule. Notably, N 2 solubility in felsic melts is also much lower than CO 2 solubility, even on a molar basis. This implies that the exsolution of nitrogen may drive vapor oversaturation in felsic melts derived from nitrogen-rich sediments. We also measured the partitioning of nitrogen between olivine, pyroxenes, plagioclase, garnet, and basaltic melt by slowly cooling MORB melts to sub-liquidus temperatures to grow large crystals. The mineral/melt partition coefficients of nitrogen range from 0.001 to 0.002, and are similar to argon partition coefficients. These new data, therefore, support the assumption that there is little fractionation between nitrogen and argon during mantle melting and that the N 2 /Ar ratio in basalts and xenoliths is, therefore, representative of the N 2 /Ar ratio in the mantle source. This assumption is essential for assessing the size of the nitrogen reservoir in the mantle. Our data also show that for an upper mantle oxidation state that is similar to the one observed today, nitrogen outgassing by partial melting is extremely efficient and even low melt fractions in the range of a few percent may extract nearly all nitrogen from the mantle source.
 
Article
Fahlores from the epithermal Mangazeyskoye Ag-Pb–Zn ore deposit (Sakha, Russia) typically occur in rhythmically zoned crystals. In contrast to typical (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores, up to about 2.6 of the 13 sulfur atoms in the formula unit may be vacant and many of them have Ag/(Ag + Cu) ratios substantially above those established for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores coexisting with miargyrite, pyrargyrite, and sphalerite by experimental, field, and theoretical investigations. The charge balance inequalities consequent on the removal of sulfur are compensated by metallization of the nominally Ag⁺ and Cu⁺ ions in the octahedral metal clusters that are formed by the removal of S²⁻ from the octahedral site at the center and corners of the unit cell of fahlore (space group I4¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{4 }$$\end{document} 3 m, Z = 2). The development of these vacancies leads to an effective ionic charge of [(Ag,Cu)66-2∗vac+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${(\mathrm{Ag},\mathrm{Cu})}_{6}^{\left(6-2*\mathrm{vac}\right)+}$$\end{document}] for these octahedral clusters, resulting in an intrinsic limit on sulfur vacancies of 3 per formula unit (pfu). The increases in the Ag/(Ag + Cu) ratio of some of the Mangazeyskoye fahlores relative to those defined for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores are readily attributed to the lowering of the activities of Ag10Zn2Sb4S13 and Ag10Fe2Sb4S13 components in fahlore by sulfur vacancies. This results in expansion of the field of fahlore stability relative to that given by the fahlore breakdown reaction Ag10Zn2Sb4S13 (fahlore) = AgSbS2 (miargyrite) + 3 Ag3SbS3 (pyrargyrite) + 2 ZnS (sphalerite) for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores. A preliminary analysis demonstrates that the increased Ag/(Ag + Cu) ratios observed in the Mangazeyskoye fahlores relative to those defined for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores are due to sulfur vacancies being almost exclusively in the octahedral sulfur site where the vacancy for sulfur substitution appears to be approximately ideal. However, it appears that a maximum in Ag/(Ag + Cu) ratios is reached around one pfu, and, to a first approximation, does not appear to increase with further increase in contents. This latter inference suggests that the for S (◻(S)-1) substitution on tetrahedral sulfur sites may be highly nonideal. When properly calibrated for sulfur fugacity based on experimental, spectroscopic, and theoretical studies, reactions such as Ag10Zn2Sb4S13=Ag10Zn2Sb4S12□OCT+1/2S2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Ag}}_{{{1}0}} {\text{Zn}}_{{2}} {\text{Sb}}_{{4}} {\text{S}}_{{{13}}} = {\text{Ag}}_{{{1}0}} {\text{Zn}}_{{2}} {\text{Sb}}_{{4}} {\text{S}}_{{{12}}}{\square}^{{{\text{OCT}}}} \, + \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\text{ S}}_{{2}}$$\end{document} and Ag10Zn2Sb4S11,□TETS=Ag10Zn2Sb4S12□OCT\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Ag}}_{{{1}0}} {\text{Zn}}_{{2}} {\text{Sb}}_{{4}} \left( {{\text{S}}_{{{11, \square}}} } \right)^{{{\text{TET}}}} {\text{S}}\, = \,{\text{Ag}}_{{{1}0}} {\text{Zn}}_{{2}} {\text{Sb}}_{{4}} {\text{S}}_{{{12}}}{\square}^{{{\text{OCT}}}}$$\end{document} will afford fahlore to serve as a petrogenetic indicator of sulfur fugacity in low-sulfidation environments. Until such calibrations are developed, the fahlore Ag/(Ag + Cu) ratios may nonetheless be used to define relative sulfur fugacities in rhythmically zoned fahlores from ore deposits such as Mangazeyskoye.
 
Article
In terrestrial magmas titanium is predominantly tetravalent (Ti 4+), in contrast, lunar magmas are more reduced (IW-1) and hence approximately 10% of their bulk Ti content is trivalent (Ti 3+). Changes in oxidation state and coordination number are both important parameters that can serve to drive Ti stable isotope fractionation. As such, mineral-mineral and mineral-melt Ti stable isotope fractionation factors determined for terrestrial samples may not be appropriate for lunar samples that formed under more reducing conditions. To address this issue, several experiments were carried out in gas mixing furnaces over a range of fO 2 (air to IW-1) to determine Ti stable isotope fractionation factors for minerals, such as ilmenite, clinopyroxene and rutile that are highly abundant on the Moon. Results show that the extent of Ti stable isotope fractionation significantly increases with decreasing fO 2. For example, the isotopic difference between ilmenite and residual melt (Δ 49 Ti ilmenite-melt) is resolvably lower by ~ 0.44 ‰ from terrestrial-like FMQ-0.5 to lunar-like IW-1 at an intermediate precision of ± 0.003 ‰ (95% c.i. OL-Ti). This confirms that fractionation factors determined for terrestrial conditions are indeed not applicable to lunar settings. Our new fractionation factors for ilmenite, clinopyroxene and silicate melt are mostly consistent with those previously determined by ab initio modelling based on density-functional theory. Using our new experimental data in conjunction with previously published high-precision HFSE data and Ti stable isotope data of lunar basalts, we modelled the solidification of the Lunar Magma Ocean (LMO). The model for LMO solidification included fractionation of Ti stable isotopes not only by Ti-oxides, but also by typical lunar silicate minerals as pyroxene or olivine. The resulting δ 49 Ti for urKREEP and ilmenite-bearing cumulates are within error of previous estimates, but also indicate that ilmenite-bearing cumulates must have contained around 15% ilmenite.
 
Article
Amphibolite clasts in the suevite of the Ries impact crater contain shock-induced melt veins (SMVs) with high-pressure phases such as majoritic garnet, jadeitic clinopyroxene and others. In addition, heat conduction from hot SMVs into adjacent rock portions locally produced further high P–T melt pools. These melts were preferentially generated in rock domains, where the SMVs cross older (‘pre-Ries’) veinlets with analcime or prehnite and larger grains of sericitized plagioclase. Melting of such chemically different local bulk systems (Na-, Ca-, Ca-Na- and K-Na-rich) was facilitated by low solidus temperatures of the original secondary OH-bearing phases. From the resulting shock-induced melts, liebermannite, kokchetavite, jadeite, nonstoichiometric and albitic jadeite, grossular, vuagnatite, lawsonite + coesite, and clinozoisite crystallized during pressure release. Vuagnatite is now proven to be a genuine high-pressure phase. Its ubiquitous distance of 20–35 μm from the hot shock veins suggests a temperature sensitivity typical for an OH-bearing phase. In local Na-rich melts albitic jadeite appears instead of the assemblage jadeite + SiO2. Liebermannite, a dense polymorph of K-feldspar was identified by Raman spectroscopy. After stishovite, liebermannite constitutes the second known high-pressure phase in the Ries that contains silicon exclusively in six-fold coordination. The KAlSi3O8-polymorph kokchetavite was formed in alkali-rich melt glasses. Pressure and temperature values in the range of about 8–11 GPa and ~ 800–1100 °C were estimated from the chemical compositions of locally occurring majoritic garnets (Si = 3.21–3.32 and 3.06–3.10 apfu), respectively, and the presence of fine-grained aggregates of lawsonite and coesite. Generally, the neighboring areas of the veins are characterized by a sequence of variable high-pressure phases documenting strongly falling P–T conditions with increasing distance from the vein. These novel features enlighten the dynamic event during passage of a shock wave.
 
Article
The South Aegean Volcanic Arc overlies a slowly subducting, cool slab of oceanic-to-transitional crust, and hosts the hazardous Christiana–Santorini–Kolumbo volcanic field. In order to investigate the primitive melts feeding the volcanic field, we present major and trace element analyses of 130 olivine-hosted melt inclusions from Santorini, integrated with previously published H2O and CO2 data. Following post-entrapment corrections, we identify four endmember primitive melt types preserved in Fo ≥ 80 olivines, ranging from low-K island-arc basalts with La/Yb ~ 1.5 and 1.5–3.0 wt% H2O to andesites with La/Yb ~ 6–10 and 3.0–3.5 wt% H2O. They are consistent with melting at 1.3 to 2.3 GPa and 1350–1440 °C of variably depleted peridotitic mantle fluxed by slab-derived melts and fluids. The chemical signatures of sediment melts dominate, while those of fluids derived from the ocean crust are low compared to global datasets. This is consistent with thick sediment accumulations observed in the Hellenic trench, and with low calculated fluid fluxes from the downgoing slab. The low H2O contents estimated for the primary melts (0.8–1.8 wt%) may imply a component of decompression melting beneath the arc. Coupled with a well-constrained chronostratigraphic context, the melt inclusion archive provides a time series of mantle-derived input into the silicic crustal magmatic system over the last 530 ka. Primitive melts with La/Yb ≤ 5 have been erupted encased in olivines over the last 530 ky, without any evident time variation. Melt inclusions with La/Yb > 5 have, on the other hand, been restricted to two periods: (1) prior to the onset of major explosive volcanism at ~ 360 ka, and (2) the products of the 3.6 ka Late-Bronze-Age eruption and the 22-to-3.6 ka inter-Plinian period immediately preceding it. The observations may be explained by time-varying differential extraction of melts from deep storage zones in the mantle or lower crust, related to lithospheric rifting and caldera collapse events. Temporal variations in the supplies of slab-derived melts and fluids may also play a role.
 
Article
Knowing the pressure of crystallization of peraluminous felsic rocks is essential to understanding their petrogenesis and implications for the tectonic histories of orogenic belts. Phase relations studies in the haplogranite system reveal that the stability field of quartz enlarges while the stability field of feldspars shrinks with increasing pressure. The result is that the compositions of the cotectic melts (saturated in quartz and feldspar) become less silicic with increasing pressure. Furthermore, the position of the quartz − feldspar cotectic is insensitive to water activity in the melt. This relationship provides an opportunity to estimate the pressure at which melt, quartz, and feldspar equilibrated. The accuracy of pressure estimation depends on accurately estimating the composition of the cotectic melt. However, additional components drive the composition of the cotectic melt away from that in the simple haplogranite system. For example, anorthite component (An) shifts the cotectic curve towards the Qz–Or join, and excess aluminum in the melt (i.e., peraluminous composition) drives the cotectic curve toward the Qz apex. A majority of felsic magmas (> ~ 70 wt% SiO2) are at least slightly peraluminous, and strongly peraluminous magmas are common in orogenic settings. A reasonable estimation of the pressures (depths) of crystallization of peraluminous magmas, which is critical for understanding accompanying tectonic processes, requires a better understanding of the effect of excess Al on the cotectic, and specifically on quartz saturation. We have compiled 32 experimental cotectic glass compositions in the felsic system, in which ASI (= Al2O3/(CaO + Na2O + K2O)(commonly defined identically to A/CNK), in mole content)) range from 1.10 to 2.13. The scheme of (Blundy and Cashman, Contrib Mineral Petrol 140:631–650, 2001) was chosen to correct for the effect of An on the position of the cotectic. The measured Qz of the experimental cotectic melts was compared to Qz calculated using the Blundy and Cashman correction at the experimental pressures. We assumed that the discrepancy between the corrected Qz content and the experimental Qz content was caused by excess aluminum in the melt. Based on this assumption, we can regress the relationship between excess aluminum (ASI-1) and “excess” Qz content. The positive correlation between ASI-1 and increased Qz demonstrates that excess aluminum increases the cotectic Qz content, i.e. quartz solubility, in the peraluminous system. The deviation of Qz content systematically increases from 4 to 20 wt%, with ASI rising from 1.10 to 2.13; the increase is especially rapid and clearly defined between 1.1 and 1.5, the normal range for natural peraluminous melts. The departure from ASI = 1.00 (ASI–1) systematically affects normative Qz in melt at a given P. We calibrate this relationship as: δQz=-18.14×ASI-12+37.64×ASI-1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\delta Qz=-18.14\times {\left(ASI-1\right)}^{2}+37.64\times \left(\mathrm{ASI}-1\right)$$\end{document}where δQz is the deviation of normative Qz in quartz- + feldspar-saturated melts as ASI increases from 1.00. Uncertainty in our calibration is fairly high (~ 120 MPa (one sigma), based on a standard error from the regression). This uncertainty might be influenced by many factors: (1) the accuracy of the compiled experiments; (2) disequilibrium in the compiled experiments; (3) uncertainty in the estimation of quartz + feldspar-saturated melt compositions based on natural rock samples; (4) the effect of other elements on the position of the cotectic curve, such as F, P, B, Fe, Mg, Ti, Mn. The empirical barometer was applied to natural examples (compositions of peraluminous glasses and granite compositions that permit estimation of melt compositions) with other thermobarometric constraints (cordierite-saturation, GASP (garnet-Al2SiO5-quartz-plagioclase), GBPQ (garnet-biotite-plagioclase-quartz), TitaniQ barometers for estimated melts; host rock metamorphic conditions). We propose that, where natural samples provide good estimates of the compositions of peraluminous melts that were saturated in quartz + feldspars (ideally two feldspars), our calibration provides useful estimates of the pressure of equilibration. Note that whole-rock compositions of granites cannot be assumed to represent melt composition; hence, pressures calculated by our barometer may not be meaningful unless applied to rocks whose compositions are likely to represent melts on the quartz-feldspar cotectic. The most obvious candidates are rhyolite glasses that host quartz + feldspar phenocryst assemblages and phenocryst-poor, aplitic-textured granites with textures indicating simultaneous crystallization of quartz and feldspar. The pressure calculation can be easily performed with the user-friendly spreadsheet (online supplement Table S4). As anticipated, the calculated pressures for peraluminous compositions are significantly higher than current quartz-saturation-based barometers would otherwise suggest.
 
Article
The reliability of eight Fe–Mg exchange geothermobarometers for garnet-bearing peridotites, pyroxenites and eclogites has been examined using a database comprised of more than 300 published peridotite, pyroxenite and eclogite experiments conducted from 10 to 70 kbar and 850 to > 1650∘C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ{\rm C}$$\end{document}. We have tested Fe–Mg exchange geothermometers suitable for a range of mantle lithologies, including websterite, harzburgite, wehrlite and eclogite. All geothermometers maintained an average difference in experimental and calculated temperature (T) ΔT=Texp-Tcalc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\Delta {\text{T}} = {\text{T}}_{{\exp - { }}} {\text{T}}_{{{\text{calc}}}} } \right)$$\end{document} of less than ±\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document} 50 °C with a standard deviation of ΔT\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {\text{T}}$$\end{document} between ±\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document} 50 to 150 °C. Most geothermometers performed well across a narrow range in ln KdFe-MgA-B\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Kd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{A}} - {\text{B}}}}$$\end{document} (where KdFe-MgA-B=\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Kd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{A}} - {\text{B}}}} =$$\end{document}FeA×MgB(FeB×MgA)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{\left( {{\text{Fe}}_{{\text{A}}} \times {\text{Mg}}_{{\text{B}}} } \right)}}{{({\text{Fe}}_{{\text{B}}} \times {\text{Mg}}_{{\text{A}}} )}}$$\end{document}), however, systematic overestimation and underestimation of T were observed outside of the optimal range of lnKdFe-MgA-B\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{lnKd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{A}} - {\text{B}}}}$$\end{document}. Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural samples. All previously published calibrations of the garnet-clinopyroxene geothermometer were unable to reliably reproduce the experimental T for both peridotite and eclogite experimental compositions, which hinders their confident application to natural datasets. To improve the state of mantle geothermobarometry we have used our experimental database to recalibrate the (1) garnet-clinopyroxene Fe–Mg exchange geothermometer, and (2) garnet-orthopyroxene Fe–Mg exchange geothermometer. Each geothermometer has been recalibrated across an extended P, T, and compositional range. The inclusion of eclogitic experiments in the calibration for the garnet-clinopyroxene geothermometer permits application to both eclogitic and peridotitic/pyroxenitic assemblages equilibrated under a wide range of PT conditions in the upper mantle. Using multiple linear regression to solve for lnKd, we found the following expressions best reproduced the experimental T (℃) of our dataset: TFe-Mggrt-cpx(∘C)=3356.34-0.008×Pkbar+0.259×XCagrt+0.914×XMggrt+-0.159×Jdcpx+lnKdFe-Mggrt-cpx+1.265-273\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{T}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{grt}} - {\text{cpx}}}} { (}^{ \circ } {\text{C)}} = { }\frac{3356.34}{{\left( {\left( { - 0.008 \times {\text{P }}\left( {{\text{kbar}}} \right)} \right) + \left( {0.259 \times {\text{X}}_{{{\text{Ca}}}}^{{{\text{grt}}}} } \right) + \left( {0.914 \times {\text{X}}_{{{\text{Mg}}}}^{{{\text{grt}}}} } \right) + \left( { - 0.159 \times {\text{Jd}}^{{{\text{cpx}}}} } \right) + \left( {{\text{ln}}\left( {{\text{ Kd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{grt}} - {\text{cpx}}}} } \right) + 1.265} \right)} \right)}} - 273{ }$$\end{document}TFe-Mggrt-opx(∘C)=1851.85-0.007×Pkbar+-1.83×XCagrt+lnKdFe-Mggrt-cpx+1.08-273\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{T}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{grt}} - {\text{opx}}}} { (}^{ \circ } {\text{C)}} = { }\frac{1851.85}{{\left( {\left( { - 0.007 \times {\text{P }}\left( {{\text{kbar}}} \right)} \right) + \left( { - 1.83 \times {\text{X}}_{{{\text{Ca}}}}^{{{\text{grt}}}} } \right) + \left( {{\text{ln}}\left( {{\text{ Kd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{grt}} - {\text{cpx}}}} } \right) + 1.08} \right)} \right)}} - 273$$\end{document}. where, XCagrt=\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{X}}_{{{\text{Ca}}}}^{{{\text{grt}}}} =$$\end{document}CaCa+Fe+Mg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{{\text{Ca}}}}{{\left( {{\text{Ca}} + {\text{Fe}} + {\text{Mg}}} \right)}}$$\end{document}, KdFe-Mggrt-opx=\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Kd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{grt}} - {\text{opx}}}} =$$\end{document}Fegrt×Mgopx(Feopx×Mggrt),XMggrt=\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{\left( {{\text{Fe}}_{{{\text{grt}}}} \times {\text{Mg}}_{{{\text{opx}}}} } \right)}}{{({\text{Fe}}_{{{\text{opx}}}} \times {\text{Mg}}_{{{\text{grt}}}} )}} , {\text{X}}_{{{\text{Mg}}}}^{{{\text{grt}}}} =$$\end{document}MgCa+Fe+Mg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{{\text{Mg}}}}{{\left( {{\text{Ca}} + {\text{Fe}} + {\text{Mg}}} \right)}}$$\end{document}, Jdcpx=Na-Cr-2×Ti\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Jd}}^{{{\text{cpx}}}} = {\text{Na}} - {\text{Cr}} - 2 \times {\text{Ti}}$$\end{document}, KdFe-Mggrt-cpx=\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Kd}}_{{{\text{Fe}} - {\text{Mg}}}}^{{{\text{grt}} - {\text{cpx}}}} =$$\end{document}Fegrt×Mgcpx(Fecpx×Mggrt),\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{\left( {{\text{Fe}}_{{{\text{grt}}}} \times {\text{Mg}}_{{{\text{cpx}}}} } \right)}}{{({\text{Fe}}_{{{\text{cpx}}}} \times {\text{Mg}}_{{{\text{grt}}}} )}},$$\end{document} with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe²⁺ = total Fe. Our updated calibrations resolve several issues with earlier calibrations, including a poor performance at elevated P and compositional limitations. An improvement in precision and accuracy has been demonstrated through application to the experimental calibration dataset, a second independent set of published experimental data, and to natural peridotites, pyroxenites and eclogites from on and off craton settings. Iterative PT estimates on natural datasets calculated using our updated calibrations compare well with estimates from widely used calibrations such as the Taylor (1998) two-pyroxene solvus geothermometer. We anticipate that this contribution will provide an important reference for the reliability of mantle geothermometers and that our updated calibrations will be used in future studies on peridotite, pyroxenite and eclogite inclusions in diamond and mantle-derived xenoliths.
 
Article
Infiltration-driven metamorphism has produced the widespread development of forsterite in the siliceous dolomites of the Alta, Utah contact aureole. SIMS (secondary ion mass spectrometry) δ¹⁸O analyses show that in most of the middle to outer forsterite zone samples, forsterite, calcite and dolomite are homogenous in δ¹⁸O at the grain-scale, but forsterite and calcite exhibit minor intergranular heterogeneities of < 1‰ on the thin-section to hand sample-scales. In contrast, within ~ 300 m of the igneous contact (inner forsterite and periclase zones), grain-scale δ¹⁸O heterogeneities in forsterite are as large as 3.0‰, but forsterite crystals do not exhibit systematic center-to-edge decreases in δ¹⁸O due to skeletal growth and infilling. With one exception, forsterite, calcite and dolomite in all samples analyzed yield disequilibrium fractionations. The oxygen isotope disequilibrium commonly recorded among forsterite, calcite and dolomite may reflect the rapid growth of forsterite from increased reaction overstepping as temperature increased, or results from ¹⁸O/¹⁶O depletion in the matrix carbonate minerals during continued fluid infiltration after forsterite crystallization was complete, during the initial stages of cooling in the forsterite zone. In a traverse across the boundary between forsterite- and periclase-bearing marble layers in the periclase zone, forsterite and calcite SIMS δ¹⁸O profiles combined with the trend in forsterite textures indicate that these layers were not infiltrated simultaneously, nor continuously during both prograde reactions responsible for forsterite and periclase. Rather, the texture and SIMS data require infiltration and formation of forsterite first in the forsterite marble layer, followed by infiltration and formation of periclase in the periclase marble layer. As a consequence, the δ¹⁸O profile recorded by forsterite formed earlier than the δ¹⁸O profile recorded by calcite, but both profiles are prograde features—that is, both profiles formed in response to prograde reactions forming first forsterite and then periclase. These grain-scale δ¹⁸O profiles further suggest that these layers experienced significant changes in permeability that are linked to their sequential infiltration-reaction history. The asynchronous growth of these index minerals at the outcrop-scale during prograde heating may be more common during contact metamorphism of carbonate lithologies than currently recognized.
 
Article
It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite. However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120-km-long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively, and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg + Fe²⁺)], lower Cr# [Cr/(Cr + Al)], and lower Fe³⁺/R³⁺ (R³⁺ = Fe³⁺ + Cr³⁺ + Al³⁺) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher-grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher-grade conditions that facilitated the diffusion of these elements to associated sulphide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc, Ti, V, Mn, Ni, Ga, Os, Ir, Ru, and Rh from the least altered chromite cores assuming equilibrium with the parental komatiitic melt also suggest the variable effects of metamorphism when compared with global experimental and empirical values of the natural samples.
 
Article
Lithium and boron are trace components of magmas, released during exsolution of a gas phase during volcanic activity. In this study, we determine the diffusivity and isotopic fractionation of Li and B in hydrous silicate melts. Two glasses were synthesized with the same rhyolitic composition (4.2 wt% water), having different Li and B contents; these were studied in diffusion-couple experiments that were performed using an internally heated pressure vessel, operated at 300 MPa in the temperature range 700–1250 °C for durations from 0 s to 24 h. From this we determined activation energies for Li and B diffusion of 57 ± 4 kJ/mol and 152 ± 15 kJ/mol with pre-exponential factors of 1.53 × 10–7 m²/s and 3.80 × 10–8 m²/s, respectively. Lithium isotopic fractionation during diffusion gave β values between 0.15 and 0.20, whereas B showed no clear isotopic fractionation. Our Li diffusivities and isotopic fractionation results differ somewhat from earlier published values, but overall confirm that Li diffusivity increases with water content. Our results on B diffusion show that similarly to Li, B mobility increases in the presence of water. By applying the Eyring relation, we confirm that B diffusivity is limited by viscous flow in silicate melts. Our results on Li and B diffusion present a new tool for understanding degassing-related processes, offering a potential geospeedometer to measure volcanic ascent rates.
 
Article
Genetic models for the formation of K-rich magmas in subduction-related settings range from partial melting of subduction-affected mantle sources to melting of crustal rocks depending on the local tectonic framework. The Miocene high-K calc-alkaline to shoshonitic rocks of Limnos Island reflect the magmatic activity in the northern Aegean, which migrated southwards in response to trench retreat and the collision of continental terranes in the Hellenic subduction system. New whole rock and mineral data of basaltic andesites, dacites and monzonites from Limnos indicate that the magmas underwent fractional crystallization of olivine, clinopyroxene, amphibole, apatite, and Fe-Ti oxides at 1100 to 700 °C and 0.5 to 0.1 GPa without significant assimilation of crustal rocks during the magma evolution. The strong enrichment of large ion lithophile elements and light rare-earth elements relative to depleted heavy rare earth and high-field strength elements points towards a mantle source that has been extensively hybridized by subducted sedimentary material. New Sr–Nd-Pb isotope data reveal a distinct isotopic composition of the Limnos rocks with high ²⁰⁷ Pb/ ²⁰⁴ Pb at low ²⁰⁶ Pb/ ²⁰⁴ Pb and ¹⁴³ Nd/ ¹⁴⁴ Nd ratios that is likely related to the subduction of the continental crustal succession of the Apulian block which was subducted prior to the onset of magmatism on Limnos. Partial melting models assuming a hybridized mantle source suggest that the primary melts of Limnos formed by melting of a phlogopite pyroxenite at melting degrees of 5 to 10%. Compositional differences between high-K calc-alkaline and shoshonitic magmas are explained by variable melting degrees and varying amounts of sediment supply to the mantle. The magmatic and tectonic evolution of Limnos island is typical for the Oligocene and Miocene volcanic centres of the migrating western Aegean arc front.
 
Article
Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H 2 O in the form of OH groups. H 2 O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H 2 O, pyroxene from ~ 50 to ~ 310 µg/g H 2 O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H 2 O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H 2 O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H 2 O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H 2 O increase from ~ 450 to ~ 2500 µg/g H 2 O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.
 
Article
A new model describing zircon saturation in silicate melts is presented that combines the results of 196 data from new experiments with data from previous experimental studies. In the new experiments, the concentration of Zr in melts coexisting with zircon was determined at temperatures between 800 and 1500 °C for 21 compositions (with alumina saturation index, ASI, from 0.20 to 1.15), containing ~ 1 to 16 wt % FeO T and, for a subset of these conditions, at variable pressure (0.0001 to 4.0 GPa) and water content (0 to 15 wt %). The collated dataset contains 626 data, with 430 from 26 literature studies, and covers conditions from 750 to 1620 °C, (including 45 new data and 106 literature data for temperatures < 1000 °C), ASI 0.20 to 2.00, 0.0001 to 4.0 GPa and 0 to 17 wt % H 2 O. A limitation of previous models of zircon saturation is the choice of parameter used to describe the silicate melt, which may not be appropriate for all compositions and can result in differences in predicted temperatures of over 200 °C for granitic systems. Here we use optical basicity ( Λ ), which can be easily calculated from the major oxide components of a melt, to parameterise the composition. Using a non-linear least-squares multiple regression, the new zircon saturation model is: $$\log{\mathrm{Zr}} = 0.96(5) - 5790(95)/T - 1.28(8)P + 12.39(35){\varLambda} + 0.83(9)x.\text{H}{_2}\text{O} + 2.06(16)P{\varLambda} $$ log Zr = 0.96 ( 5 ) - 5790 ( 95 ) / T - 1.28 ( 8 ) P + 12.39 ( 35 ) Λ + 0.83 ( 9 ) x . H 2 O + 2.06 ( 16 ) P Λ where Zr is in ppm, T is temperature in K, P is pressure in GPa, Λ is the optical basicity of the melt, x .H 2 O is the mole fraction of water in the melt, and the errors are 1σ. This model confirms that temperature and melt composition are the dominant controls on zircon solubility. In addition, pressure and melt water content exert small but resolvable effects on the solubility and are included, for the first time, in a model. Using this new calibration, 92% of the predicted temperatures are within 10% of the experimental temperatures for the collated dataset (with an average temperature difference of 57 °C), while predicted temperatures for only 78 and 62% of the collated dataset are within 10% of the experimental temperature (with average temperature differences > 80 °C) using the widely cited Watson and Harrison (Earth Planet Sci Lett 64:295–304, 1983) and Boehnke et al. (Chem Geol 351:324–334, 2013) models, respectively. This new model can be extrapolated to temperatures below those included in the calibration with greater accuracy and when applied to melt inclusions from the Bishop Tuff, gives temperatures that are in excellent agreement with independent estimates.
 
Experimental results of the stability of antigorite and its breakdown reactions in compositions with A no added F (Fsys = 0 wt%), B 2 wt% F (Fsys = 2 wt%), and C 5 wt% F (Fsys = 5 wt%) in the starting materials. Previous results on antigorite stability (all in F-free systems, (A): U&T (1995): Ulmer and Trommsdorff (1995); W&S (1997): Wunder and Schreyer (1997); B&P (2003): Bromiley and Pawley (2003). In (C) the grey curve “FUM (2014)” depicts experimental results on chlorite stability in hydrous peridotite (Fumagalli et al. 2014). The mineralogical composition of the individual run products is given using a pie diagram. Dark parts of the pie indicate stable mineral phases found in run products. Mineral abbreviations: atg antigorite, fo forsterite, en enstatite, chu clinohumite, chn chondrodite, chl chlorite, hu humite, HGM humite group minerals. The extent of the stability fields of chlorite + HGM (stippled curve) is not well known and not the focus of our study but we assume that this stability field is similar to the chlorite stability field (Fumagalli et al. 2014)
Phase diagram depicting the main reactions in antigorite-, and F-bearing systems (i.e., 2 and 5 wt% F) as function of pressure (GPa), temperature (°C) and depth (km). Blue and red dashed lines show the P – T – depth evolution of the top part of the peridotite layer (i.e., at 7 km depth in the slab) of a warm slab (red dashed line, Mexico, Syracuse et al. 2010) and a cold slab (blue dashed line, Tonga, Syracuse et al. 2010). Points (1) and (2) indicate the P–T conditions at which antigorite breaks down to olivine and orthopyroxene in systems with low F-concentrations (i.e., 2 wt%, reaction 3), and to chlorite and HGM in F-rich systems (i.e., 5 wt%, reactions 4 and 5). Point (3) depicts the P–T conditions at which the Moho of the (in a cold subducted slab (e.g., Tonga) will intersect the chlorite-out boundary, and in this scenario HGM will be the prime carriers of F in the slab at depths greater than about 170 km depth. Point (4) displays the P – T conditions F-rich system at which chlorite and HGM break down into anhydrous forsterite and enstatite, thus releasing all the volatiles. The area colored in grey indicates the P – T – depth space in which HGM are the sole F-bearing and water bearing minerals in our system. When chlorite breaks down (chl-out), all F is transferred to HGM, that may transfer F and some water to much greater depths, perhaps even into the transition zone of the mantle (Grützner et al. 2017)
Sketch of a subduction zone modified after Schmidt and Poli (2014). The green areas show the approximate stability regions of serpentine (antigorite), chlorite and humite-group minerals (HGM) in the slab
Article
We present new experimental data on the effect of F on the stability of antigorite and its breakdown products at high pressures (2–6 GPa) and high temperatures (570–850 °C). The experiments show that F does not affect the stability of antigorite, but addition of F to the system affects which minerals are formed when antigorite breaks down. In a F-free system and in a system with intermediate F contents (2 wt% F), antigorite breaks down to olivine and orthopyroxene, but in a F-rich system (5 wt% F), antigorite breaks down to other hydrous and F-bearing mineral assemblages which include chlorite, clinohumite and humite-group minerals (HGM). Since the latter mineral phases are stable at higher pressures and temperatures, and contain more F than antigorite, significant amounts of F and potentially other halogens can be retained in the subducting slab and transported deep into the mantle and possibly even into the Earth’s transition zone.
 
Article
Cr-pyrope is one of the most abundant mineral inclusions in peridotitic diamonds and its thermal equation of state (EoS) is a prerequisite for accurately determining its entrapment pressure (Pe), which is important to understanding the physicochemical environment of diamond formation. We present in situ single-crystal X-ray diffraction (XRD) experimental results of five natural Cr-pyropes (Cr# = 0.3–22.4, Cr# = Cr/(Cr + Al)) at high pressure (P), high temperature (T), and high P–T up to 13.2 GPa and 950 K. The obtained P–volume(V)–T data were used to derive EoS parameters. The results indicated that the compressional behaviors of these Cr-pyropes are close (up to 2.8% difference), but the differences between their thermal expansivities are up to 8.8%. The thermal expansivities of the Cr-pyropes are significantly higher than that of end-member garnets (pyrope, almandine, and grossular) obtained by room-P high-T XRD, but they are consistent with the end-member thermal expansivities obtained by high P–T XRD. To investigate the compositional effects on the estimation of Pe of Cr-pyrope, the obtained EoS parameters were used to calculate the Pe in diamond. The results indicated that the variation in thermal expansion behavior plays a more significant role in influencing the Pe in comparison with the compressional behavior. In addition, the Pe of the Cr-pyropes are compared with the Pe of end-member garnets (pyrope, almandine, grossular, and uvarovite), which indicates that the low-Cr (Cr# = 0.3–4.4) pyropes are closer to pyrope in Pe while the high-Cr (Cr# = 9.3–22.4) pyropes are closer to grossular.
 
Article
Subduction of the Pacific slab has been widely credited for the destruction of the eastern North China Craton (NCC). However, the nature of the subcontinental lithospheric mantle (SCLM) beneath the central and western NCC is less well constrained, which has hindered the understanding of cratonic responses to repeated Phanerozoic circum-craton subductions. Here, we report the in situ major- and trace-element contents and Sr isotopic compositions of minerals from 27 peridotite xenoliths obtained from Cenozoic basalts from Xiyang‒Pingding, central NCC, to gain new insights into how plate subduction affected the Archean lithospheric keel. On the basis of olivine major-element and clinopyroxene trace-element contents, as well as clinopyroxene Sr isotopic compositions, we divide the xenoliths into four types. The high-Mg# types 1 and 2 peridotites show high olivine forsterite (Fo) contents, convex-upward rare-earth element patterns, and high ⁸⁷Sr/⁸⁶Sr (0.7053‒0.7062) in clinopyroxene, and they could be relicts of an old cratonic root. The high-Mg# type 3 peridotites exhibit high (La/Yb)N and low Ti/Eu and ⁸⁷Sr/⁸⁶Sr (0.7038‒0.7044), and they are interpreted as reaction products of Archean SCLM and carbonatitic melts from the asthenosphere with a contribution of recycled oceanic components. The low-Mg# type 4 peridotites display low ⁸⁷Sr/⁸⁶Sr (0.7029‒0.7033) and represent fragments of modified SCLM that has been recently refertilized by asthenospheric melts. Complex architectures of the SCLM underneath Xiyang‒Pingding reflect multistage overprinting of mantle metasomatism. Recent influxes of asthenospheric melts played a critical role in transforming refractory SCLM to fertile SCLM. Combined with previous studies, our findings suggest that the SCLM beneath the central NCC became progressively more refertilized from the center toward both the northern and southern margins, a spatial pattern that was likely generated by N‒S-trending subductions of the circum-craton plates.
 
Article
Voluminous moderate- to high-magnesian [Mg# = molar Mg/(Mg + Fe²⁺) = 44–64] andesitic and dacitic rocks with high silica (mostly 61–66 wt%) adakitic affinity (Y = 13–22, Yb = 1.3–2.1, Sr/Y = 18–44, La/Yb = 10–25) and common mafic magmatic enclaves (MMEs) are first reported in the Keçiboyduran stratovolcano (KSV) from the Cappadocia volcanic province (CVP), Central Anatolia, Turkey. We present comprehensive whole-rock geochemistry and Sr–Nd–Pb isotope data, mineral chemical compositions and ⁴⁰Ar–³⁹Ar ages for KSV samples. Based on the volcanostratigraphy and ⁴⁰Ar–³⁹Ar dating results, two successive eruption ages of 2.2–1.6 Ma (stage I: amphibole-rich) and 1.6–1.2 Ma (stage II: pyroxene-rich) were established for the KSV, corresponding to the Gelasian and Calabrian stages of Early Pleistocene, respectively. Textural and geochemical evidence indicates that the KSV magnesian andesites–dacites are products of a hybrid magma formed by mixing between mantle-derived mafic and crust-derived felsic magmas with further fractionation and minor contamination during magma storage and ascent. Our new data, combined with previous geological and geophysical results suggest that parental magnesian mafic melts of the KSV rocks originated from a heterogenous mantle source generated through the metasomatism of mantle wedge material by subducted sediment-derived melts, and then partially melted through asthenospheric upwelling in response to slab break-off. The mafic magma underplated the overlying lower crust, resulting in its partial melting to generate crustal felsic magma. Both magmas mixed at lower crustal levels creating MME-rich hybrid magmas. Subsequently, the hybrid magmas were emplaced at different depths of the crust (c. 4–11 and 11–15 km for the stage I and II, respectively), where they crystallized at moderate temperatures (c. 1180–840 °C) and under relatively high oxygen fugacity (LogƒO2 = − 11.4 to − 9.2), water-rich (H2Omelt = 5.6–3.6 wt%) and polybaric (~ 1.2 to 5.1 kbars) conditions, and underwent fractionation of primarily amphibole ± pyroxene causing adakitic affinity. We propose a new petrogenetic model for the early Quaternary magnesian/adakitic andesites/dacites of the CVP in a post-subduction tectonic setting. Our results provide robust evidence for slab break-off of the eastern Cyprus oceanic lithosphere and put further constraints on the tectonic evolution of the eastern Mediterranean collision zone during the Early Quaternary.
 
Article
Arsenopyrite (FeAsS) is one of the sulfide minerals of seafloor massive sulfide deposits. The presence of sodium chloride and high-temperature and high-pressure (HTHP) geological conditions seriously affect the process of arsenopyrite weathering. However, electrochemical oxidative dissolution has never been considered in the context of seafloors, though it has already been shown to increase dissolution significantly in terrestrial deposits. In this work, in situ electrochemical techniques and surface analysis were used to investigate the behaviors of oxidative arsenopyrite dissolution in different concentrations of NaCl at temperatures ranging from 280 to 360 °C and pressures ranging from 12.0 to 20.0 MPa. In the initial stage, arsenopyrite was oxidized to S⁰, As(III), and Fe(II). The S⁰ and As(III) were ultimately converted into SO4²⁻ and AsO4³⁻ and entered the solution. The Fe(II) was converted into α-FeOOH, γ-FeOOH, and Fe2O3 as a passivation film. The presence of Cl⁻ ions promoted the oxidative dissolution of arsenopyrite without changing its oxidation mechanism. Higher temperatures or greater pressures promoted the oxidative dissolution of arsenopyrite by enhancing charge migration and ion diffusion. Under the experimental HTHP conditions, the oxidative arsenopyrite dissolution rate constant was 8.0 × 10–5 mol∙m⁻²∙s⁻¹. This work expands the understanding of the geochemical cycles of Fe, As and S and provides an experimental basis for the formation of secondary minerals from arsenopyrite weathering under the hydrothermal solution conditions of the seafloor.
 
Article
Volcanic rocks commonly display complex textures acquired both in the magma reservoir and during ascent to the surface. While variations in mineral compositions, sizes and number densities are routinely analysed to reconstruct pre-eruptive magmatic histories, crystal shapes are often assumed to be constant, despite experimental evidence for the sensitivity of crystal habit to magmatic conditions. Here, we develop a new program (ShapeCalc) to calculate 3D shapes from 2D crystal intersection data and apply it to study variations of crystal shape with size for plagioclase microlites (l < 100 µm) in intermediate volcanic rocks. The smallest crystals tend to exhibit prismatic 3D shapes, whereas larger crystals (l > 5–10 µm) show progressively more tabular habits. Crystal growth modelling and experimental constraints indicate that this trend reflects shape evolution during plagioclase growth, with initial growth as prismatic rods and subsequent preferential overgrowth of the intermediate dimension to form tabular shapes. Because overgrowth of very small crystals can strongly affect the external morphology, plagioclase microlite shapes are dependent on the available growth volume per crystal, which decreases during decompression-driven crystallisation as crystal number density increases. Our proposed growth model suggests that the range of crystal shapes developed in a magma is controlled by the temporal evolution of undercooling and total crystal numbers, i.e., distinct cooling/decompression paths. For example, in cases of slow to moderate magma ascent rates and quasi-continuous nucleation, early-formed crystals grow larger and develop tabular shapes, whereas late-stage nucleation produces smaller, prismatic crystals. In contrast, rapid magma ascent may suppress nucleation entirely or, if stalled at shallow depth, may produce a single nucleation burst associated with tabular crystal shapes. Such variation in crystal shapes have diagnostic value and are also an important factor to consider when constructing CSDs and models involving magma rheology.
 
Article
To interpret modern-day unrest at Yellowstone Caldera, timescales leading up to its most common type of eruption—effusively emplaced rhyolite—must be quantified. This work takes advantage of the different rates of elemental diffusion in clinopyroxene to calculate the magmatic timescales of events preceding eruption of the ca. 262 ka Scaup Lake rhyolite, which ended ~ 220,000 years of dormancy in this high-silica system. Here, we present diffusion chronometry timescales accounting for various sources of error and using multiple elements from NanoSIMS measurements of clinopyroxene rims. We combine these with previously published timescales from sanidine rims to better understand the relationship between timescales captured by different minerals from the same volcanic event. We show that timescales archived by rims of different types of phenocrysts from the same lava may not be concomitant. The Scaup Lake rhyolite appears to have undergone several rejuvenation events over ~ 5000 years before its eruption, and the last events (< 40 years before eruption) were not recorded by clinopyroxene. This work highlights the importance of using multiple methods to determine a timescale for a given process. Although many studies use Fe–Mg zonation from BSE images to calculate diffusive timescales alone, we show that these are maximums or overestimates if not referenced to the appropriate initial condition. Instead, we demonstrate that diffusion chronometry conducted with multiple elements in multiple mineral phases with rigorous error propagation produces the most robust and accurate temporal results. In addition, we recommend that diffusion chronometry results not be interpreted in isolation, but rather in a holistic petrological approach that includes consideration of the relevant phase equilibria and crystal growth and dissolution rates.
 
Article
We examine the connected history of dacite-dominant volcanic rocks of the Tschicoma Formation, erupted between 5.5 and 2 Ma from the Jemez Mountains volcanic field, western USA. Zircon samples from two separate eruptions have continuous SHRIMP U–Pb age spectra spanning 0.84–1.08 Myr duration (3.12–3.96 Ma and 3.50–4.58 Ma, respectively), following an episode of zircon crystallization 0.28–0.50 Myr earlier (at 4.46 Ma and 4.86 Ma, respectively). Zircon chemical variations, as well as ubiquitous resorption textures that commonly show large core-rim age differences (up to 720–740 kyr), suggest that they grew in separate melt lenses. Zircons were likely stored at near-solidus or even sub-solidus conditions after crystallization, but may have been reactivated in response to at least four major magma recharge events every 300–400 kyr and smaller events in between. A cycle of zircon dissolution (from heating), recrystallization (during cooling), and storage repeated in different locations in the Tschicoma mush system throughout its lifespan; each recharge-induced heating stage may last for several hundred to more than a thousand years based on calculations of zircon dissolution. We envisage the melt lenses to be distributed in a crystal mush zone, coalescing into a single magma batch as magma recharge occurs shortly before eruption. Once active, increasing magma supply rates may trigger large-scale partial melting of the pre-existing mush and caldera-forming eruptions.
 
Article
Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P–T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.
 
Article
This study presents new petrological and fluid inclusion datasets of migmatites from the Karakorum Shear Zone, Ladakh, India, to know the P–T–fluid evolution of mineral assemblages and the associated tectonic history. The presence of plagioclase, quartz, and biotite inclusions in the coarse-grained poikiloblastic pargasite (amphibole) is indicative of the hydration reaction bt+pl+qtz+H2O=prg+melt\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$bt+pl+\mathrm{qtz}+\mathrm{H}2\mathrm{O} = \mathrm{prg}+\mathrm{melt}$$\end{document}, which is consistent with diffusive H2O-fluxed melting. Phase equilibria calculations are consistent with migmatization at 0.85–1.02 GPa and 640–670 °C in water-saturated conditions (i.e., 0.7 wt% H2O). The monophase primary and secondary carbonic fluid inclusions present in quartz display eutectic temperatures between − 56.9 and − 56.6 °C, suggesting pure CO2 composition. The isochores of primary CO2 inclusions reveal that the post-peak migmatization event took place between 0.59–0.55 GPa and 550–670 °C, which occurred due to density reversal during the re-equilibration. The fluid inclusion microtextures preserved the signature of isothermal decompression, which is well corroborated with mineralogical P–T calculations. Primary inclusions were preserved initially as carbonic-aqueous fluids; however, the H2O phase diffused out subsequently and dissolved with the melt such that the inclusions became pure carbonic. Fluid infiltration along the Karakorum Shear Zone played a critical role in forming migmatites. The P–T path derived from thermodynamic modeling and fluid inclusion data are consistent with isothermal decompression during exhumation following crustal thickening of the Asian continent (or South Tibaten Crust) between 18 and 15 Ma.
 
Article
The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 510–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite–ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite–chloritoid-bearing rocks.
 
Article
We provide a simple geometric theory of crystal growth which predicts the shape and final dihedral angle of three-grain junctions of an augite crystal with two plagioclase grains. The predicted dihedral angle $$\Delta $$ Δ depends on the initial impingement angle $$\psi $$ ψ formed by the plagioclase grains, and also on the relative growth rates of the augite and the plagioclase, and shows reasonable agreement with data obtained from natural samples. We show that the two augite-plagioclase grain boundaries will normally curve towards each other, which is consistent with the first two types of junction described in the companion paper. However, the third type, the eagle’s beak, is formed by the meeting of grain boundaries which curve in the same direction. Although it is possible to account for this type of junction by invoking the localised dissolution of one of the plagioclase grains, this is unlikely to occur. A more plausible explanation involves the late impingement of the two plagioclase grains, consistent with the observation that eagles’ beaks are common in gabbros and strongly orthocumulate troctolites, in which the plagioclase framework has not been established by the time augite is growing in substantial quantities. An observed flattening of the curve of $$\Delta $$ Δ values at high values of $$\psi $$ ψ can be explained by taking into account the importance of interfacial energy in late-stage crystallisation.
 
Article
Atoll garnets are uncommon features that have been recognized in contrasting metamorphic environments worldwide, but their origin remains largely debated. Several models have been proposed to explain their formation, including preferential dissolution of garnet cores by fluid infiltration, polymetamorphism, and the coalescence of subgrains. We report atoll-shaped garnets in an amphibolite facies schist from the Palaeoproterozoic New Quebec Orogen, Canada, and investigate their textural and chemical zoning through petrography, electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) maps. Textural evidence indicates a subhedral poikiloblastic core, an inclusion ring composed of matrix minerals, and a euhedral rim. Major element distribution maps show flat zoning, whereas trace elements show concentric growth zoning. Such characteristics are consistent with rapid, post-kinematic growth involving Rayleigh fractionation of trace elements and coeval with accessory phase breakdown. Our observations rule out the preferential dissolution, polymetamorphism and coalescence models, and support that the formation of atoll garnet in these rocks is best explained by a kinetic control and rapid growth. Our study concludes that the term “atoll” is more descriptive than genetic, and that the physio-chemical mechanisms leading to its formation should be assessed on a case-by-case basis using complementary tools, primarily including trace element mapping.
 
--Sequence of five fossil transverse ribs (arrowed) in Late Holocene alluvium, Depot Creek. Paleoflow right to left. Modern channel in foreground (flow fight to left); front of Flinders Ranges in background.
Article
Rapid flow structure in Quaternary alluvial fan
 
Article
The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of ~ 70 °C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of ~ 730 °C, 6.6 kbar in the migmatitic gneisses and ~ 500 °C, 4.2 kbar in the serpentinites and Tambo schists 2–4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the ~ 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2–1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45–65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U–Pb dating of zircons shows that partial melting and diking occurred at 29.0–31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North–South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex.
 
Article
The thermodynamic equilibrium dihedral angle at grain junctions in crystalline rocks is set by the grain boundary interfacial surface energies, but the long times required to attain equilibrium mean that the observed dihedral angles in igneous rocks are generally set by the kinetics of crystallisation. We distinguish three types of augite–plagioclase–plagioclase dihedral angle in mafic igneous rocks. In the first, augite grows in the pores of a pre-existing plagioclase framework accompanied by little to no inwards-growth of the plagioclase pore walls. In the second, the plagioclase pore walls grow inwards simultaneously with the augite, and the dihedral angle is generally larger than the original angle at which the two plagioclase grains impinged except when the impingement angle itself is large. The first type is seen in rapidly crystallised rocks, whereas the second is observed in slowly cooled rocks. The third type is highly asymmetric and resembles (and so we call) an eagle’s beak: it is only seen in slowly cooled rocks. It is common in gabbroic cumulates, and is also present in strongly orthocumulate troctolites. Using the mode of interstitial phases to calculate the amount of interstitial liquid present in a series of mafic cumulates from the Rum and Skaergaard layered intrusions, we show that the asymmetry of three-grain junctions in troctolites increases as the rocks progress from adcumulate to orthocumulate (i.e. as the olivine–plagioclase crystal mush becomes more liquid-rich), with eagles’ beaks becoming the dominant three-grain junction geometry for troctolitic mushes containing ∼ 12 vol.% interstitial material (corresponding to ∼ 30 vol.% liquid in the mush). The geometry of three-grain junctions in mafic rocks is thus a function not only of cooling rate, but also of the progression along the liquid line of descent during fractionation. The first two types of junction are formed in relatively primitive liquids, during which the crystal mushes on the margins of the solidifying magma body are formed predominantly of plagioclase and olivine, whereas the eagle’s beak geometry occurs once augite forms an important component of the crystal framework in the accumulating mush, either because it is a framework-forming primocryst phase or because it grows from highly abundant interstitial liquid.
 
Article
All available volume and elasticity data for the garnet end-members grossular, pyrope, almandine and spessartine have been re-evaluated for both internal consistency and for consistency with experimentally measured heat capacities. The consistent data were then used to determine the parameters of third-order Birch–Murnaghan EoS to describe the isothermal compression at 298 K and a Mie–Grüneisen–Debye thermal-pressure EoS to describe the PVT behaviour. In a full Mie–Grüneisen–Debye EoS, the variation of the thermal Grüneisen parameter with volume is defined as $$\gamma = {\gamma }_{0}{\left(\frac{V}{{V}_{0}}\right)}^{q}$$ γ = γ 0 V V 0 q . For grossular and pyrope garnets, there is sufficient data to refine q which has a value of q = 0.8(2) for both garnets. For other garnets, the data do not constrain the value of q and we therefore refined a q- compromise version of the Mie–Grüneisen–Debye EoS in which both γ / V and the Debye temperature θ D are held constant at all P and T , leading to $$\left( {{\raise0.7ex\hbox{${\partial C_{{\text{V}}} }$} \!\mathord{\left/ {\vphantom {{\partial C_{{\text{V}}} } {\partial P}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${\partial P}$}}} \right)_{{\text{T}}} = 0$$ ∂ C V ∂ P T = 0 , parallel isochors and constant isothermal bulk modulus along an isochor. Final refined parameters for the q- compromise Mie–Grüneisen–Debye EoS are: Pyrope Almandine Spessartine Grossular V 0 (cm ³ /mol) a 113.13 115.25 117.92 125.35 K 0T (GPa) 169.3 (3) 174.6 (4) 177.57 (6) 167.0 (2) $$K^{\prime}_{{0{\text{T}}}}$$ K 0 T ′ 4.55 (5) 5.41 (13) 4.6 (3) 5.07 (8) θ D0 771 (28) 862 (22) 860 (35) 750 (13) γ 0 1.185 (12) 1.16 (fixed) 1.18 (3) 1.156 (6) for pyrope and grossular, the two versions of the Mie–Grüneisen–Debye EoS predict indistinguishable properties over the metamorphic pressure and temperature range, and the same properties as the EoS based on experimental heat capacities. The biggest change from previously published EoS is for almandine for which the new EoS predicts geologically reasonable entrapment conditions for zircon inclusions in almandine-rich garnets.
 
Article
Tibetan Cenozoic syn-collisional potassic–ultrapotassic igneous rocks provide unique insights into the processes and origins of metasomatism in the upper mantle, as well as continental subduction and plateau uplift. Crustal recycling in the magma source of the Tibetan potassic–ultrapotassic rocks has been well-documented. However, the nature of the metasomatic agents and the timing of mantle metasomatism are still disputed. Oxygen isotopes are a powerful tracer for identifying any recycled crustal material in the mantle due to the significant fractionation caused by surface water–rock interaction. Here we present an integrated in-situ study of oxygen isotopes and the major/trace elements of olivine in the Cenozoic potassic–ultrapotassic rocks and mantle xenoliths from the western Kunlun area of the northwestern Tibet. Olivines from mantle xenoliths have oxygen isotope compositions that range from elevated δ¹⁸O values (5.83 ± 0.78–5.97 ± 0.40‰) to values (5.09 ± 0.56–5.10 ± 0.46‰) that are indistinguishable from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., Earth Planet Sci Lett 128:231–241, 1994). Elevated olivine δ¹⁸O values reflect mantle metasomatic processes by an ¹⁸O-rich agent, although a few olivine rims show higher δ¹⁸O values caused by reactions with host lavas. In contrast, olivine oxygen isotope compositions of potassic–ultrapotassic rocks are higher than typical “mantle” values and those of the mantle xenoliths (6.51 ± 0.74–7.52 ± 0.24‰). There is no sign of crustal contamination, and fractional crystallization did not change the isotopic compositions of the studied potassic–ultrapotassic igneous rocks, thus their high-δ¹⁸O signature reflects the nature of the primary melts and their source region. Enrichments in olivine ¹⁸O and clinopyroxene Sr isotopic compositions, as well as the enriched trace element compositions of clinopyroxene and orthopyroxene in the mantle xenoliths, indicate that they have been highly metasomatized by silicate materials with a minor carbonate component from recycled Indian continental crustal component, and the enriched lithospheric mantle is the magma source of potassic–ultrapotassic rocks. High olivine δ¹⁸O values are a common feature of Tibetan Cenozoic potassic–ultrapotassic rocks, consistent with the mantle metasomatic agents that were derived directly from recycled continental crust material via Indian continental subduction. Our study reinforces the profound link between continental subduction, mantle processes and generation of Tibetan Cenozoic potassic–ultrapotassic rocks.
 
Article
The H2O solubility in peralkaline haplogranitic melts has been experimentally determined as a function of pressure (27–200 MPa) and temperature (1123–1523 K). The compositions were based on Ab38Or34Qz28 (AOQ) with 4 and 8 wt% Na2O in excess. H2O solubility experiments were performed in an internally heated pressure vessel and quenched to glasses for analysis. For quantification of H2O contents in the glasses using FTIR analysis, the linear molar absorption coefficients as a function of Na2O excess with respect to AOQ composition were determined, as well as the glass densities as a function of H2O concentration. The H2O solubility increases with increasing pressure, decreasing temperature, and with increasing peralkalinity. A linear dependence between Na2O excess (wt%) and H2O solubility (wt%) was found. It has been previously shown that on a molar basis the different alkalis contribute similarly to the H2O solubility increase so that H2O solubility increases linearly with excess alkali (difference between mole fractions of alkalis and that of alumina). Thus, the dependence of H2O solubility on pressure, temperature and excess alkali obtained from the new data of this study allow a simple prediction of H2O solubility for peralkaline rhyolitic melts based on the excess alkali content. This new empirical model was tested with H2O solubility data from literature for peralkaline haplogranitic and natural peralkaline rhyolitic melt compositions, yielding good agreement (< 10% deviation) between predicted and observed H2O solubility, which is an improvement compared to previous models. The model can be applied to natural peralkaline rhyolitic melts that occur, e.g. on Pantelleria, Gran Canaria, or the East African Rift.
 
Article
Mount Early and Sheridan Bluff (87°S) are the above-ice expression of Earth’s southernmost volcanic field and are isolated by > 1000 km from any other exposed Cenozoic volcano in Antarctica. These monogenetic, Early Miocene volcanoes consist of olivine-phyric basaltic pillow lavas and breccias (Mount Early) and pāhoehoe lavas (Sheridan Bluff) whose differentiation is controlled by the fractional crystallization of olivine with lesser quantities of clinopyroxene, plagioclase and magnetite. Fractional crystallization or contamination by crust cannot account for the coexistence of olivine tholeiite and alkaline compositions but their relationship can be explained by change from higher (5-6%) to lower (1.5-2%) degrees of partial melting concurrent with a decrease in peridotite‒melt reaction in a mantle that is heterogeneous on a small-scale. Both magma types have geochemical and isotopic signatures that differentiate them from most of the volcanism found within the West Antarctic rift system. Data trends in Sr-Nd-Pb isotope space indicate mixing of at least two-distinct mantle sources; 1) a relatively depleted component similar to sources for mid-ocean ridge basalt from the extinct Antarctic-Phoenix spreading center, and 2) an enriched component similar to sources for mafic magmas of the Jurassic Karoo‒Ferrar large igneous provinces. The availability of these mantle source types was facilitated by the detachment, sinking and heating of metasomatized continental lithosphere (enriched source) that released volatiles into the surrounding asthenosphere (depleted source) to promote flux melting. Volcanism triggered by lithospheric detachment is therefore explicitly applied to Mount Early and Sheridan Bluff to explain their isolation and enigmatic tectonic setting but also to account for source heterogeneity and the ephemeral change in degree of mantle partial melting recorded in their mafic compositions.
 
Article
Continental flood basalt lavas often contain deeply-sourced, thermo-chemically anomalous material that can provide a potential probe of inaccessible reservoirs. However, continental flood basalts interact with geochemically diverse domains within the continental lithosphere, which may complicate interpretations of deep mantle signatures. We examine the role of continental lithospheric mantle in continental flood basalts erupted as part of the 1.1 Ga Keweenawan large igneous province, centered on the Lake Superior region of North America. We show that flood basalts at Mamainse Point exhibit a range of εHf 1100 from −14.1 to +6, plotting along the global εHf—εNd mantle array. Lithospheric mantle melts represented by alkaline rocks from the Coldwell and Seabrook Lake Complexes yield positive εNd 1100 (+0.7 to +4.3) and εHf 1100 from −6.9 to +2.4, placing them below the mantle array. Mamainse Point lavas are interpreted to be variably crustally contaminated melts of the Keweenawan plume and ambient upper mantle; there is no clear evidence for contributions from an enriched lithospheric mantle.
 
Article
High-temperature conditions for ore deposition and anomalous abundances of base metals in orogenic gold deposits are frequently attributed to magmatic-hydrothermal fluids instead of metamorphic orogenic fluids. Zona Basal is a shear zone-related gold and base metals-rich mineral occurrence recently discovered in the northwestern portion of the Quadrilátero Ferrífero mining district in southeast Brazil. Mineralogical and geochemical characterization has revealed a two-stage mineralizing system. An Au-W-As stage is marked by a late- to post-tectonic arsenopyrite-pyrrhotite-pyrite assemblage (crystallized from ca. 491 to 404 °C), associated with native gold and scheelite, which was formed during the transition from the peak of greenschist facies metamorphism to retrograde metamorphism. A following, clearly post-tectonic stage comprises a pyrrhotite-pyrite-sphalerite-chalcopyrite-galena-ullmannite ± meneghinite ± fahlore ± miargyrite ± pyrargyrite assemblage crystallized from ca. 400 to < 170 °C, corresponding to the Ag and base metals stage. Fahlores from the Zona Basal mineral occurrence have been found to be sufficiently enriched in Ag, due to the Ag–Cu cation-exchange reaction 13PbS (galena) + 10AgSbS2 (in galena) = 3Ag3SbS3 (pyrargyrite) + Ag(Cu)−1 (exchange in fahlore) + CuPb13Sb7S24 (meneghinite), to jump across the (Cu, Ag)10(Fe,Zn)2Sb4S13 miscibility gap. Two mechanisms may explain the abrupt chemical change between the two mineralization stages: (1) participation of magmatic-hydrothermal fluids of relatively low temperature (acting mostly as a chemical source for fluids) associated with a syn- to late tectonic granite intrusion; or (2) local re-concentration of base metals by unmixing of an early metamorphic orogenic fluid due to fluid immiscibility after the transition from ductile to brittle conditions, generating coexisting fluids with different salinities (i.e., extremely low and moderate salinity). The findings in this paper illustrate the importance of considering retrograde or cooling reactions and mineral re-equilibrium while addressing complex ore-formation.
 
Article
Oxidized fluids in the subduction zone may convert polyvalent elements in the mantle to their higher valence states. The most abundant polyvalent element in the mantle is Fe, a significant part of which is contained in olivine as Fe²⁺. Results of the study of arc mantle xenoliths, in lab high-pressure–high-temperature experiments, and thermodynamic modeling have shown that at pressures of ~ 50–2000 MPa and temperatures of 1000–1250 °C, well above the serpentine stability field, Fe²⁺ from olivine reacts with free aqueous fluid according to the following simplified reaction: 3Fe2SiO4 + 2H2O ⇆ 3SiO2 + 2Fe3O4 + 2H2. The resulting ferric iron is preserved in spinel of a certain composition, Mg,Fe2+Fe23+O4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {{\text{Mg,Fe}}_{{}}^{{2 + }} } \right){\text{Fe}}_{{2}}^{{3 + }} {\text{O}}_{{4}}$$\end{document}, whereas new high-Mg olivine, with magnesium number up to 96 in natural samples and 99.9 in experiments, forms in the reaction zone. SiO2 produced in the reaction either dissolves in the fluid or, with a small amount of water, reacts with olivine to form orthopyroxene as follows: (Mg,Fe)2SiO4 + SiO2 = (Mg,Fe)2Si2O6. The released H2 may decrease the oxidation state of polyvalent elements present in the fluid (e.g., S⁴⁺, S⁶⁺). Traces of high-temperature water–olivine interaction appear as swarms of fluid-spinel inclusions and are ubiquitous in olivine from ultramafic arc xenoliths. The described process is similar to serpentinization but occurs at higher pressure and temperature conditions and yields different reaction products. The reducing capacity of olivine is relatively low; however, given the large volume of mantle (and crustal) peridotites, the overall effect may be significant.
 
Article
Boron geochemistry can track fluid–rock interaction during metamorphic evolution and provides important insights into mass transfer processes in subduction zones. This study presents boron concentration and isotopic data for white mica (phengite and paragonite) and tourmaline in an ultrahigh-pressure (UHP) metapelitic schist from the western Tianshan (Xinjiang Province, China). The pelitic schist experienced dehydration during heating related to the onset of exhumation, which is recorded by phengite and tourmaline formed during this stage. Boron isotope ratios in phengite decreased from –8.5 ‰ to –16.0 ‰ (relative to NIST SRM 951) with increasing temperature from 525 °C to 575 °C. This is recorded in a correlated decrease of 11B/10B ratios, B content and Si content of phengite. Thus, the B isotopes of released fluids during decompression–heating evolved from 0 ‰ to –6.9 ‰, consistent with a preferential loss of isotopically heavy B during dehydration. The formation of BSE-dark zones in tourmaline with relatively light δ11B values (–9 to –6‰) and high Mg# (0.65-0.68) could be related with fluids released during this stage. In a second stage, paragonite formed in a rehydration process during advanced exhumation. During interaction with external fluids, boron concentrations and isotopic values in paragonite increased: B concentration range from 72 to 232 μg/g, and δ11B increased from –15.6 to –2.5 ‰. Fluid-fractionation modeling demonstrates that the external fluid ([B] = 340 ±20 μg/g; δ11B = +8 ±2 ‰) may have been derived from high-δ11B serpentinites that occur in the study area (δ11B between –1 and +8 ‰). Respond to this hydration process, the tourmaline is featured by developing BSE-light zones with heavier δ11B values (-4 to -2‰) and lower Mg# (0.62-0.64). Boron geochemistry of white micas and tourmaline improve our understanding of mass transfer during metamorphic processes in subduction zones; it allows us to identify the influence of both closed-system recrystallization events and the effect and likely source of externally-derived fluids.
 
Article
Crust–mantle interaction induced by continental subduction occurs through metasomatic reaction of continental crust-derived materials with mantle wedge peridotite in continental subduction channels. Mantle-derived igneous rocks above continental subduction zones can provide insights into the nature of crust–mantle interaction and the types of metasomatism. The Laixi region is located on the southeastern margin of the North China Craton adjacent to the Sulu orogen. Here, we present K–Ar dating, whole-rock major and trace elements, Sr–Nd–Pb isotopes, and olivine and clinopyroxene major and trace elements of the Laixi basalt to investigate the nature of metasomatism at the crust–mantle interface. These basalts exhibit geochemical inheritance of island arc basalt (IAB)-like trace-element patterns, moderately enriched Sr–Nd isotopic compositions, and variable Pb isotopic compositions from upper-crustal materials of the subducted South China Block. The olivine and clinopyroxene contents of the Laixi basalts indicate a mixed source composed of ~ 51% pyroxenite and ~ 49% peridotite which is the result of carbonate metasomatism. On the basis of the typical differences in MgO, CaO and Al2O3 contents between the Laixi basaltic magmas and experimental melts, the metasomatic agents are identified as carbonated eclogite-derived melts. These features of the IAB-like basalts suggest metasomatic reaction of mantle-wedge peridotite with carbonated silicate melt derived from the subducted upper crust in the continental subduction channel, with olivine being consumed to generate ultramafic metasomatites enriched in pyroxene.
 
Article
Devolatilization of subducting lithologies liberates COH-fluids. These may become partially sequestered in peridotites in the slab and the overlying forearc mantle, affecting the cycling of volatiles and fluid mobile elements in subduction zones. Here we assess the magnitudes, timescales and mechanism of channelized injection of COH-fluids doped with $${\mathrm{Ca}}_{\mathrm{aq}}^{2+}$$ Ca aq 2 + , $${\mathrm{Sr}}_{\mathrm{aq}}^{2+}$$ Sr aq 2 + and $${\mathrm{Ba}}_{\mathrm{aq}}^{2+}$$ Ba aq 2 + into the dry forearc mantle by performing piston cylinder experiments between 1–2.5 GPa and 600–700 °C. Cylindrical cores of natural spinel-bearing harzburgites were used as starting materials. Based on mineral assemblage and composition three reaction zones are distinguishable from the rim towards the core of primary olivine and orthopyroxene grains. Zone 1 contains carbonates + quartz ± kyanite and zone 2 contains carbonates + talc ± chlorite. Olivine is further replaced in zone 3 by either antigorite + magnesite or magnesite + talc within or above antigorite stability, respectively. Orthopyroxene is replaced in zone 3 by talc + chlorite. Mineral assemblages and the compositions of secondary minerals depend on fluid composition and the replaced primary silicate. The extent of alteration depends on fluid CO 2 content and fluid/rock-ratio, and is further promoted by fluid permeable reaction zones and reaction driven cracking. Our results show that COH-fluid induced metasomatism of the forearc mantle is self-perpetuating and efficient at sequestering $${\mathrm{Ca}}_{\mathrm{aq}}^{2+}$$ Ca aq 2 + , $${\mathrm{Sr}}_{\mathrm{aq}}^{2+}$$ Sr aq 2 + , $${\mathrm{Ba}}_{\mathrm{aq}}^{2+}$$ Ba aq 2 + and CO 2aq into newly formed carbonates. This process is fast with 90% of the available C sequestered and nearly 50% of the initial minerals altered at 650 °C, 2 GPa within 55 h. The dissolution of primary silicates under high COH-fluid/rock-ratios, as in channelized fluid flow, enriches SiO 2aq in the fluid, while CO 2aq is sequestered into carbonates. In an open system, the remaining CO 2 -depleted, Si-enriched aqueous fluid may cause Si-metasomatism in the forearc further away from the injection of the COH-fluid into peridotite.
 
Article
Accessory and minor phases are frequently used to infer the composition and intensive parameters of magmas. In granites, the most common ferromagnesian phases are biotite and amphibole, which raises the question as to what compositional domains are present among these phases and how do those domains relate to granite composition. Here we present a characterization of biotite and amphibole compositions from S-, I-, and A-type granites. A database of biotite (1215 data points) and amphibole (525 data points) compositions from previously classified S-, I-, and A-type granites has been compiled. Three characteristics can be used to describe the variations in biotite composition including XAnniteBt\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${X}_{\mathrm{Annite}}^{\mathrm{Bt}}$$\end{document} (fraction of Fe²⁺ in the octahedral site), XFeVI∗Bt\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${X}_{{\mathrm{Fe}}^{\mathrm{VI}*}}^{\mathrm{Bt}}$$\end{document} (fraction of total iron in the octahedral site), and total aluminum (apfu). Three characteristics can also be used to describe variations in amphibole composition including, NK/CNK [i.e., (Na + K)/(Ca + Na + K)], Fe/(Fe + Mg), and total aluminum. Utilizing, for the first time, a random forest (machine learning) model the three characteristics for biotite and amphibole could discriminate the inferred source region of the granites with 82 and 96% accuracy, respectively. Biotite composition can also be used to broadly characterize fO2,\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${f}_{{\mathrm{O}}_{2}},$$\end{document}fH2OfHF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{f}_{{\mathrm{H}}_{2}\mathrm{O}}}{{f}_{\mathrm{HF}}}$$\end{document}, fH2OfHCl\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{f}_{{\mathrm{H}}_{2}\mathrm{O}}}{{f}_{\mathrm{HCl}}}$$\end{document}, and fHFfHCl\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{f}_{\mathrm{HF}}}{{f}_{\mathrm{HCl}}}$$\end{document} of each granite type at a given temperature and pressure. Thus, biotite and amphibole compositions can be used to further characterize granites; however, biotite and amphibole should not solely be used to infer the source of a granite.
 
Article
The New Caledonia ophiolite represents a rare example of proto-arc section originated during subduction infancy. The sequence is dominated by refractory harzburgites overlain by ultramafic (dunites and wehrlites) and mafic (gabbronorites) lithologies. In this contribution, we report the first occurrence of amphibole-bearing intrusives in the New Caledonia forearc sequence. This study deals with a petrological and geochemical investigation of a pyroxenite intrusion cut by mafic-ultramafic dikes. The intrusion consists of medium grain websterites, composed of orthopyroxene (30-75 vol.%), clinopyroxene (20-50 vol.%) and amphibole (2-30 vol.%), which occurs as interstitial or poikilitic phase. Whole rocks display moderate Mg# (71-82) and concave downward REE patterns, bearing depleted to flat LREE (LaN/NdN = 0.5-1) and flat HREE segments (DyN/LuN = 0.8-1.1). Bulk rocks mirror clinopyroxene at higher absolute values. Fluid mobile element (FME) enrichments, coupled to Zr-Hf depletion, are observed for both clinopyroxene and bulk rock. Mineral major element variations and textural relationships indicate that the investigated lithotypes derived from primitive hydrous magmas, which underwent extensive fractional crystallisation and post-cumulus processes. Geochemical modelling shows that the parental melts in equilibrium with the pyroxenites share remarkable similarities with the New Caledonia CE-boninite. However, they significantly differ from the equilibrium melts previously reported for the other intrusive rocks of the sequence. As a whole, our new results highlight a greater compositional variability for the liquids ascending into the Moho transition zone and lower crust. This may be also related to the involvement of a highly heterogeneous mantle source during subduction initiation.
 
Article
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.
 
Article
The magmatic history of the early Izu-Bonin-Mariana (IBM) arc forms a gap between a growing understanding of Eocene subduction and IBM arc initiation in the western Pacific, and Miocene- recent IBM arc processes. Fresh volcanic minerals in lapilli tuffs drilled at DSDP Site 296 on the northern Kyushu Palau Ridge (KPR) provide an opportunity to understand the early–late Oligocene magmatic evolution of the IBM arc leading up to arc rifting and opening of the Shikoku back-arc basin. In this study, we use major and trace element compositions of feldspar, amphibole and pyroxene, with melt inclusions, to infer KPR magma compositions, crystallization temperatures and pressures, and temporal sequence. A major finding of this approach is that inferred magma compositions span a wider range of trace element variation than that inferred from basaltic to dacitic glass shards within the tuffs. Elemental and thermobarometric data for clinopyroxene indicate the presence of mafic, incompatible element-depleted (Nb/Yb < 0.3 and La/SmN < 1.4) magmas that crystallized at shallow depths, and incompatible element-enriched (Nb/Yb = 8.1 and La/SmN = 6.5), mafic, amphibole-bearing arc magmas that either crystallized over a range of pressures or without reaching plagioclase saturation. We interpret the incompatible element-depleted magmas as decompression melts of a shallow BABB source mantle and the incompatible element-enriched type as mature, water-rich arc magmas. The occurrence of both types of magma in several lapilli tuff intervals in the drilled section suggests that arc extension and rifting was a gradual process leading to multiple events of decompression melting interspersed with the eruption of mature arc magma.
 
Article
The solubility of CO2 in mafic magmas is strongly dependent on magma composition, which ultimately affects magma storage conditions and eruptive behavior. Recent experimental work showed that previously published volatile solubility models for mafic magmas are not well calibrated at mid-crustal pressures (400–600 MPa). Using a simple thermodynamic model, here we construct a general CO2 solubility model for mafic magmas by establishing the compositional dependence of two key thermodynamic parameters. The model is calibrated using experimental data from 10 magma compositions that span a range of pressures as well as silica (44–53 wt.%) and total alkali (2–9 wt.%) contents. We also survey the experimental literature for relevant H2O solubility data to determine how to model H2O solubility for these magmas. We combine these separate CO2 and H2O solubility models into a single general model for mixed-fluid (H2O–CO2) solubility in mafic magmas called MafiCH. We test the MafiCH model using experiments from three compositions that fall both within and beyond the calibrated range, and find that the model accurately constrains the CO2 solubility of depolymerized magmas. Sensitivity tests identify that Na, Ca, and Al have the largest effect on CO2 solubility while Si and Mg do not play a strong role in CO2 solubility in mafic, depolymerized melts. Overall, saturation pressures calculated using the new model presented here are typically lower than those predicted by previous models. The model provides a new framework to interpret volcanic data from mafic magma compositions for which no experimental data is available.
 
Article
Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.
 
Article
Water degassing plays a major role in magma transport and eruption by increasing liquidus temperatures, bubble and crystal volume fractions, and strongly affecting the viscosity of bulk magma. High spatial resolution textural analysis detailing the dynamics of bubble and crystal growth is key to unravelling the swift changes in magma crystallinity and gas content that affect the conditions of magma flow, fragmentation, and eruption. Ex situ observation of samples from a previous experimental study of magma degassing reveals that vesicles are surrounded by chemically heterogeneous residual glass that may be produced by newly formed minerals that are not observable at the microscale. Here, we present new in situ high-temperature (500–1100 °C), time-elapsed (every ~ 20 min at 200–800 °C, ~ 10 min at 900–1000 °C, and ~ 5 min at 1100 °C) observations of degassing of synthesised, hydrous (4.2 wt.% H2O) dacite glasses using scanning transmission electron microscopy at 0.4 nm resolution. The experiments reproduce degassing of a silicic melt by high-temperature heated stage mounted in the analytical instrument. We monitor the dynamics of nucleation and growth of nanobubbles that experience coalescence and formation of microbubbles and trigger the nucleation and growth of nanolites of plagioclase, clinopyroxene, Fe-Ti oxides, and quartz, at the expense of the residual melt. The ability to image degassing and crystallisation at nanoscale reveals a sequence of complex physical and chemical changes of the residual melt and shows that the kinetics of crystallisation in silicic melts is modulated by the melt’s ability to exsolve fluids that help form mineral nuclei and nanolites. Finally, we highlight that the competition between gas retention and crystallisation is initiated at the nanoscale and may anticipate the role of microlites in controlling rates of magma ascent in a volcanic conduit and modulating the style of the consequent volcanic eruption.