Volume 103, Issue 7

Analyses by the CheMin X-ray diffraction instrument on Mars Science Laboratory show that gypsum, bassanite, and anhydrite are common minerals at Gale crater. Warm conditions (~6 to 30 °C) within CheMin drive gypsum dehydration to bassanite; measured surface temperatures and modeled temperature depth profiles indicate that near-equatorial warm-season surface heating can also cause gypsum dehydration to bassanite. By accounting for instrumental dehydration effects we are able to quantify the in situ abundances of Ca-sulfate phases in sedimentary rocks and in eolian sands at Gale crater. All three Ca-sulfate minerals occur together in some sedimentary rocks and their abundances and associations vary stratigraphically. Several Ca-sulfate diagenetic events are indicated. Salinity-driven anhydrite precipitation at temperatures below ~50 °C may be supported by co-occurrence of more soluble salts. An alternative pathway to anhydrite via dehydration might be possible, but if so would likely be limited to warmer near-equatorial dark eolian sands that presently contain only anhydrite. The polyphase Ca-sulfate associations at Gale crater reflect limited opportunities for equilibration, and they presage mixed salt associations anticipated in higher strata that are more sulfate-rich and may mark local or global environmental change. Mineral transformations within CheMin also provide a better understanding of changes that might occur in samples returned from Mars.

The iron (oxyhydr)oxides hematite (α-Fe 2 O 3 ) and goethite (α-FeOOH) are natural and reactive minerals common in soils and sediments, and their adsorption of Fe(II) produces reactive surface sites that facilitate reduction of oxidized environmental pollutants. Single-exposure experiments with 4-chloronitrobenzene showed that hematite is more reactive than goethite, when normalized by surface area loading. Interestingly, the product of Fe(II) oxidation is a mixture of goethite and hematite, and the goethite to hematite ratio depends on the distribution of Fe(II) activated surface sites, which is a function of aqueous Fe(II) concentration, surface area loading, and pH. More goethite is produced under conditions of higher Fe(II), lower surface area loading, and higher pH. Recurrent-exposure experiments showed a substantial decrease in reaction rate after one to three exposures, a trend suggestive of reaction contributions from the increasing goethite surface area over time. Using known atomic surface geometry for goethite and hematite, the hematite {012} facet is proposed as the site of primary mineral growth with goethite {021} at the interface between the two minerals. These results have implications in contaminant fate modeling, where the mineral phases present in the environment, the minerals likely to form, and the surrounding aqueous conditions all have an impact on contaminant reaction rate.

We have measured the effect of alkalis on S concentration at sulfide saturation (SCSS) in an underexplored compositional space of natural hydrous arc melts (basaltic andesite to shoshonite) at 1270 °C and 1 GPa. At an oxygen fugacity approximately 2.5 log units below the fayalite-magnetite-quartz (FMQ) buffer, SCSS increases with Na 2 O (562 ppm S/wt% Na 2 O), K 2 O (98 ppm S/wt% K 2 O), and total alkalis (88 ppm S/wt% Na 2 O+K 2 O) over the compositional range we have studied (1.6–3.1 wt% Na 2 O; 0–6.5 wt% K 2 O; 1.9–6.3 wt% FeO tot ). Experiments with ~1.3 wt% H 2 O show approximately half the increase in SCSS with alkalinity compared to those with ~3.0 wt% H 2 O. Our results show a possible limit to the increase in SCSS solely by increasing alkali concentration at ~7.5 wt% total alkali concentration. Using our results and published data, we retrained earlier SCSS models to provide a better fit to test data. We also developed a new empirical model using theoretical optical basicity as a compositional parameter that predicts SCSS in the overall data set with slightly better accuracy compared to previous models: In(SCSSppm)=16.34− 5784T− 339.4PT+10.85ln(Λ )+3.750XFeO+6.703XH2O$$\begin{array}{} \displaystyle \rm{In(SCSS_{ppm}) = 16.34 - \frac{5784}{{\it T}}-339.4 \frac{{\it P}}{{\it T}} + 10.85 ln (\Lambda)+ 3.750 {\it X}_{FeO} + 6.703 {\it X}_{H_{2}O}} \end{array}$$ with temperature ( T ) in Kelvin, pressure ( P ) in GPa, the optical basicity (Λ) and mole fractions ( X ) of FeO (calculated from Kress and Carmichael 1991), and H 2 O in the melt. The discrepancies between observed and predicted SCSS for our experiments of varying alkalinity reflects the heavy bias toward anhydrous, alkali-poor basalt compositions in the underlying data sets on which most models are developed.

Fibrous erionite is classified by the International Agency for Research on Cancer (IARC) as carcinogenic substance to humans (Group 1). In the areas where it is present in the bedrock, it may cause environmental exposure, and both professional and environmental exposures are possible when the bedrock is used for industrial applications (e.g., building materials). For health and environment protection, prevention is a priority action. In this framework, the recent guidelines of the Consensus Report of the Weinman International Conference on Mesothelioma suggest identifying locations where potentially hazardous mineral fibers (like erionite) are found in the environment, to prevent environmental exposure. The present study will show that one such potentially hazardous mineral fiber might be fibrous ferrierite. Here, the mineralogy, chemical-physical properties, and surface activity of a hydrothermal fibrous ferrierite from Monte Lake British Columbia (Canada) and a diagenetic fibrous ferrierite from Lovelock, Nevada (U.S.A.), were investigated using a combination of “state of the art” experimental methods including optical microscopy, electron microscopy and microprobe analysis, laser ablation-inductively coupled plasma-mass spectrometry (for the trace elements), vibrational spectroscopy, electron paramagnetic resonance, and synchrotron powder diffraction. The chemical-physical properties of these fibrous ferrierites (morphometric parameters, specific surface area, chemical composition with special attention to metals, mainly iron) that prompted adverse effects in vivo were compared to those of the positive carcinogenic standard fibrous erionite-Na from Jersey, Nevada (U.S.A.). The results of our study have demonstrated that, although there are differences in the crystal chemistry and genetic environment, ferrierite samples exhibit outstanding similarities with fibrous erionite samples: both fibrous erionite and fibrous ferrierite may occur in large amounts as microcrystalline fibrous–asbestiform phases in diagenetic rocks with fibers of breathable sizes. For both zeolites, iron is not structural but is associated with impurities lying at the surface of the fibers. Moreover, data useful to understand the surface activity of these fibrous ferrierites were collected. As far as hydrothermal sample is concerned, the EPR data indicate the presence of hydrophilic (SiO-, AlO-, SiOH) and hydrophobic (Si-O-Si) interacting surface groups able to bind the charged CAT1 probes at close sites and attract the probes in the water pools formed into the fiber aggregates. A high percentage of CAT1 probes weakly interacting with the surface due to competition with metal ions were observed for surface of the diagenetic sample. CAT8 probes were less adsorbed by its surface if compared to the diagenetic sample but the more charged surface provided a stronger binding strength for the diagenetic sample compared to the hydrothermal one. In summary, the results of this study indicate that fibrous ferrierite may represent a potential health hazard and, applying the precautionary principle, it should undergo a procedure of toxicity testing.

Peridotites dredged from mid-ocean ridges and glassy mid-ocean ridge basalts (MORB) transmit information about the oxygen fugacity f o 2 ) of Earth’s convecting upper mantle to the surface. Equilibrium assemblages of olivine+orthopyroxene+spinel in abyssal peridotites and Fe 3+ /ΣFe ratios in MORB glasses measured by X-ray absorption near-edge structure (XANES) provide independent estimates of MORB source region f o 2 , with the former recording f o 2 approximately 0.8 log units lower than the latter relative to the quartz-fayalite-magnetite (QFM) buffer. To test cross-compatibility of these oxybarometers and examine the compositional effects of changing f o 2 on a peridotite plus melt system over a range of Earth-relevant f o 2 , we performed a series of experiments at 0.1 MPa and f o 2 controlled by CO-CO 2 gas mixes between QFM-1.87 and QFM+2.23 in a system containing basaltic andesite melt saturated in olivine, orthopyroxene, and spinel Oxygen fugacities recorded by each method are in agreement with each other and with the f o 2 measured in the furnace. Measurements of f o 2 from the two oxybarometers agree to within 1σ in all experiments. These results demonstrate that the two methods are directly comparable and differences between f o 2 measured in abyssal peridotites and MORB result from geographic sampling bias, petrological processes that change f o 2 in these samples after separation of melts and residues, or abyssal peridotites may not be residues of MORB melting. As f o 2 increases, spinel Fe 3+ concentrations increase only at the expense of Cr from QFM-1.87 to QFM-0.11. Above QFM, Al is also diluted in spinel as the cation proportion of Fe 3+ increases. None of the three spinel models tested, MELTS (Ghiorso and Sack 1995), SPINMELT (Ariskin and Nikolaev 1996), and MELT_CHROMITE (Poustovetov and Roeder 2001), describe these compositional effects, and we demonstrate that MELTS predicts residues that are too oxidized by >1 log unit to have equilibrated with the coexisting liquid phase. Spinels generated in this study can be used to improve future thermodynamic models needed to predict compositional changes in spinels caused by partial melting of peridotites in the mantle or by metamorphic reactions as peridotites cool in the lithosphere. In our experimental series, where the ratio of Fe 2 O 3 /FeO in the melt varies while other melt compositional parameters remain nearly constant, experimental melt fraction remains constant, and Fe 3+ becomes increasingly compatible in spinel as f o 2 increases. Instead of promoting melting, increasing the bulk Fe 3+ /ΣFe ratio in peridotite drives reactions analogous to the fayalite-ferrosilite-magnetite reaction. This may partly explain the absence of correlation between Na 2 O and Fe 2 O 3 in fractionation-corrected MORB.

Performing well-controlled metal-silicate partitioning experiments at conditions directly simulating those of a deep magma ocean is difficult. It is therefore common to perform experiments at lower pressures and temperatures, which are used to determine the effects of salient variables. Often, these effects are determined by multiple linear regression of a data set covering a large range of P - T -composition space. In particular, these data sets often contain the results of experiments performed both with and without sulfur in the system. Data are often regressed, however, using a relationship based only upon the formation of oxide species in the silicate melt. Several studies have suggested that when sulfur is present in the system, siderophile trace metals may also dissolve into silicate melt as S-bearing species. We have derived a relationship for regressing experimental metal-silicate partitioning data that considers the formation of both oxide and sulfide species in the silicate melt. Using model data sets, we have assessed the ability of this relationship, and the more typical single-species relationship, to accurately parameterize data in which the formation of S-bearing species is important. We have also applied this new relationship to experimental results on the metal-silicate partitioning of gold and find it is able to reconcile the conflicting pressure dependencies of lnDAumet/sil$\begin{array}{} D{\rm _{Au}^{met/sil}} \end{array} $ found in previous studies.

We employ large mineralogical data resources to investigate the diversity and spatial distribution of vanadium minerals. Data for 219 approved species (http://http://rruff.info/ima, as of April 15, 2016), representing 5437 mineral species-locality pairs (http://http://mindat.org and other sources, as of April 15, 2016), facilitate statistical evaluation and network analysis of these vanadium minerals. V minerals form a sparse, moderately centralized and transitive network, and they cluster into at least seven groups, each of which indicates distinct paragenetic process. In addition, we construct the V mineral-locality bipartite network to reveal mineral diversity at each locality. It shows that only a few V minerals occur at more than three localities, while most minerals occur at one or two localities, conforming to a Large Number of Rare Events (LNRE) distribution. We apply the LNRE model to predict that at least 307 ± 30 (1σ) vanadium minerals exist in Earth’s crust today, indicating that at least 88 species have yet to be discovered—a minimum estimate because it assumes that new minerals will be found only using the same methods as in the past. Numerous additional vanadium minerals likely await discovery using micro-analytical methods. By applying LNRE models to subsets of V minerals, we speculate that most new vanadium minerals are to be discovered in sedimentary or hydrothermal non-U-V ore deposits other than igneous or metamorphic rocks/ore deposits.

Ultramafic rocks exposed at the Earth’s surface generally record multiple stages of evolution that may include melt extraction, serpentinization, carbonatization, and metamorphism. When quantitative thermometry based on mineral chemistry is applied to such rocks, it is often unclear what stage of their evolution is being observed. Here, in peridotites with extensive replacement of silicate minerals by carbonates (listwaenites), we present a case study that addresses the timing of carbonate formation relative to closure of exchange reactions among relict primary minerals. Massive and schistose serpentinized peridotites of Neoproterozoic age outcrop at Gabal Sirsir, South Eastern Desert, Egypt (northwestern corner of the Arabian-Nubian Shield or ANS). Petrography, bulk composition, and mineral chemistry are all consistent with a strongly depleted mantle harzburgite protolith for the serpentinites. Bulk compositions are low in Al 2 O 3 and CaO and high in Mg# [molar Mg/(Mg+Fe) = 0.89–0.93]. Relict spinel has high Cr# [molar Cr/(Cr+Al)] and low Ti, while relict olivine has high Mg# and NiO contents. Based on compositions of coexisting relict olivine and chromian spinel, the protolith experienced 19 to 21% partial melt extraction. Such high degrees of partial melting indicate a supra-subduction zone environment, possibly a forearc setting. Along thrust faults and shear zones, serpentinites are highly altered to form talc-carbonate rocks and weathering-resistant listwaenites that can be distinguished petrographically into Types I and II. The listwaenitization process took place through two metasomatic stages, associated first with formation of the oceanic crustal section and near-ridge processes (~750–700 Ma) and subsequently during obduction associated with the collision of East and West Gondwana and escape tectonics (~650–600 Ma). In the first stage, Mg# of chromian spinel in the serpentinites continuously changed due to subsolidus Mg–Fe 2+ redistribution, while the Mg# of chromian spinel in the Type I listwaenites was frozen due to the absence of coexisting mafic silicates. During the second stage, the Type II listwaenites formed along shear zones accompanied by oxidation of relict chromian spinel to form ferritchromite and Cr-bearing magnetite in both serpentinites and listwaenites. The high Cr# of chromian spinel relics in both serpentinites and listwaenites preserves primary evidence of protolith melt extraction, but divalent cations are more easily mobilized at low temperature. Hence, relict chromian spinel in listwaenites shows significantly higher Mg# and lower MnO than that in serpentinite, suggesting that nearly complete alteration of ultramafic rocks to form listwaenite took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Furthermore, the ferritchromite in the serpentinites has higher Mn content (1.1–2.1 wt%) than that in the listwaenites (0.6–0.9 wt%), indicating its formation after carbonatization since carbonate minerals are a favorable sink for Mn.

In this work, we investigate the oxidation behavior of a nearly end-member riebeckite, ideally Na 2 (Fe32+Fe23+)$\begin{array}{} (\text{Fe}^{2+}_3\text{Fe}^{3+}_2) \end{array} $ Si 8 O 22 (OH) 2 , by using vibrational FTIR and Raman spectroscopies. Combining these results with previous studies performed on the same sample by single-crystal structure refinement and Mössbauer spectroscopy, we conclude that iron oxidation in riebeckite is a multi-step process. (1) In the ~523 K < T < 623 K temperature range, the O-H bond lengthens and both the electrons and the hydrogen cations delocalize. Raman analysis shows that this step is reversible upon cooling to room temperature. (2) In the 623 K < T < 723 K range, the kinetic energy increases so that the electrons can be ejected from the crystal; beyond 723 K an irreversible oxidation of Fe occurs that couples with irreversible changes in the SiO 4 double-chains leading to a contraction of the unit-cell volume, i.e., to structural changes detectable at the long-range scale. (3) Beyond 823 K, the irreversible oxidation is completed and H + ions are forced to leave the crystal bulk. Because of this multi-step process, the onset of the deprotonation process is detected at ~700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mössbauer spectroscopy on powders (and by changes in the cation distribution observed by structure refinement). Also, Raman scattering shows that the release of H + from the crystal surface starts ~100 K before the complete deprotonation of the crystal bulk is witnessed by FTIR absorption. Hence, the oxidation of Fe starts at the crystal surface and induces electron and H + migration from the crystal interior to the rim and thus subsequent oxidation through the crystal bulk. No deprotonation is observed by FTIR either in powders embedded in KBr or in crystals heated in N 2 atmosphere, implying that the release of H + needs surficial (atmospheric) oxygen to form H 2 O molecules. Fe 2+ → Fe 3+ oxidation produces a flux of electrons throughout the crystal matrix, which generates electrical conductivity across the amphibole. An important implication of this work, which might have interesting applications in material science, is that iron oxidation in riebeckite (and possibly in other Fe-rich silicates) is reversible in a given range of temperature. Also, this work shows that complex processes cannot be fully understood or even monitored accurately without using a proper combination of independent techniques.

In this work, we have studied the relationships between mass concentration and unit formula of amphibole using 114 carefully selected high-quality experimental data, obtained by electron microprobe (EMP) + single-crystal X-ray structure refinement (SREF) ± secondary-ion mass spectrometry (SIMS) analyses, of natural and synthetic Li-free monoclinic species belonging to the Ca and Na-Ca subgroups, and 75 Li-free and Mn-free C 2/ m end-members including oxo analogs of Ca amphiboles. Theoretical considerations and crystal-chemical driven regression analysis allowed us to obtain several equations that can be used to: (1) calculate from EMP analyses amphibole unit-formulas consistent with SREF±SIMS data, (2) discard unreliable EMP analyses, and (3) estimate W O 2– and Fe 3+ contents in Li-free C 2/ m amphiboles with relatively low Cl contents (≤1 wt%). The AMFORM approach mostly relies on the fact that while the cation mass in Cl-poor amphiboles increases with the content of heavy elements, its anion mass maintains a nearly constant value, i.e., 22O + 2(OH, F, O), resulting in a very well-defined polynomial correlation between the molecular mass and the cation mass per gram (R 2 = 0.998). The precision of estimating the amphibole formula [e.g., T Si ± 0.02, C Al ± 0.02, A (Ca+Na+K) ± 0.04 apfu] is 2–4 times higher than when using methods published following the last IMA recommended scheme (2012). It is worth noting that most methods using IMA1997 recommendations (e.g., PROBE-AMPH) give errors that are about twice those of IMA2012-based methods. A linear relation between W O 2– and the sum of C (Ti, Fe 3+ ) and A (Na+K) contents, useful to estimate the iron oxidation state of highly oxidized amphiboles typical of post-magmatic processes, is also proposed. A step by step procedure (Appendix Deposit item AM-18-76385, supplemental Material. Deposit items are free to all readers and found on the MSA web site, via the specific issue’s Table of Contents (go to http://www.minsocam.org/MSA/AmMin/TOC/2018/Jul2018_data/Jul2018_data.html). 1) and a user-friendly spreadsheet (AMFORM.xlsx, provided as supplementary material Deposit item AM-18-76385, supplemental Material. Deposit items are free to all readers and found on the MSA web site, via the specific issue’s Table of Contents (go to http://www.minsocam.org/MSA/AmMin/TOC/2018/Jul2018_data/Jul2018_data.html). ) allowing one to calculate amphibole unit-formulas from EMP analyses are presented. This work opens new perspectives on the unit-formula calculation of other minerals containing OH and structural vacancies (e.g., micas).

Fe-kaolinite has been detected in granite saprolite beneath sedimentary kaolin deposits in the Seto district of central Japan. Granite saprolite, which was found underneath sedimentary kaolin deposits formed in fluvial and lacustrine environments, had been subjected to kaolinization. The clay fractions of granite saprolite consist mostly of kaolinite with subordinate micaceous clay, quartz, and feldspars. Electron probe microanalysis (EPMA) showed that the kaolinite in clay fractions contained an average 3.30–3.72 wt% of Fe 2 O 3 , indicative of Fe-kaolinite. Fe+Si was inversely proportional to Al in Fe-kaolinite, indicating coupled substitution between Fe+Si and Al. The K 2 O contents of Fe-kaolinite increased with increasing Fe 2 O 3 up to 0.77 wt%, whereas K did not correlate with other elements, suggesting that K was not contained with the structure of kaolinite but was present in its interlayers. X-ray absorption near-edge structure (XANES) spectroscopy showed that about 60 to 70% of Fe in the clay fractions is ferric iron, and extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that Fe is situated in octahedral sites replacing Al. Fe-kaolinite was likely precipitated by the infiltration of acidic groundwater with higher Fe and alkali contents into granite saprolite, accompanied by the intense kaolinization of sedimentary kaolin deposits.

The rare mineral kalistrontite, K 2 Sr(SO 4 ) 2 , has been discovered in exceptional quantities in exploration boreholes targeting Permian polyhalite [K 2 Ca 2 Mg(SO 4 ) 4 ·2(H 2 O)]-bearing evaporite deposits in North Yorkshire, U.K. The kalistrontite is associated with anhydrite, polyhalite, halite, magnesite, and traces of celestine in the Fordon (Evaporite) Formation, English Zechstein 2 cycle, at depths of ~1.5 to 1.7 km below surface. It was first encountered here during quantitative X-ray diffraction assays of composited drill-core samples over an identified ~50 m interval in York Potash Ltd.’s boreholes SM6, SM9, and deflections SM9A and 9B. X-ray diffraction including structural refinement, thermal analysis, Raman spectroscopy, petrographic examination, quantitative microanalysis, and Sr isotopic analysis have been employed to fully characterize the kalistrontite and determine its genesis to understand its distribution and significance for the polyhalite deposits. Petrographic examination reveals that the kalistrontite is present in two general forms. First as irregularly shaped, poikilotopic millimeter-scale patches of subhedral, equant to elongate millimeter-scale crystals that enclose fine, rounded, irregular anhedral and rarely euhedral crystals of anhydrite, halite, and magnesite. Second as a vein-fill formed of an interlocking mosaic of elongate sub-millimeter scale, euhedral crystals that are compositionally zoned and again enclose fine rounded anhydrite and halite crystals at vein margins. Kalistrontite displays largely replacive contact relationships with both the earlier and generally simultaneously formed anhydrite and halite but before at least some of the polyhalite. Vein-fill kalistrontite was deposited by mineralizing fluids proceeding along fractures, patchily replacing the pre-existing low-porosity anhydrite and halite. EDX microanalysis of the North Yorkshire kalistrontite indicates a purer composition than previously reported but some (5–12% stoichiometric) substitution of Ca for Sr is identified and directly linked to petrographic textures identified during backscattered scanning electron imaging. Improved resolution XRD data for the kalistrontite is comparable to that previously published, with similar unit-cell dimensions [ a = 5.45826(5) Å, c = 20.8118(2) Å, α = 90°, β = 90°, γ = 120°, V = 536.968(3) Å 3 ] and space group R 3 m (166), despite the limited Ca substitution for Sr. Thermal behavior, published for the first time, shows that kalistrontite is essentially stable from ambient to ~960 °C. Melting occurs from ~960 to 1430 °C with a resulting weight loss of 62.57%, accompanied by the evolution of SO 2 . Minor endothermic features are tentatively ascribed to the boiling of K from surface sites. The first published Raman spectrum for kalistrontite shows a major frequency shift at 968 cm –1 with minor features of decreasing intensity at 458, 617, 1095, 1152, 650, 170, and 127 cm –1 . Consistent 87 Sr/ 86 Sr values for kalistrontite and anhydrite (mean, 0.707014 ± 0.000010, 2 S.E. and 0.707033 ± 0.000020, 2 S.E., respectively) along with very similar values obtained for the polyhalite are indicative of Late Permian seawater in an open environment with very limited evidence of basin constriction or Sr contribution from hydrothermal or meteoric source(s). When compared to the LOWESS global curve, the 87 Sr/ 86 Sr values suggest a consistent formation date of 255 ± 2 Ma (late Wuchiapingian), the first published date for the EZ2 deposits in North Yorkshire. Diagenetic processes, particularly the late-stage supply of K- and Sr-rich fluid, must have proceeded extensively in the North Yorkshire deposits. However these show only limited spatial development, within the shelf zone on the margins of the main polyhalite deposit. The K-rich nature (26.3 wt% K 2 O) of kalistrontite, compared to other K-bearing evaporite minerals (e.g., kainite 18.9 wt% K 2 O, carnallite 17.0 wt% K 2 O, polyhalite 15.6 wt% K 2 O), has a significant effect on borehole γ-ray response (303 compared to 229, 200, and 185 API units, respectively) and therefore considerable implications for evaporite deposit modeling and the determination of deposit-grade. Understanding the character and distribution of kalistrontite is necessary for modeling the nature, extent, and grade of the world’s richest-known deposit of polyhalite. York Potash Ltd. has recently commenced construction of the $3.0 bn Woodsmith Mine to support large-scale polyhalite production, promising the creation of thousands of jobs and a boost to both local and national economies. First production is scheduled for late 2021.

The Amami Plateau, Daito Ridge, and Oki-Daito Ridge of the northwestern Philippine Sea Plate are remnants of Mesozoic island arcs. We have newly recovered samples of peridotite and peridotite-derived minerals from the Daito Ridge. The peridotite samples are composed of serpentinized/altered olivines, orthopyroxene porphyroclasts, small clinopyroxenes, and spinels, indicating a harzburgitic origin. Chondrite- and primitive mantle-normalized trace-element patterns for clinopyroxenes are characterized by a steep positive slope from middle rare earth elements to heavy rare earth elements (HREEs) plus yttrium. The light rare earth elements (LREEs) and Sr and Zr contents of clinopyroxenes vary in abundance, and some crystals have high LREE/HREE ratios coupled with positive Sr and Zr anomalies. These petrological and geochemical characteristics are not consistent with the Daito peridotites being the residue of a single partial melting event including melt extraction expected for mid-ocean ridge mantle. Instead, the peridotite source must have been enriched with slab-derived components, which are associated with arc-related magma. Thus, it is concluded that the studied peridotite fragments belong to an exhumed mantle section of a remnant proto-Philippine Sea island arc.

We apply an experimentally based thermodynamic model of Si+O saturation for the core to determine the saturation level of these elements under the conditions when the core formed. The model limits the bulk Si content of the core to between 0.4 and 3.1 wt% depending on the pressure, temperature, and oxygen content of the metal when it segregated from silicate. With knowledge of the core’s Si content, the measured 30 Si content of the silicate Earth, and the experimentally determined metal-silicate fractionation factor, we can calculate the core’s δ 30 Si, which is between –0.92 to –1.36‰. SiO 2 cycled through the core and then released into the mantle might be trapped in inclusions in diamond formed in the lower mantle. These would be characterized by significantly lighter δ 30 Si values of –1.12 ± 0.13‰ (1σ), compared to bulk silicate earth values of –0.29‰ and a potentially key indicator of mass transfer from the core to the mantle.

Heat capacity ( C P ) data of Al-F-bearing titanite are presented that yield the standard entropy S298.15∘ $\begin{array}{} {\text S}_{298.15}^{\circ} \end{array} $ of F-Al-titanite CaAlFSiO 4 (FAT). C P of synthetic FAT was measured with relaxation calorimetry and differential scanning calorimetry between 5 and 764 K. The results constrain S298.15∘ $\begin{array}{} S_{298.15}^{\circ} \end{array} $ to be 115.4 ± 2.0 J/(mol·K) and subsequently the standard Gibbs free energy of formation from the elements, Δ f G °, of CaAlSiO 4 F to be between –2583 ± 3.0 and –2588 ± 3.0 kJ/mol, and the standard enthalpy of formation from the elements, Δ f H °, to lie between –2728 ± 3.0 and –2733 ± 3.0 kJ/mol depending on the thermodynamic data retrieval approach. These data, in turn, can be used to quantitatively model high-grade and UHP fluid-rock interaction. The calculation of future petrogenetic grids involving F-bearing minerals and titanite solid solutions in the system CaTiSiO 4 O–CaAlSiO 4 F will only be possible by expanding existing internally consistent thermodynamic databases to the F-system.

Two errors are identified in the implementation of a recently published thermometer based on the partitioning of trace elements between garnet and clinopyroxene. The errors compromise comparisons with other thermometers and experimental results. Using the same methodology, a new, simplified procedure is presented to rectify the errors and test the consequences. In general, the corrected thermometer gives temperatures that are 30–50 °C higher than uncorrected values.