Recovering ancient records of Earth's magnetic field is essential for determining the role of the magnetosphere in protecting early Earth from cosmic radiation and atmospheric escape. We present paleomagnetic field tests hinting that a record of Earth's 3.7-billion-year (Ga) old magnetic field may be preserved in the northeastern Isua Supracrustal Belt as a chemical remanent magnetization acquired during amphibolite-grade metamorphism in the banded iron formation. Multiple petrological and geochronological lines of evidence indicate that the northernmost part of Isua has not experienced metamorphic temperatures exceeding 380 degrees C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use "pseudo" baked contact tests (intrusions emplaced 3.26-3.5 Ga ago) and a fold test (folding 3.6 Ga ago) to demonstrate that some samples preserve a ca. 3.7 Ga record of the magnetic field. We recover a field strength of >15 mu T. This suggests that Earth's magnetic field may have been weak enough to enhance atmospheric escape during the Archean. Plain Language Summary Recovering ancient records of Earth's magnetic field is challenging because the magnetization in rocks is often reset by heating during tectonic burial over their long and complex geological histories. We show that rocks from the Isua Supracrustal Belt in West Greenland have experienced three thermal events throughout their geological history. The first event was the most significant, and heated the rocks up to 550 degrees C 3.7-billion-years-ago. The subsequent two events did not heat the rocks in the northernmost part of the area above 380 degrees C. We use multiple lines of evidence to test this claim, including paleomagnetic field tests, the metamorphic mineral assemblages across the area, and the temperatures at which radiometric ages of the observed mineral populations are reset. We use these lines of evidence to argue that an ancient, 3.7 billion year old record of Earth's magnetic field may be preserved in the banded iron formations in the northernmost part of the field area. The magnetization was acquired during mineral transformation associated with the first thermal event and therefore only a lower limit on the strength of the ancient magnetic field was constrained. However, we are able to conclude that the ancient magnetic field was likely comparable with the strength of Earth's magnetic field today.

We present paleomagnetic field tests that hint that a record of Earth’s 3.7-billion-year (Ga) old magnetic field may be preserved as a chemical remanent magnetization acquired during amphibolite-grade metamorphism in the banded iron formation from the northeastern Isua Supracrustal Belt. Multiple petrological and geochronological lines of evidence indicate that the northern most part of Isua has not experienced metamorphic temperatures exceeding 350◦C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use a ‘pseudo’ baked contact test to assess paleodirections in the banded iron formation that pre-date the intrusion of the 3.26-3.5 Ga Ameralik dyke swarm. We demonstrate that specimens that pass this test also go on to pass a fold test and may also pass a reversal test. We recover what appears to be the oldest known whole rock record of the geomagnetic field, and oldest known records of reversals suggesting that Earth’s magnetic field behaviour in the Eoarchean may have been similar to that observed today.

The ferrimagnetic mineral magnetite (Fe3O4) is abundant in banded iron formation (BIFs), and has the potential to provide U-Pb or Pb-Pb age information on these rocks because it incorporates small amounts of U during growth. Combined with age measurements, paleomagnetic studies of BIF magnetites may also yield insight into the history of Earth’s magnetic field and its relationship to early evolution of Earth’s interior and atmosphere. Reliable magnetite ages utilizing Pb isotopes require knowledge of Pb diffusion in the magnetite structure. For this reason, we undertook an experimental investigation of Pb diffusion in magnetite by diffusing Pb2+ ions into pre-polished slabs of natural magnetite oriented parallel to {001} or {111}. A mixture of PbSO4 and Fe2O3 was used as a surface powder source to supply Pb2+ diffusant at the sample surface and at the same time buffer the oxygen fugacity of the system at magnetite-hematite (MH) - a typical fO2 for banded iron formations (BIFs) due to the common presence of both iron oxides (and where Pb2+ is stable relative to other Pb valence states). Diffusion experiments spanned temperatures of 500−675°C and durations of 75 to 2035 h. Following each experiment, in-diffused Pb was depth-profiled using Rutherford backscattering spectroscopy (RBS) and Pb diffusivities were calculated from the profiles using an infinite half-space diffusion model. The following diffusion law for Pb2+ in magnetite is based upon 12 independent diffusivity measurements: DPb (m2·s-1) = (9×10-17 m2·s-1) exp(-98,000 J·mol-1)/RT) where the uncertainties in the pre-exponential constant and activation energy are ±6% and ±15%, respectively. Pb diffusion in magnetite over the temperature range of our study is orders of magnitude slower than projected for other divalent cations based on down-temperature extrapolation of previously measured diffusion laws (e.g., for Mn2+, Fe2+, Co2+, and Ni2+). This finding is encouraging in terms of the potential suitability of magnetite for U-Pb age determinations of BIFs and other magnetite-bearing rocks. Indeed, classical Dodson closure temperatures well above 500°C are not unrealistic in cases where magnetite crystals having large diffusion domains (e.g., >100µm in radius) are cooled relatively rapidly (e.g., at 100°C/MYr). This is of particular significance for paleomagnetic studies, since the Curie temperature of magnetite is 580°C and therefore the age of magnetization in magnetite-bearing rocks may be directly dated. However, slow cooling of magnetites having small diffusion domains can lead to Pb loss at temperatures of 200°C or lower. Pb mobilization is evaluated for various time-temperature scenarios that involve both heating and cooling as well as “closed-loop” time-temperature paths. We conclude that U-Pb or Pb-Pb age determinations of BIF magnetites are potentially reliable, but isotopic results should be assessed in concert with knowledge of the thermal history of the host rock and the effective grain size ofthe magnetites.

Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (f(O2); iron-wustite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 degrees C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing f(O2). Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: [GRAPHICS] The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: [GRAPHICS] The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by [GRAPHICS] The Eu valence-state partitioning techniques based on (root SmxGd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or root SmxGd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or root(SmxGd) techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 +/- 0.25 (as determined by lattice strain) to DLu3+ = 0.69 +/- 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor f(O2). These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts.

The origin of life on earth requires the synthesis of protobiopolymers in realistic geologic environments along strictly abiotic pathways that rely on inorganic phases (such as minerals) instead of cellular machinery to promote condensation. One such class of polymer central to biochemistry is the polynucleotides, and oligomerization of activated ribonucleotides has been widely studied. Nonetheless, the range of laboratory conditions tested to date is limited and the impact of realistic early Earth conditions on condensation reactions remains unexplored. Here, we investigate the potential for a variety of minerals to enhance oligomerization using ribonucleotide monomers as one example to model condensation under plausible planetary conditions. The results show that several minerals differing in both structure and composition enhance oligomerization. Sulfide minerals yielded oligomers of comparable lengths to those formed in the presence of clays, with galena being the most effective, yielding oligonucleotides up to six bases long. Montmorillonite continues to excel beyond other clays. Chemical pretreatment of the clay was not required, though maximum oligomer lengths decreased from ~11 to 6 bases. These results demonstrate the diversity of mineral phases that can impact condensation reactions and highlight the need for greater consideration of environmental context when assessing prebiotic synthesis and the origin of life.

Nitrogen is a primary constituent of Earth’s atmosphere and an essential component of many biological processes. Recent developments in understanding N incorporation into silicate minerals suggest that the solid earth and lithosphere could provide a substantial reservoir for the planetary N budget. Furthermore, determining the concentration and distribution of low abundances of N in silicates presents a significant challenge in microanalysis. We tested capabilities for high spatial resolution quantification of low levels of N using EPMA with the goal of developing standards to be used in coupled N and noble gas analyses of silicate minerals. We investigated the concentration and distribution of N in a suite of ammonium-bearing silicate minerals that reflect the dominant N-bearing phases of the lithosphere: tobelite, buddingtonite, ammonioleucite, tsaregorodtsevite, and hyalophane. These minerals form during NH4-rich alteration of K-bearing silicate minerals, and the bonding environment of N in each mineral is likely to differ based on crystal structure. Using an optimized analytical routine for N quantification enables sensitivity approaching 0.02 weight% N and high-resolution quantitative mapping of sub-micron variations in N contents.

Earth is the only known inhabited world in our solar system. Criteria essential for planetary habitability include surface liquid water, a stable atmosphere, and a magnetic field. While the rock record suggests Earth has fulfilled these criteria for at least 4 billion years (Ga), both its environment and life have evolved over time. The Great Oxygenation Event (GOE), which occurred ~2.5 Ga ago, drastically altered the chemistry of the oceans and atmosphere. Decoding environmental and magnetic signals recorded in rocks prior to the GOE is essential for understanding the conditions under which life first emerged.An ideal target for investigating surface conditions prior to the GOE are banded iron formations (BIFs), which precipitated directly from ancient oceans. However, BIFs have been significantly altered since their formation, and it is unclear whether a record of their depositional environment remains. The present day mineralogy is dominated by magnetite, but it remains to be established how this relates to the precipitates deposited on the seafloor. Additionally, in spite of magnetite's ideal magnetic properties, BIFs are avoided for paleomagnetic analysis because the timing of magnetization is uncertain. It is vital to constrain the magnetic field record leading up to the GOE because it may have influenced atmospheric hydrogen loss, contributing to rapid surface oxidation.We present paleomagnetic field tests from the Isua Supracrustal Belt that suggest a record of Earth’s 3.7-billion-year (Ga) old (Eoarchean) magnetic field is preserved in the banded iron formation in the northernmost northeast region of the belt. Our results are supported by radiometric Pb-Pb dating of magnetite from the same banded iron formation. We show that the Pb-magnetite system has a closure temperature below 400 °C for the magnetite grain size range observed in the banded iron formation, suggesting the rocks have not been significantly heated since magnetization was acquired. This temperature range is well below the Curie temperature of magnetite (580 °C), suggesting Eoarchean magnetization has not been thermally overprinted by subsequent metamorphism. Passed paleomagnetic field tests suggest the rocks have also avoided chemical overprints. We recover an ancient magnetic field strength, supporting previous studies that argue Earth’s magnetic field has been active throughout most of its history although variations in its strength remain poorly constrained.

The solubility of titanite (CaTiSiO5) in Si-rich melts was measured experimentally through growth experiments at 800–1000 °C, 0.5–1.0 GPa, log fO2 ~ CCO–0.8, t = 72–168 h, and H2O = 0 to 4 wt.%, and in dissolution experiments at 925–1300 °C, 0.8 GPa, t = 18–118 h, and H2O = 1–10 wt.% in a piston-cylinder apparatus. Run product glasses in growth experiments were homogeneous, and iron loss suppressed ilmenite crystallization. Saturation concentrations in dissolution experiments were estimated by fitting measured diffusion profiles. Titanite solubility increases with increasing temperature and melt composition parameter $$M = {\text{ molar }}\left( {{\text{Na }} + {\text{ K }} + {\text{ 2Ca}}} \right)/\left( {{\text{Al }} \times {\text{ Si}}} \right)$$ . Multiple linear regression of glass composition data from growth and dissolution experiments (n = 29) plus 39 experiments from the LEPR database (Hirschmann et al. 2008) yielded the titanite solubility equation (adj. r2 = 0.95): $$\left( {TiO_{2} } \right)^{melt} \left( {wt.\% } \right) = 0.978 \times M + 0.0048 \times T\left( K \right){-}5.90$$ . This model correctly predicted undersaturation in 95% of 2344 experiments from the LEPR database that did not have titanite or rutile. Application to natural rocks yields saturation temperatures that are similar to independent temperature estimates. This equation should be useful for constraining the temperatures of titanite-saturated melts, for determining whether titanite saturation in magmatic source regions is likely, and for determining when titanite can crystallize and begin to exert an influence on melt geochemistry.

New experiments to study titanium solubility in quartz were conducted at conditions not previously explored to extend and improve existing Ti-in-quartz solubility models for thermobarometric applications. Starting materials for experiments included silica glass, anatase, synthetic and natural rutile, Ti-enriched silica gel, Ti-enriched melts, zirconia, and HF and H2O fluids. Additional experimental data enabled us to characterize Ti-in-quartz solubility across much of the α- and β-quartz stability fields from 2 to 30 kbar and 550 to 1050 °C. Mutual occurrences of mineral inclusions in one another and Raman spectroscopy of mineral phases confirmed co-crystallization of quartz, rutile, and zircon. Electron microprobe measurements and cathodoluminescence images show that Ti concentrations in quartz crystals from all experiments are relatively uniform, and Ti concentrations of quartz crystals grown at the same experimental conditions using several Ti–rich starting materials and several different growth media are the same within experimental and analytical uncertainties. There are no significant differences in Ti concentrations of quartz across the α–β quartz transition. The Ti concentration in quartz crystals, $${X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}$$ , systematically increases with temperature, but the quantity $$RT \mathrm{ln}{X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}$$ is a constant at fixed pressure. The Ti concentration in quartz decreases non-linearly with pressure. To account for the observed P–T dependent changes to Ti in quartz, we developed the Ti-in-quartz solubility model: $$RT \mathrm{ln}{X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}=- 55.287-[P\left(\mathrm{kbar}\right)\bullet (-2.625+0.0403 P\left(\mathrm{kbar}\right))]+RT \mathrm{ln}{a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ where R is the gas constant 0.0083145 kJ/K, T is temperature in Kelvin, P is the pressure in kbar, $${X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}$$ is the mole fraction of TiO2 in quartz, and $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ is the activity of TiO2 in the growth media (e.g., fluid, melt) referenced to rutile at standard state conditions of 1 bar and 25 °C. Experiments that co-crystallized quartz, rutile, and zircon permitted us to cross-check thermobarometric results from our Ti-in-quartz solubility models against the widely accepted Zr-in-rutile solubility models. We further tested our Ti-in-quartz solubility models using experiments that co-crystallized quartz, wollastonite, and titanite to fix $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ < 1. Concentrations of Ti in quartz crystallized from the sub-unity $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ experiments in the α- and β-quartz fields predict activities that match those calculated using the mineral reaction equilibrium and available thermodynamic data. Demonstrated agreement between calculated and measured experimental P–T conditions using the Zr-in-rutile and Ti-in-quartz solubility models and the consistent reduction of Ti concentrations in systems with $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ < 1 provide evidence that our experimental results accurately describe the equilibrium solubility of Ti in quartz.

The solubility of aluminum in rutile has been studied in the systems Al2O3–TiO2–H2O and Al2O3–SiO2–TiO2–H2O at 700–1200 °C and 0.075–3.3 GPa. Electron probe microanalysis (EPMA) measurements of rutile crystals grown in equilibrium with corundum show that the concentration of Al increases with increasing temperature, pressure, and oxygen fugacity. Solubility is enhanced by the addition of Nb and reduced by the addition of trivalent cations. These results are consistent with the substitution of Al3+ for Ti4+ on normal cation sites and the simultaneous incorporation of positively charged hydrous defects for charge compensation. Parameters for a thermodynamic model of Al substitution in rutile are calculated and the merits of using the model as a geochemical tool are discussed.

Diffusion of N has been measured in natural calcite. For diffusion normal to the {10 (I) over bar4} cleavage surface, an activation energy for diffusion of 222 +/- 17 kJ mol(-1) and pre-exponential factor of 1.92 x 10(-9) m(2)sec(-1) are obtained. Diffusion parallel to c is slightly faster than diffusion normal to {10 (I) over bar4}. These data indicate that N contained in inclusions in calcite will diffuse through the calcite lattice at Earth's upper crustal and surface conditions slowly enough so that calcite may faithfully retain a passive signature of its formation environment for billions of years, providing much needed insight into the cycling of nitrogen between atmosphere, crust, and mantle.

Elemental abundance and isotopic fractionation profiles across zoned minerals from a martian meteorite (Shergotty) and from a lunar olivine-normative mare basalt (Apollo 15555) were used to place constraints on the thermal evolution of their host rocks. The isotopic measurements were used to determine the extent to which diffusion was responsible for, or modified, the zoning. The key concept is that mineral zoning that is the result of diffusion, or that was significantly affected by diffusion, will have an associated diagnostic isotopic fractionation that can quantify the extent of mass transfer by diffusion. Once the extent of diffusion was determined, the mineral zoning was used to constrain the thermal history. An isotopic and chemical profile measured across a large zoned pigeonite grain from Shergotty showed no significant isotopic fractionation of either magnesium or lithium, which is evidence that the chemical zoning was dominantly the result of crystallization from an evolving melt and that the crystallization must have taken place at a sufficiently fast rate that there was not time for any significant mass transfer by diffusion. Model calculations for the evolution of the fast-diffusing lithium showed that this would have required a cooling at a rate of about similar to 150 degrees C/h or more. Measurable isotopic fractionation across a zoned olivine grain from lunar mare basalt 15555 indicated that the chemical zoning was mainly due to crystallization that was modified by a small but quantifiable amount of diffusion. The results of a diffusion calculation that was able to account for the amplitude and spatial scale of the isotopic fractionation across the olivine grain yielded an estimate of 0.2 degrees C/h for the cooling rate of 15555. The results of an earlier study of zoned augite and olivine grains from martian nakhlite meteorite NWA 817 were reviewed for comparison with the results from Shergotty. The isotopic fractionations near the edges of grains from NWA 817 showed that, in contrast to Shergotty, the lithium zoning in augite and of magnesium in olivine was due entirely to diffusion. The isotopic fractionation data across zoned minerals from the martian meteorites and from the lunar basalt were key for documenting and quantifying the extent of mass transfer by diffusion, which was a crucial step for validating the use of diffusion modeling to estimate their cooling rates. (C) 2020 Elsevier Ltd. All rights reserved.

We investigate the retention of volatile species and we analyze the volatile species released on low temperature crushing of a microcrystalline quartz (< 5 mu m)-dominated chert from the Petrified Forest National Park, known to be ca. 215 million years old, in order to learn more about the atmospheric composition during the Late Triassic. Diffusion laws for nitrogen and carbon in the Petrified Forest chert were determined using both in-diffusion of C (believed to diffuse as CO2) and N (believed to diffuse as N-2) from a C-O-H-N vapor and ion implantation followed by heating. Arrhenius relationships in the form of D = D(0)exp(-E-a/RT) were established: the log (D-0, m(2)/s) values are -14.1 and -17.2 and the E-a (kJ/mol) values are 68.9 and 37.9 for nitrogen and carbon, respectively. Although this chert sample has not experienced temperatures above 100 degrees C, the diffusion results indicate that it is similar to 70% retentive to carbon and similar to 92% retentive to nitrogen at 100 degrees C for over 1 billion years, given a diffusion domain size of a sphere with a 1 cm radius. This suggests that at ambient conditions this material is a very good container for volatiles over geologically long timescales. In addition, the major atmospheric gases (N-2, O-2, Ar, CO2) are released when this sample is crushed incrementally into a quadrupole mass spectrometer, indicating that microcrystalline quartz dominated phases can provide a record of past atmospheric compositions. The combination of the retentiveness of atmospheric gases as well as the release of inclusion gas upon crushing makes this a reliable proxy for Earth's volatile history extending at least to the Late Triassic and potentially even earlier Earth history.

Slip on the active Mai'iu low‐angle normal fault in Papua New Guinea that dips 15–24° at the surface has exhumed in its footwall a single, continuous fault surface across a >25‐km‐wide dome. Derived from a metabasaltic protolith, the fault zone consists of a <3‐m‐thick zone of gouges and cataclasites that overprint a structurally underlying carapace of extensional mylonites. Detailed microstructural and geochemical data, combined with chlorite‐based geothermometry, reveal changing deformation processes and conditions in the Mai'iu fault rocks as they were exhumed. The microstructure of nonplastically deformed actinolite grains inherited from the fine‐grained (6–35 µm) metabasaltic protolith indicates that shearing at depth was controlled by diffusion creep accompanied by grain‐boundary sliding of these grains together with chlorite neo‐crystallization at T > 275°C–370°C. In a foliated cataclasite unit at shallower crustal levels (T ≈ 150°C–275°C), metasomatic reactions accompanied fluid‐assisted mass transfer processes that accommodated aseismic, distributed shearing; pseudotachylites and ultracataclasites in the same unit indicate that such creep was punctuated by episodes of seismic slip—after which creep resumed. At the shallowest levels (T < 150°C), gouges contain abundant saponite, a frictionally weak mineral that promotes creep on the shallowest dipping (≤24°), most poorly oriented part of the Mai'iu fault. Our field, microstructural and geochemical data of freshly exhumed fault rocks support geodetic, seismological, and geomorphic evidence for mixed seismic‐to‐aseismic slip on this active low‐angle normal fault.

Chemical diffusion of Ti has been measured in natural K-feldspar and plagioclase. The sources of diffusant used were TiO2 powders or pre-annealed mixtures of TiO2 and Al2O3. Experiments were run in crimped Pt capsules in air or in sealed silica glass capsules with solid buffers (to buffer at NNO). Rutherford backscattering spectrometry (RBS) was used to measure Ti diffusion profiles. From these measurements, the following Arrhenius relations are obtained for diffusion normal to (001): For oligoclase, over the temperature range 750-1050 degrees C: D-Olig = 6.67 x 10(-12) exp(-207 +/- 31 kJ/mol/RT) m(2)s(-1) For labradorite, over the temperature range 900-1150 degrees C: D-Lab = of 4.37 x 10(-14) exp(-181 +/- 57 kJ/mol/RT) m(2)s(-1) For K-feldspar, over the temperature range 800-1000 degrees C: D-Ksp = 3.01 x 10(-6) exp(-342 +/- 47 kJ/mol/RT) m(2)s(-1). Diffusivities for experiments buffered at NNO are similar to those run in air, and the presence of hydrous species appears to have little effect on Ti diffusion. Ti diffusion also shows little evidence of anisotropy. In plagioclase, there appears to be a dependence of Ti diffusion on An content of the feldspar, with Ti diffusing more slowly in more calcic plagioclase. This trend is similar to that observed for other cations in plagioclase, including Sr, Pb, Ba, REE, Si, and Mg. In the case of Ti, an increase of 30% in An content would result in an approximate decrease in diffusivity of an order of magnitude. These data indicate that feldspar should be moderately retentive of Ti chemical signatures, depending on feldspar composition. Ti will be more resistant to diffusional alteration than Sr. For example, Ti zoning on a 50 mu m scale in oligoclase would be preserved at 600 degrees C for durations of similar to 1 million years, with Sr zoning preserved only for similar to 70 000 yr at this temperature. These new data for a trace impurity that is relatively slow-diffusing and ubiquitous in feldspars (Hoff and Watson 2018) have the potential to extend the scope and applicability of t-T models for crustal rocks based on measurements of trace elements in feldspars.

Rationale High‐precision determination of silicon isotopes can be achieved by in situ multi‐collector secondary ion mass spectrometry (MS‐SIMS). The accuracy of the analyses is, however, sensitive to ion yields and instrumental mass fractionations (IMFs) induced by the analytical procedure. These effects vary from one instrument to another, with the analytical settings, and with the composition and nature of the sample. Because ion yields and IMF effects are not predictable and rely on empirical calibrations, high‐accuracy analyses require suitable sets of standards. Methods Here, we document calibrations of ion yields and matrix effects in a set of 23 olivine standards and 3 low‐Ca pyroxene for silicon isotopic measurements in both polarities using Cameca IMS 1270 E7 and IMS 1280 HR2 ion probes set with the cesium (Cs) or radiofrequency (RF) source. Results Silicon ion yields show (i) strong variations with the chemical composition, and (ii) an opposite behavior between the secondary positive and negative polarities. The magnitude of IMF along the fayalite‐forsterite (olivine) series shows a complex behavior, increasing overall by ≈7‰ (secondary positive) and ≈15‰ (secondary negative) with increasing olivine Mg#. A drastic change in olivine IMF occurs at Mg# ≈ 70 in both polarities. The magnitude of IMF for low‐Ca pyroxene from Mg# = 70–100 is almost constant in both polarities, i.e. ≈0.1‰ in secondary positive and ≈0.15‰ in secondary negative. The analytical uncertainties on individual analyses were ± 0.05–0.15‰ (2 S.E.) with both sources, and the external errors for each standard material were ≈ ±0.05–0.5‰ (2 S.E.) with the Cs source and ≈ ±0.03–0.15‰ (2 S.E.) with the RF source. Conclusions The IMF effect of Si isotopes in silicates shows complex behaviors that vary with the chemistry and the settings of the instrument. We developed a suitable set of standards in order to perform high‐accuracy in situ measurements of Si isotopes in olivine and low‐Ca pyroxene characterized by varying chemical compositions by MC‐SIMS.

Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that coesite crystals are relatively homogenous. The Ti concentrations of coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006. https://doi.org/10.1007/s00410-006-0132-1 ; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010. https://doi.org/10.1007/s00410-010-0505-3 ). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in coesite by using a least-squares method to fit Ti concentrations in coesite to the simple expression: $$RT\ln X_{{{\text{TiO}}_{2} }}^{\text{coesite}} = - 55.068 + 0.00195 \times T\;({\text{K}}) - 1.234 \times P\;({\text{kbar}}) + RT\ln a_{{{\text{TiO}}_{2} }}^{\text{rutile}} ,$$ where R is the gas constant 8.3145 × 10−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, $$X_{{{\text{TiO}}_{2} }}^{\text{coesite}}$$ is the mole fraction of TiO2 in coesite, and $$a_{{{\text{TiO}}_{2} }}^{\text{rutile}}$$ is the activity of TiO2 in the system referenced to rutile. Ti-in-coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.