Volume 103, Issue 10

The 3000 km 2 Marcy anorthosite massif dominates the Adirondack Highlands (Grenville Province, New York). The Marcy massif was metamorphosed to granulite facies conditions, in places preserving igneous textures with metamorphic coronas and is most deformed near its margins. Historically, the relationship between anorthosite emplacement and metamorphism has been controversial, and many workers have argued that anorthosite emplacement coincided with metamorphism. Valley and O’Neil (1982) proposed that high-pressure metamorphic mineral assemblages in the anorthosite could not reflect the same event that formed wollastonite skarns adjacent to anorthosite, which have low δ 18 O and formed in the presence of meteoric water during shallow emplacement. This study presents new in-situ geochronology that constrains the timing of metamorphic mineral growth in Adirondack anorthosite to 1050–1035 Ma. The Zr source for metamorphic zircon growth was the breakdown of hemoilmenite and is texturally linked to high-pressure mineral assemblages. These data are consistent with previously determined ca. 1155 Ma magmatic ages and later granulite facies metamorphism during the 1090–1020 Ma Ottawan phase of the Grenvillian orogeny.

In rare-metal granites, niobium and tantalum are generally hosted by Nb–Ta oxides. However, in SE China, the Nb-specialized Huangshan granites are a unique occurrence in which Nb is essentially hosted by Li–Fe micas. The Huangshan granites are part of the Early Cretaceous (Late Yanshanian) Lingshan granite complex and belong to the A-type granite series, with two facies differing by their mica compositions: medium-grained “protolithionite” granite and medium-grained lithian (lithium-rich) annite granite. The granites are characterized by elevated whole-rock Nb contents (average 144 ppm in “protolithionite” granite and 158 ppm in annite granite), quite low Ta contents (average 9 and 4 ppm, respectively), leading to very high Nb/Ta ratios (average 15.3 and 31.2). Niobium is mainly hosted in the micas, with an average Nb content of 1347 ppm in the lithian annite and 884 ppm in the “protolithionite,” which is the highest ever reported in granitic mica. With an estimated endowment of ~80 kt Nb, the Huangshan granites represent a new style of potential Nb resource. Contrasting with the great rarity of columbite, there is abundant Hf-rich zircon, Y-rich fluorite, and Th-rich fluocerite included in the Huangshan micas. Such accessory minerals being typical of alkaline rhyolitic magmas and niobium enrichment in the Huangshan granites results from A-type melt. The extreme Nb enrichment in the micas results from the highly compatible behavior of Nb in this melt, combined with the high magma temperature (estimated at 790–800 °C) and possibly enhanced magma oxidation.

We have modeled the genesis of primary mantle-derived magma to explore the controls exerted on its Ni-Cu ore potential by water content, pressure, and mantle potential temperature ( T p ). During decompression melting, Ni concentration in primary magma decreases with an increasing degree of melting, which is in contradiction to long-held understanding obtained from previous isobaric melting models. Pressure exerts a first-order control on the ore potential of primary plume-derived melt, such that plumes rising beneath thick lithosphere with melting paths terminating at relatively high pressure generate Ni-rich melts. Additionally, as plumes with higher T p produce more Ni-rich melt at a higher pressure, the magmatism related to hotter plume-centers may have the greatest ore potential. On the other hand, the strong dependence of Cu behavior upon the presence or absence of residual sulfide is partly countered in decompression melting. Significant influences of mantle-contained water on Ni and Cu partitioning are restricted to low-degree melting. While release of H 2 O in lithosphere delamination may trigger voluminous magmatism, the Ni concentration in the melt is far lower than in melt generated from plumes. Furthermore, if isobaric melting dominates when the subcontinental lithospheric mantle (SCLM) is heated by underlying hotter plumes, the plume-lithosphere interaction plays no active role in the Ni ore potential of primary magma because derived melt volumes are relatively small. In subduction zones, flux-melting of the mantle wedge tends to generate cool Ni-poor melts, however hot subduction zones may produce magmas with increased metal concentrations. Overall, the anticipated ranges of Ni contents in primary melts are strongly controlled by tectonic setting, with a range of 100–300 ppm in subduction zones, 230–450 ppm in mid-ocean ridges, and 500–1300 ppm in plume suites. There are only minor differences in the Cu concentrations of primitive magmas generated from diverse tectonic settings, despite the variations in Cu partitioning behaviors.

Carbon has been suggested as one of the light elements existing in the Earth’s core. Under core conditions, iron carbide Fe 7 C 3 is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe 7 C 3 , however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe 7 C 3 up to 80 GPa and 800 K. Our diffraction data indicate that Fe 7 C 3 adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe 7 C 3 were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V 0 = 745.2(2) Å 3 , bulk modulus K 0 = 167(4) GPa, its first pressure derivative K0′ $\begin{array}{} \displaystyle K_{0}^{\prime} \end{array}$ = 5.0(2), d K /d T = -0.02(1) GPa/K, and thermal expansion relation α T = 4.7(9) × 10 –5 + 3(5) × 10 –8 × ( T – 300) K –1 . We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe 7 C 3 has strong anisotropic compressibilities with the linear moduli M a > M c > M b from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c 3 is approximately four times larger than that of a 3 and b 3 at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe 7 C 3 . Therefore, the effect of high temperature needs to be considered when using Fe 7 C 3 to explain the anisotropy of the Earth’s inner core.

Magma mixing at arc volcanoes is common, but the manner in which mixing or mafic recharge may trigger volcanic eruptions is unclear. We test ideas of eruption triggering for the 1103 ± 13 years B.P. Chaos Crags eruption at the Lassen Volcanic Center, Northern California. We do so by applying mineral-melt and two-mineral equilibria from mafic enclaves and host lavas from six eruptive units of the Chaos Crags eruption to calculate crystallization conditions. Understanding that Chaos Crags are a type location for magma mixing, we estimate these P - T conditions by employing some apparently new methods to reconstruct pre-eruptive liquid compositions—which can be independently verified using various mineral-melt equilibria. We find that crystallization of “host” rhyodacite magmas occurs within the upper crust (at pressures of 0–1.7 kbar) over an approximate 300 °C interval (temperatures ranging from 669–975 °C) and that mafic magmas (which occur as enclaves within the host felsic samples) crystallized over an approximate 250 °C temperature interval (ranging from 757–1090 °C), also within the upper crust, though extending to middle-crust depths (0–3.9 kbar). Notably, both host lavas and mafic enclaves contain crystals that are inherited from their opposing end-member, and both magma types contain plagioclase crystals that appear to have equilibrated with the resulting intermediate composition magmas; these intermediate composition plagioclase crystals indicate that some amount of time passed between both the recharge of magma into a felsic reservoir and the mixing event that caused an exchange of crystals before eruption. We propose that mafic recharge—though it may have been the ultimate triggering event—did not immediately precede any of the eruptive events at Chaos Crags. The most mafic (least mixed) enclaves in our collection are nearly aphyric, indicating that they were likely the first melts to enter the system, and quenched upon intrusion into a cold, upper-crust felsic magma. Many high- T olivine grains in enclaves also coexist with clinopyroxene, plagioclase, and amphibole crystals that crystallized from only slightly more evolved liquids, at temperatures that are low enough (e.g., 800–900 °C) to have possibly quenched earlier-formed, high- T Ol crystals, perhaps negating the use of Ol diffusion profiles as a record of mixing-to-eruption timescales (at Chaos Crags, at least, they would only provide minimum times, which could be orders of magnitude less than actual times). And more crystalline enclaves record more mixing and more cooling. It thus appears that recharge is required to reinvigorate an otherwise dormant Chaos Crags system, as described by Klemetti and Clynne (2014), but ~250 °C of cooling and crystallization, as recorded by many enclaves, provides the immediate cause of eruption—through increased magma overpressure by the exsolution of a fluid phase and increased buoyancy.

A redox reaction in which Sn 2+ oxidizes to Sn 4+ is thought to occur during the precipitation of cassiterite (SnO 2 ) and stannite (Cu 2 FeSnS 4 ) from high-temperature hydrothermal solutions. In four stanniferous regions with differing mineralization environments (South Dakota, U.S.A.; Cornwall, England; Erzgebirge, Germany/Czech Republic; Andean tin belt, Bolivia), the tin isotope composition in stannite (mean value δ 124 Sn = –1.47 ± 0.54‰, n = 21) is consistently more fractionated toward negative values than that of paragenetically earlier cassiterite (mean value δ 124 Sn = 0.48 ± 0.62‰, n = 50). Given the oxidation-dependent mechanism for cassiterite precipitation, this isotopic shift is most likely attributable to the oxidation of Sn in solution; precipitation of heavy-Sn-enriched cassiterite results in residual dissolved Sn with lighter isotopic composition, which is expressed in the negative δ 124 Sn values of later-formed stannite. Equally important is that the mean values for the cassiterite from the various deposits are slightly different and may indicate that the initial Sn isotope composition in early-formed cassiterite relates to variations in the source or magmatic processes. Therefore, the Sn isotopes may provide information on both redox reactions and petrologic sources and processes.

The surface energy (hydrated surfaces) of fayalite (α-Fe 2 SiO 4 ) was determined to be 2.47 ± 0.25 J/m 2 using high-temperature oxide melt solution calorimetry. This is larger than the surface energy of magnetite (Fe 3 O 4 ), but lower than that of forsterite (α-Mg 2 SiO 4 ). The changes in the positions of the quartz-fayalite-magnetite (QFM) and quartz-iron-fayalite (QIF) buffers with particle size reduction were calculated. QFM is lowered in f O 2 by 3–7 log units as a function of temperature for 30 nm particles while QIF is raised by 1–2 log units. The estimated surface energy difference between olivine and spinel polymorphs decreases the pressure of the olivine-spinel transition in Fe 2 SiO 4 by about 1 GPa.

Naturally and experimentally deformed gouges from sliding surfaces within the Galera Fault Zone were analyzed using scanning and transmission electron microscopy (SEM, TEM) to identify changes in the fault rocks as a consequence of ongoing deformation. The two gouges studied have a particular mineral association that includes planar (mainly smectite and illite) and fibrous clay minerals (sepiolite and palygorskite). Microstructural findings include a radical difference in grain alignment between the two gouges, a phenomenon that strongly influences gouge permeability. Smectite crystals are aligned on the same orientation and show a great number of layer terminations and delamination on the basal planes that contribute to a distributed mode of deformation in the gouge. In contrast, the sepiolite-rich gouge exhibits a grid-like microfabric that results in localized deformation limited to small areas where the needle-like crystals are bent and broken producing “feather-like” structures, without the presence of lattice distortions. Meanwhile, significant chemical results include: (1) Al content identified in sepiolite fibers through analytical electron microscopy (AEM), together with variability in the (110) d -spacing of sepiolite across single fibers, suggest the existence of a progressive transformation from sepiolite to palygorskite. (2) Mg content in smectite suggests that a portion of the smectites within the fault plane could have an authigenic origin and may be the result of a transformation reaction from palygorskite, however, the similarity of the 2:1 layer compositions between the smectites in the two contexts do not allow to either confirm nor deny such possibility. (3) Chemical continuity of Mg-decrease and Al+Fe-increase in the octahedral cation content of the sepiolites, palygorskites, and smectites within the gouges indicate a sequence of mineral transformations that is favored by a depleted Mg content and an increase of Al content in the fluid. In this setting, deformation promotes grain size reduction and fluid-rock interaction with the wall rocks resulting in a local supply of Al to the fault gouge that drives phase transformations. Structural differences between smectites and fibrous clay minerals affect important chemical and physical properties of the gouge including their mechanical properties. We propose that the permeability of the gouges in the Galera Fault is strongly affected by their mineralogy. Furthermore, the extent of the mineral authigenesis and mineral transformations could be a controlling factor that progressively changes both the permeability and the strength of the fault.

The high-pressure behavior of dravite tourmaline [Na(Mg 3 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 (OH)] has been studied using luminescence spectroscopy and synchrotron-based single-crystal diffraction up to ~65 and ~24 GPa, respectively. Two emission bands associated with Cr 3+ /V 2+ substitution are constant in energy up to ~9.0 GPa, and they shift to longer wavelength at higher pressures, suggesting that a change in compressional mechanism could occur at this pressure. Single-crystal diffraction data show subtle changes in ring ditrigonality occur near 9.0 GPa, which could cause the observed change in luminescence. Near 15 GPa, a splitting of one of the emission bands is observed, suggesting that a phase transition occurs at this pressure and that two unique octahedral sites are present in the high-pressure phase. Hysteresis is not observed on decompression, which indicates that this is a second-order transition, and the high-pressure structure appears to be metastable up to ~65 GPa. Single-crystal diffraction measurements show that a phase transition from rhombohedral R 3 m to rhombohedral R 3 occurs at pressures near 15.4 GPa. The high-pressure phase is characterized by a distorted Si 6 O 18 ring (e.g., the Si-Si-Si angles deviate from 120°), and the Si, Al, O6, O7, and O8 sites of the low-pressure phase split, implying that the high-pressure phase of tourmaline is a higher entropy phase. The large X -site exerts the primary control on compressibility, and the substitution of larger cations into this site will likely lower the pressure at which this transition occurs. Dravite tourmaline shows anisotropic compression with the c -axis being more compressible than the a -axis. The pressure and volume data up to ~15.4 GPa were fit with second- and third-order Birch-Murnaghan equations of state. We obtain a bulk modulus, K 0 = 109.6(3.2) GPa, and a pressure derivative, K0′ $\begin{array}{c} \displaystyle K^{\prime}_{0} \end{array} $ = 4.6(8) GPa, and with the pressure derivative set to 4, a bulk modulus of 112.0(1.0) GPa is derived. Moreover, our high-pressure results show that massive overbonding of the X and Y sites can be accommodated by the tourmaline structure. This unexpected result may explain the extraordinary structural tolerance with respect to chemical substitution on the X , Y , and Z sites.

The role of fluid(s) in the formation of different lithological facies of kimberlites is still poorly understood. The uncertainty in the composition of kimberlite melts hampers understanding the composition of volatiles, the depth of exsolution, and the effect on magma ascent and fragmentation. Recent estimates of H 2 O and CO 2 solubility in kimberlite-like magmas suggest very shallow exsolution of fluid, while many features of kimberlites indicate the presence of significant fluid fraction at depth. Deep magmatic fluid produces negative trigonal etch pits on natural diamonds, the characteristics of which depend on the temperature and composition of the fluid. Positively oriented trigonal etch pits are very rare on natural diamonds and are likely a feature of resorption events unique to only some kimberlite magmas. Here we present the first systematic study of positively oriented trigonal etch pits on natural diamonds from Snap Lake kimberlite dike, Northwest Territories, Canada. The study used 91 micro-diamonds selected from a population of 251 diamonds representative of all six kimberlite litho-facies identified in the Snap Lake dike. We established that unlike the majority of diamonds from kimberlite pipes in the Northwest Territories, every studied Snap Lake diamond shows positively oriented trigons. These trigons cover the whole diamond surface starting from the {111} faces and continuing over the resorbed edges. They overprint negatively oriented trigons and modify them into hexagons. Atomic force microscopy obtained detailed geometry of 154 positive trigons on 14 diamonds. Three distinct trigon morphologies dependent on the type of the crystal lattice defect were recognized. The point-bottomed shape and positive correlation between the depth and diameter of the individual pits suggest a high CO 2 content in the fluid. Comparison with the existing experimental data on positive trigons implies resorption at low-pressure conditions in the 800–1000 °C temperature range by trapped magmatic fluid after the dike emplacement. The intensity of this late resorption event (and the size of the positive trigons) increases from the dike contact with the country rock into the interior of the dike. Such a late resorption event is absent in the majority of kimberlites, which form pipes, and might be a specific feature of hypabyssal kimberlite bodies (dikes). The absence of positive trigons on diamonds from the majority of kimberlites suggests very quick magma cooling below ~800 °C after the pipe emplacement, precluding the development of any late resorption features. Our study shows that for kimberlitic magmas, for which mineral chemistry is unable to provide a robust record of magmatic fluid, morphological details of dissolution features on the surface of diamond and other mantle-derived minerals can serve as a fluid proxy. Better constraints of the pressure, temperature, and oxygen fugacity of the reversal in the trigon orientation on diamond may help to reconstruct the emplacement path of geologically diverse kimberlite bodies.

The carbonation of ultramafic rocks, including tailings from ultramafic-hosted ore deposits, can be used to remove CO 2 from the atmosphere and store it safely within minerals over geologic timescales. Quantitative X-ray diffraction (XRD) using Rietveld refinements can be employed to estimate the amount of carbon sequestered by carbonate minerals that form as a result of weathering of ultramafic rocks. However, the presence of structurally disordered phases such as serpentine minerals, which are common in ultramafic ore bodies such as at the Woodsreef chrysotile mine (New South Wales, Australia), results in samples that cannot be analyzed using typical Rietveld refinement strategies. Previous investigations of carbon sequestration at Woodsreef and other ultramafic mine sites typically used modified Rietveld refinement methods that apply structureless pattern fitting for disordered phases; however, no detailed comparison of the accuracy (or precision) of these methods for carbon accounting has yet been attempted, making it difficult to determine the most appropriate analysis method. Such an analysis would need to test whether some methods more accurately quantify the abundances of certain minerals, such as pyroaurite [Mg 6 Fe23+$\begin{array}{} \rm Fe^{3+}_{2} \end{array}$ (CO 3 )(OH) 16 ·4H 2 O] and other hydrotalcite group minerals, which suffer from severe preferred orientation and may play an important role in carbon sequestration at some mines. Here, we assess and compare the accuracy, and to a lesser extent the precision, of three different non-traditional Rietveld refinement methods for carbon accounting: (1) the PONKCS method, (2) the combined use of a Pawley fit for serpentine minerals and an internal standard (Pawley/internal standard method), and (3) the combined use of PONKCS and Pawley/internal standard methods. We examine which of these approaches represents the most accurate way to quantify the abundances of serpentine, pyroaurite, and other carbonate-bearing phases in a given sample. We demonstrate that by combining the PONKCS and Pawley/internal standard methods it is possible to quantify the abundances of disordered phases in a sample and to obtain an estimate of the amorphous content and any unaccounted intensity in an XRD pattern. Eight artificial tailings samples with known mineralogical compositions were prepared to reflect the natural variation found within the tailings at the Woodsreef chrysotile mine. Rietveld refinement results for the three methods were compared with the known compositions of each sample to calculate absolute and relative error values and to evaluate the accuracy of the three methods, including whether they produce systematic under- or overestimates of mineral abundance. Estimated standard deviations were also calculated during refinements; these values, which are a measure of precision, were not strongly affected by the choice of refinement method. The abundance of serpentine minerals is, however, systematically overestimated when using the PONKCS and Pawley/internal standard methods, and the abundances of minor phases (<10 wt%) are systematically underestimated using all three methods. Refined abundances for pyroaurite were found to be increasingly susceptible to error with increasing abundance, with an underestimation of 6.6 wt% absolute (60.6% relative) for a sample containing 10.9 wt% pyroaurite. These significant errors are due to difficulties in mitigating preferred orientation of hydrotalcite minerals during sample preparation as well as modeling its effects on XRD patterns. The abundances of hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O], another important host for atmospheric CO 2 during weathering of ultramafic rocks, was consistently underestimated by all three methods, with the highest underestimation being 3.7 wt% absolute (or 25.0% relative) for a sample containing 15.0 wt% hydromagnesite. Overall, the Pawley/internal standard method produced more accurate results than the PONKCS method, with an average bias per refinement of 6.7 wt%, compared with 10.3 wt% using PONKCS and 12.9 wt% for the combined PONKCS-Pawley/internal standard method. Furthermore, the values for the refined abundance of hydromagnesite obtained from refinements using the Pawley/internal standard method were significantly more accurate than those for refinements done with the PONKCS method, with relative errors typically <25% for hydromagnesite abundances between 5 and 15 wt%. The simpler and faster sample preparation makes the PONKCS method well-suited for rapid carbon accounting, for instance in the field using a portable XRD; however, the superior accuracy gained when using an internal standard make the Pawley/internal standard method the preferable means of undertaking a detailed laboratory-based study. As all three methods displayed an underestimation of carbonate phases, applying these methods to natural samples will likely produce an underestimate of hydromagnesite and hydrotalcite group mineral abundances. As such, crystallographic accounting strategies that use modified Rietveld refinement methods produce a conservative estimate of the carbon sequestered in minerals.

Three types of polyphase solid-inclusions (PSIs) with distinct mineral assemblages and microstructures were found in garnet of an ultrahigh-pressure (UHP) eclogite-vein system from the Dabie Shan, east-central China. Type-1 PSI contains variable volumes of quartz, K-feldspar, plagioclase ± other phases, whereas Type-2 PSI contains variable volumes of quartz, calcite ± other phases. Both types display shapes that are compatible with those of euhedral coesite inclusions. Type-3 PSI always contains a rutile core that is surrounded by plagioclase ± quartz and generally displays the morphology of the rutile core. Variable amounts of K-feldspar are embedded within the plagioclase of Type-3 PSIs. The three PSI types developed fluid-mediated microstructures that include wedge-like offshoot and protrusion textures and inclusion-garnet interfaces controlled by the crystallographic structure of garnet. PSIs in peak minerals of UHP rocks have been previously thought to represent primary supercritical fluid or melt inclusions. Here we propose that the studied PSIs were formed under high-pressure (HP) eclogite-facies conditions during exhumation and represent reaction products between an enclosed mineral, such as coesite and rutile, and external fluids infiltrating the host garnet along fractures that have been healed later on. Two immiscible aqueous fluids (i.e., a siliceous and a carbonaceous) were involved in the formation of these PSIs. The siliceous fluid was rich in various large ion lithophile elements like Cs, Rb, Ba, K, Pb, Li, and Sr, whereas the carbonaceous fluid was rich in Pb and Sr. The new PSI formation mechanism proposed in this study brings significant implications for tracing fluid evolution and post-entrapment modifications of mineral inclusions in HP and UHP metamorphic rocks.

Pyrrhotite, Fe 7 S 8 , is a common sulfide mineral in the Earth’s crust and mantle, as well as in a range of meteorites and is of interest to a wide variety of disciplines including economic geology, geophysics, and material science. The 4 C variety of pyrrhotite shows a dramatic change in magnetic properties at T ≈ 30 K, known as the Besnus transition. Although this transition is frequently used to detect pyrrhotite in geologic samples, the underlying mechanism driving the transition has not yet been identified. This study presents a high-resolution view of the changes in heat capacity, magnetic, and electronic properties of a natural single crystal of nearly pure, monoclinic 4 C pyrrhotite across the Besnus transition. Contrary to previous studies, all of these properties show clear evidence of the Besnus transition, specific heat, in particular, revealing a clear transition at 32 K, apparently of second-order nature. Small-angle neutron scattering data are also presented, demonstrating an unusual change in short-range magnetic scattering at the transition. Furthermore, a magnetic field dependence of the transition temperature can be seen in both induced magnetization and electrical resistivity. These new observations help narrow the possible nature of the phase transition, clearly showing that interactions between intergrown coexisting 4 C and 5 C ∗ superstructures, as suggested in some literature, are not necessary for the Besnus transition. In fact, the changes seen here in both the specific heat and the electronic transport properties are considerably larger than those seen in samples with intergrown superstructures. To further constrain the mechanism underlying the Besnus transition, we identify five separate potential models and evaluate them within the context of existing observations, thereby proposing experimental approaches that may help resolve ongoing ambiguities.

Quantitative X-ray mapping using an electron probe enables quantitative evaluation of inhomogeneities within rocks. Recent studies have proposed methods to construct quantitative chemical maps by combining X-ray maps with referential spot analyses within a mapped area. These approaches address matrix effects by assuming each pixel in the mapped area represents a single phase. In such cases, the spatial resolution of the X-ray maps must be sufficiently high to separate mineral phases. This study proposes a new procedure to reliably quantify centimeter-scale X-ray maps even if the maps contain an ineligible number of pixels analyzing multiple phases because of a large mapping probe diameter. Such multi-phase pixels are statistically classified into their constituent phases by introducing a distribution-based clustering analysis. Furthermore, based on referential spot analyses, we implemented corrections for matrix effects and the backgrounds of single- and multi-phase pixels. Our technique, termed QntMap, was developed as an open source R package and distributed on a social coding platform, GitHub (https://github.com/atusy/qntmap). We applied QntMap to calculate local bulk compositions within an ultrahigh-pressure eclogite from Nové Dvory, Czech Republic. The studied sample is a garnet-rich bimineralic eclogite that includes a 3 mm thick pyroxene-rich layer. A mapped area is approximately 3 × 1 cm in size and oriented normal to the layer. A profile normal to the layer shows increases in Cr 2 O 3 (0.0 to 0.3 wt%) and X Mg [Mg/(Fe+Mg) = 0.5 to 0.8] from the garnet-rich matrix toward the pyroxene-rich layer. A large variation in X Mg and high-Cr 2 O 3 contents in the pyroxene-rich layer are inconsistent with a cumulate origin. We suggest that the pyroxene-rich layer was derived from a pyroxenitic melt that intruded the eclogite.

The new mineral stracherite, BaCa 6 (SiO 4 ) 2 [(PO 4 )(CO 3 )]F [ R 3 m , a = 7.0877(5) Å, c = 25.201(2) Å, V = 1096.4(1) Å 3 , Z = 3], belongs to the zadovite group, which also includes zadovite, BaCa 6 [(SiO 4 ) (PO 4 )](PO 4 ) 2 F; aradite, BaCa 6 [(SiO 4 )(VO 4 )](VO 4 ) 2 F; and gazeevite, BaCa 6 (SiO 4 ) 2 (SO 4 ) 2 O. All minerals of this group exhibit single-layer antiperovskite modules, which are intercalated with tetrahedral layers. In stracherite, the first CO 3 -bearing intercalated hexagonal antiperovskite, about 38% of the (PO 4 ) 3- tetrahedra are randomly substituted by planar (CO 3 ) 2– groups. The mineral was discovered in spurrite rocks of the Hatrurim Complex in the Negev Desert near Arad, Israel. Associated minerals are spurrite, calcite, brownmillerite, shulamitite, CO 3 -bearing fluorapatite, fluormayenite-fluorkyuygenite, and ariegilatite. The empirical formula of stracherite is: (Ba 0.96 K 0.02 Na 0.01 ) Σ0.99 Ca 6.01 [(SiO 4 ) 1.86 (PO 4 ) 0.12 (AlO 4 ) 0.01 (TiO 4 ) 0.01 ] Σ2 [(PO 4 ) 1.05 (CO 3 ) 0.75 (SO 4 ) 0.18 (VO 4 ) 0.02 ] Σ2 (F 0.95 O 0.03 ) Σ0.98 . Poikilitic crystals of stracherite are up to 0.5 mm in size and are confined to re-crystallization zones of spurrite marbles under the influence of by-products (gases, fluids) of combustion metamorphism.

Earth’s core is essentially composed of a light-element bearing iron-nickel alloy (Birch 1964). The nickel content in the core has negligible effects on physical properties such as density and compressibility (e.g., Lin et al. 2003; Kantor et al. 2007; Martorell et al. 2013; Badro et al. 2014). This deters any attempt to determine or even estimate the nickel content of the core using seismological models, as in the case of light elements. It was recently proposed that the presence of nickel should fractionate iron isotopes in small planetary cores (Elardo and Shahar 2017), but the effect for a large (hot) planet such as the Earth would not be measurable; this observation, however, opens up the possibility that Ni can have an effect on element partitioning between the metallic alloy and the silicate melt during core formation. In this case, the siderophile trace-element composition of the mantle would, in turn, constrain the Fe/Ni ratio in the core. Here, we investigated the effect of nickel concentration in the metallic alloy on the partitioning of other elements at conditions directly relevant to core formation, using the laser-heated diamond-anvil cell. We found no measurable effect of nickel concentration on the partitioning of Ni, Cr, and V; the Fe-Ni alloy is chemically ideal over a broad range of Ni concentrations (3.5 to 48.7 wt%). The ideality of the Fe-Ni solution across a wide range of nickel concentration shows that Fe and Ni are not only twins from the standpoint for material properties, but also from that of chemical properties in those high P - T conditions.