Subsolidus Oxidation Research Articles

First widespread occurrence of rare phosphate chladniite in a meteorite, winonaite Graves Nunataks (GRA) 12510: Implications for phosphide–phosphate redox buffered genesis in meteorites

Abstract The first widespread occurrence of rare Na-, Ca-, and Mg, Mn, Fe-bearing phosphate chladniite was observed in meteorite Graves Nunataks (GRA) 12510, which is a primitive achondrite that sits within the winonaite class. Numerous 1–500 µm chladniite grains were found, often on the margins between silicate clasts and the kamacite portions of the large metal veins that permeated through the sample. The largest chladniite grains are associated with merrillite, kamacite, taenite, troilite, albite, forsterite, diopside, and enstatite, with a few tiny chladniite grains and an apatite grain enclosed within merrillite. GRA 12510s average chladniite composition is Na2.7Ca1.25(Mg10.02Mn0.69Fe0.20)Σ10.91(PO4)9. Electron backscattered diffraction (EBSD) patterns indicate varying degrees of nucleation and growth of chladniite grains. Additionally, the first pure Raman spectrum of chladniite is described here, revealing primary ν1 bands at 954, 974, and especially 984 cm–1. The co-occurrence and close association of merrillite, apatite, chladniite, and P-bearing metallic phases within GRA 12510 suggests that the fO2 of IW-2 to IW-4 is an intrinsic property of the precursor chondritic material, and the phosphate-phosphide reaction may have buffered the final winonaite and IAB iron meteorite phase assemblages. Altogether, chladniite appears to form alongside other phosphates, with their chemistries reflecting the diverse environment of their formation. Meteoritic chladniite likely formed through subsolidus oxidation of schreibersite, scavenging Na from albite, Ca from diopside, Mg from enstatite/forsterite, Fe from kamacite/taenite, and Mn from alabandite/chromite when available. A P0-P5+ redox-buffered environment also has implications for thermometry and fast cooling rates, although more experiments are needed to extrapolate powder reaction rates to those of larger crystals. Furthermore, phosphide-phosphate buffered experiments may aid in investigating equilibrium chemistry at fO2 values between IW-2 and IW-4, which have been challenging to explore experimentally due to the limited availability of solid metal-metal oxide buffers between IW (Fe-FeO) and IW-5 (Cr-Cr2O3) at temperatures and pressures relevant to planetary interiors. Future investigations of phosphide-phosphate redox-buffered genesis at fO2 values between IW-2 and IW-4 have important implications for primitive meteorite constituents (e.g., CAI values), partially differentiated planetesimals and planets, including Mercury and core formation on Earth.

Neoformation of magnetite during selective metasomatism controlling large-scale positive magnetic anomalies within the Brunovistulian unit (Bohemian Massif)

An airborne magnetometer survey together with field measurements of magnetic susceptibility have distinguished two main granitic suites within the eastern zone of the Brno batholith (the exposed part of the Brunovistulicum). Highly magnetic biotite-amphibole tonalites, forming widespread positive magnetic anomalies (up to 300 nT), have susceptibility (χ) regularly above 5 × 10−3. Less magnetic biotite granites display lower χ values on the order of 1 × 10−4. The metaluminous tonalites significantly differ in whole-rock chemistry (enriched in FeO, MgO, and depleted in SiO2) and degree of fractionation (decreased Rb/Sr and K/Ba ratios) reflecting the distinct microchemistry of Fe-rich mafic silicates (chamosite ~24 wt% FeO). Conversely, more felsic metaluminous to peraluminous biotite granites exhibiting a higher degree of fractionation characteristically contain mafic silicates depleted in Fe2+ (Mg-clinochlore ~20 wt% FeO). The geochemistry of granitoids and chemical composition of mafic phases suggest that increased contents of pure magnetite (TiO2 <0.15 wt%) along with diamond-shaped titanite originated as a result of the reaction between primary magmatic Fe-Ti oxides, annite and anorthite during late oxidation of granitic rocks. The subsolidus oxidation of granitoid has led to releasing of Fe2+ from mafic silicates representing a substantial source of Fe2+ for the formation of pure magnetite in an evolved and crystallized magmatic system under relatively oxidative conditions. The Curie temperature (585–601 °C) and the Verwey transition (-160 °C) demonstrate the absence of titanomagnetite in both types and the overall magnetic properties of granitoids dominated by pure magnetite slightly affected by oxidation. Nevertheless, the more extensive magnetite hematitization in biotite granites indicates a higher degree of oxidation caused by dissociation of a greater amount of water phase in the post-magmatic stage of evolution of a more fractionated felsic melt. The metasomatic transformation of paramagnetic (Fe-rich biotite and amphibole) to ferromagnetic (pure magnetite) phases, well described on granitoid varieties of the Brno batholith, seems to be responsible for the extraordinarily geomagnetic behavior of the entire eastern part of the Brunovistulicum (Slavkov terrane) mostly covered by Paleozoic sediments.

The Northwest Africa 8159 (NWA 8159) Martian Meteorite Part 2. Spinel-orthopyroxene intergrowths. A record of fO2 and crust-basalt interactions

In part 1 of our examination of Martian meteorite Northwest Africa 8159 (NWA 8159) we illustrated many interesting mineralogical and textural attributes that make this martian basalt unique. Unlike the shergottites that illustrate a clear relationship between the extent of trace element and isotopic characteristics and oxygen fugacity (reduced, depleted magmas; oxidized, enriched magmas), NWA 8159 illustrates a decoupling of this relationship as it has oxidized and depleted signatures. In part 2, using a series of new observations and measurements (Cl isotopes, XANES, TEM, empirical modeling) we use NWA 8159 to explore the interaction between mantle-derived magmas and the martian crust. The magnetite-orthopyroxene intergrowths associated with olivine are a product of a martian subsolidus oxidation event near the QFM buffer and not a magmatic reaction in an oxidizing magma (>QFM + 3). This subsolidus event is further supported by Cr valence in the olivine, alteration of P-rich olivine, and end-member magnetite in the matrix of the meteorite. Although this subsolidus alteration makes it extremely difficult to determine the original fO2 of the parental magma for NWA 8159, there is evidence that during the initial stages of crystallization the fO2 was modestly reducing (∼IW + 1). Potential manifestations of more reducing magmatic conditions include P-rich cores in the olivine and low Fe3+ in silicates (plagioclase, pyroxene). Further, if analogous to all other depleted shergottites, NWA 8159 initially crystallized under reducing conditions. This decoupling between oxygen fugacity and isotopic-trace element characteristics suggests that basalts derived from the martian mantle interacted with the crust in ways that significantly influenced redox history and volatile element isotopic composition (Cl, S), without dramatically modifying many of its radiogenic isotope and trace element mantle fingerprints.

Solubility of gold in the reduced carbonic fluid

The first experimental data on the gold solubility in the fluid with composition CO-CO2 and C-O-S with small water content at the pressure 200 MPa and temperature 950°С are reported. Solubility in the fluid C-O-S is about 27 ppm. Estimation of the solubility of gold in fluid CO-CO2 with 10–15 mol.% CO is less accurate: its value is at least 2–3 ppm, but it probably can reach 200–300 ppm. The high solubility of gold found in this work, and previously Pt (Simakin et al., 2016), in the reduced carbon dioxide can explain the formation of these noble metal ore occurrence in the Gluli intrusion (Polar Siberia) by their fluid extraction and re-deposition at temperatures below the solidus. The reducing of a substantially oxidized carbon dioxide fluid, established by mineralogical sensors, was probably caused by subsolidus oxidation of olivine.

Gold Solubility in Reduced Carbon-Bearing Fluid

The paper presents first experimental data on gold solubility in CO–CO2 and C–O–S fluids with a low H2O concentration under reduced conditions, a pressure of 200 MPa, and a temperature of 950°C. Gold solubility in C–O–S fluid is approximately 27 ppm. The estimate of gold solubility in CO–CO2 fluid with 10–15 mol % CO is less accurate, and the solubility is no lower than 2–3 ppm but may reach 200–300 ppm. The high gold solubility in reduced CO2 fluid determined in the course of this research, and our earlier estimates of high platinum solubility (Simakin et al., 2016), may explain the deposition of mineralization of these noble metals in the Guli intrusion, polar Siberia, as a result of fluid extraction of these metals and their redeposition at a temperature slightly below the solidus. The reduction of the largely oxidized CO2 fluid, which was determined using mineralogical sensors, was likely related to the subsolidus oxidation of olivine.

Majindeite, Mg2Mo3O8, a new mineral from the Allende meteorite and a witness to post-crystallization oxidation of a Ca-Al-rich refractory inclusion

Majindeite (IMA 2012-079), Mg_2Mo_3O_8, is a new mineral, occurring as submicrometer-sized crystals with Ni-Fe and Ru-Os-Ir alloys, ± apatite and Nb-oxide. The observed assemblages are partially or wholly enclosed by MgAl_2O_4 spinel in a Type B1 Ca-Al-rich inclusion, ACM-2, from the Allende CV3 carbonaceous chondrite. The type majindeite has an empirical formula of (Mg_(1.57)Fe_(0.43))Mo_(3.00)O_8, and a nolanite-type P6_3mc structure with a = 5.778 A, c = 9.904 A, V = 286.35 A3, and Z = 2, leading to a calculated density of 5.54 g/cm^3. Majindeite likely formed during the subsolidus oxidation of Mo-rich precursor phase(s) included in Fe-Ni rich alloys in a system that was open to O, Mg, and Ca, which were derived externally and introduced via cracks, subgrain boundaries, and/or surfaces exposed at the exterior of the spinel. If magnetite existed in the phase assemblage, it was lost due to Fe volatilization prior to the formation of majindeite. The immediate precursor to majindeite was likely kamiokite. Majindeite formed during an oxidation event contemporaneous with or postdating the formation of grossular-rich veins in melilite. Kamiokite, the Fe-rich analog of majindeite, also occurs in ACM-2 but only within phase assemblages that contain magnetite and which are entirely enclosed in melilite ± alteration products. Here, grossular-rich veins are not observed and the coexisting awaruites are more Fe-rich than those observed with majindeite. As with majindeite, the precursors for kamiokite grains were also likely to have been Mo-rich alloys, but the Mo-oxide remained magnetite-saturated throughout the alteration process and therefore remained Fe-rich.

Generation of Forsteritic Olivine (Fo99·8) by Subsolidus Oxidation in Basaltic Flows

We identify olivine grains with compositions up to Fo99·8, which are found in multiple primitive basaltic lava flows from a monogenetic volcano in the Big Pine Volcanic Field, California, USA. In this study, we show that the forsterite in these basalts formed by subsolidus recrystallization in a high-fO2 environment. Olivine compositions are bimodal, with flows having either all normal compositions (Fo74·9^94·4) or highly forsteritic (Fo97·2^99·8) compositions. In many grains, the subhedral forsteritic olivine has a hematite and clinopyroxene rim, and internal parallel-oriented planes of hematite, clinopyroxene and orthopyroxene. Results of isotopic, chemical, crystallographic, petrographic and mineralogical analyses show that the forsterite formed through subsolidus oxidation of olivine phenocrysts. The forsteritic olivines generally occur in the thinner flows. We infer that a rapidly emplaced sequence of thin, vesicular, spatter-fed flows allowed the original olivine phenocrysts to become repeatedly reheated while exposed to air. Our study required sampling each flow, so it was difficult to avoid the altered portions of the thinner flows. Other studies would tend to avoid such flows, which may account for why such forsteritic olivines have not been more widely recognized.

Relationship of amphibole composition to host-rock geochemistry: the A-type gabbro-granite Wentworth pluton, Cobequid shear zone, eastern Canada

The Wentworth pluton is the most alkaline of a series of Late Devonian metaluminous to mildly peralkaline plutons in the Cobequid shear zone. It includes numerous granitoid bodies of slightly differing composition, which may contain sodic, sodic-calcic or calcic amphiboles. This study examines the relative importance of host rock composition including volatiles, f O2, and temperature in controlling the type of amphibole present. Electron microprobe analyses and back-scattered electron images were used to characterize the amphiboles and whole-rock geochemical analyses were made from the host granites. Magmatic sodic amphiboles are arfvedsonite, ferrorichterite and katophorite. Ferrowinchite and riebeckite developed by subsolidus oxidation reactions with loss of fluorine. Sub-solidus changes in Fe-Ti oxides make reliable estimation of f O2 difficult. Regionally, sodic amphiboles are found only in the Wentworth pluton, suggesting the importance of peralkaline geochemistry and perhaps higher temperature as a result of voluminous gabbro in the pluton. However, the presence of sodic or calcic amphibole in granite with >73% SiO2 in the Wentworth pluton shows no systematic difference with either host-rock geochemistry or distance from the gabbroic heat source, except that rocks with sodic amphibole have high Li, suggesting abundance of free water.

Opaque mineralogy and magnetic properties of selected banded iron‐formations, Hamersley Basin, Western Australia

The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study.

Mineralogy and geothermometry of high-temperature rhyolites from the central and western Snake River Plain

Voluminous mid-Miocene rhyolitic ash-flow tuffs and lava flows are exposed along the northern and southern margins of the central and western Snake River Plain. These rhyolites are essentially anhydrous with the general mineral assemblage of plagioclase ±sanidine ± quartz + augite + pigeonite ± hypersthene ± fayalitic olivine + Fe-Ti oxides + apatite + zircon which provides an opportunity to compare feldspar, pyroxene, and Fe-Ti oxide equilibration temperatures for the same rocks. Estimated pyroxene equilibration temperatures (based on the geothermometers of Lindsley and coworkers) range from 850 to 1000°C, and these are well correlated with whole-rock compositions. With the exception of one sample, agreement between the two-pyroxene thermometers tested is well within 50°C. Fe-Ti oxide geothermometers applied to fresh magnetite and ilmenite generally yield temperatures about 50 to 100°C lower than the pyroxene temperatures, and erratic results are obtained if these minerals exhibit effects of subsolidus oxidation and exsolution. Results of feldspar thermometry are more complicated, and reflect uncertainties in the thermometer calibrations as well as in the degree of attainment of equilibrium between plagioclase and sanidine. In general, temperatures obtained using the Ghiorso (1984) and Green and Usdansky (1986) feldspar thermometers agree with the pyroxene temperatures within the respective uncertainties. However, uncertainties in the feldspar temperatures are the larger of the two (and exceed ±60°C for many samples). The feldspar thermometer of Fuhrman and Lindsley (1988) produces systematically lower temperatures for many of the samples studied. The estimated pyroxene temperatures are considered most representative of actual magmatic temperatures for these rhyolites. This range of temperatures is significantly higher than those for rhyolites from many other suites, and is consistent with the hypothesis that the Snake River Plain rhyolitic magmas formed by partial fusion of relatively dry (e.g. granulitic) crustal lithologies.

Transmission electron microscopy of subsolidus oxidation and weathering of olivine

Olivine crystals in basaltic andesites which crop out in the Abert Rim, south-central Oregon have been studied by high-resolution and analytical transmission electron microscopy. The observations reveal three distinct assemblages of alteration products that seem to correspond to three episodes of olivine oxidation. The olivine crystals contain rare, dense arrays of coherently intergrown Ti-free magnetite and inclusions of a phase inferred to be amorphous silica. We interpret this first assemblage to be the product of an early subsolidus oxidation event in the lava. The second olivine alteration assemblage contains complex ordered intergrowths on (001) of forsterite-rich olivine and laihunite (distorted olivine structure with Fe3+ charge balanced by vacancies). Based on experimental results for laihunite synthesis (Kondoh et al. 1985), these intergrowths probably formed by olivine oxidation between 400 and 800°C. The third episode of alteration involves the destruction of olivine by low-temperature hydrothermal alteration and weathering. Elongate etch-pits and channels in the margins of fresh olivine crystals contain semi-oriented bands of smectite. Olivine weathers to smectite and hematite, and subsequently to arrays of oriented hematite crystals. The textures resemble those reported by Eggleton (1984) and Smith et al. (1987). We find no evidence for a metastable phase intermediate between olivine and smectite (“M” — Eggleton 1984). The presence of laihunite exerts a strong control on the geometry of olivine weathering. Single laihunite layers and laihunite-forsteritic olivine intergrowths increase the resistance of crystals to weathering. Preferential development of channels between laihunite layers occurs where growth of laihunite produced compositional variations in olivine, rather than where coherency-strain is associated with laihunite-olivine interfaces.

Vermicular orthopyroxene-magnetite symplectites from the Wateranga layered mafic intrusion, Queensland, Australia

Orthopyroxene-magnetite intergrowths (symplectites), partly or completely surrounding olivine, are described from the Wateranga layered mafic intrusion, Queensland, Australia. The texture occurs in unmetamorphosed plagioclase-rich norites, olivine gabbros and troctolites in which the primary minerals are olivine (Fo 63–69) orthopyroxene (En 66–72), clinopyroxene (Wo 42En 42Fs 16), plagioclase (An 49–65), hornblende, ilmenite, magnetite and sulphides. Symplectites range from incipient fine grained developments around corroded olivine grains to intricately formed pseudomorphs after olivine and slow a consistent orthopyroxene/magnetite ratio. Orthopyroxene in symplectites is commonly in optical continuity with surrounding magnetite-free orthopyroxene rims. Later intercumulus hornblended has replaced orthopyroxene. There is marked chemical similarity between primary and simplectite, orthopyroxenes and magnetites. Textures similar to those described here are considered elsewhere to have formed at a late magmatic stage or by solid state reactions involving subsolidus oxidation of olivine. In the Wateranga intrusion textural relations, the chemical similarity between primary and symplectite phases, and the consistent volume proportions of magnetite and orthopyroxene in the intergrowths suggest that they developed during late magmatic crystallization.

On the spectral reflectance and maturation darkening of lunar soils

Optical absorption and diffuse reflectance spectra were obtained for simulated lunar glasses of four different compositions, both in their as-quenched (reduced) states and following mild subsolidus oxidation. The transmission spectra, when normalized by the FeO content of the glasses, differed from one another only in the relative intensity of an unresolved band in the UV. For fixed melting conditions the strength of this band in the as-quenched glasses increased with increasing FeO, or with increasing TiO2 for a fixed FeO content. Electron spin resonance (ESR) experiments have demonstrated the absence of Fe3+ or Ti3+ and the presence of metallic iron in these materials; all other transition-group elements were excluded in preparation. The unresolved UV absorption edge in the as-quenched reduced glasses is therefore tentatively ascribed to Fe2+→Ti4+ intervalency charge transfer transitions. A similar UV edge was also produced by oxidation, leading to the conclusion that the assignment of this band would be ambiguous in the absence of an independent determination of the valence states of Fe and Ti. The relationship between the transmission spectra of polished samples and the reflectance spectra of sieved powders of the same materials is shown to be well described by the Kubelka-Munk approximation. Using this insight, it is possible to understand the spectral characteristics both of oxidation darkening of synthetic glass powders and of maturation darkening of lunar soils in terms of (1) the growth of the aforementioned charge transfer band(s) and (2) the development of opaque surface phases. It is shown that mechanism (1) is of primary importance in lunar highland materials and that mechanism (2) dominates in mare materials. The present results, coupled with previous findings, suggest that lunar soil maturation darkening may result from vitrification only if accompanied by (a) enrichment in the elements Fe and Ti, (b) changes in valence states of these elements, (c) partial crystallization of opaque phases such as iron, ilmenite or magnetite, or (d) a combination of (a), (b), and (c).

Ilmenite-orthopyroxene intergrowths from the moon and the Skaergaard intrusion

Myrmekitic or eutectic-like intergrowths of ilmenite and orthopyroxene, which texturally resemble intergrowths from nodules in kimberlites have been observed in lunar breccia 60016, 92 and in Lower Zone a of the Skaergaard intrusion. The lunar sample is similar in mineral chemistry to kimberlite occurrences, while the Skaergaard sample is not as Mg-rich. In all cases, however, there is a systematic distribution of Mg between orthopyroxene and ilmenite. The similar textures were formed by different processes: the lunar intergrowth is probably a eutectic-type texture resulting from crystallization of a melt, while the Skaergaard intergrowth formed by a process of sub-solidus oxidation.

Magnetites and ilmenites in the Somerset dam layered basic intrusion, southeastern Queensland

16 chemical analyses and 71 microprobe analyses of magnetites and ilmenitesin the Somerset Dam layered basic intrusion demonstrate that the chemistry of these oxides is a sensitive indicator of crystallisation and subsolidus conditions. Microprobe analyses prove the existence of two chemically distinct types of ilmenite — secondary ilmenite formed by subsolidus oxidation of magnetite-ulvöspinel ss, and primary ilmenite, which has been only partially affected by the subsolidus oxidation. Application of experimental Tf O a X relations shows that the inferred crystallisation conditions approximately the NNO buffer. Subsolidus cooling appears to have occurred approximately parallel to the NNO buffer, and was probably buffered partly an H 2O-rich fluid, and partly by the Fe-Ti oxides. The proportions of H 2O and of sulphur compounds appear to have determined the temperatures at which subsolidus oxidation of magnetite ceased.

The Eulogie Park Gabbro, a layered basic intrusion from Eastern Queensland

Abstract The Eulogie Park Gabbro comprises approximately 65 saucer‐shaped layers consisting of olivine gabbros, ferrigabbros, troctolites, leucogabbros, gabbros and magnetite‐rich rocks. The intrusion is divisible into three main parts: a lower portion consisting mainly of ferrigabbros and magnetite‐rich layers, a middle portion composed of troctolites and olivine gabbros, and an upper portion consisting mainly of olivine gabbros and leucogabbros. The minimum stratigraphic thickness is 3,000 ft (900 m). Modal compositions and textures exhibit marked changes across the boundaries of the layers. Mineral compositions vary irregularly with height in the layered sequence, and small oscillations are common. Compositional data suggest that temperature increased during the first half of the crystallisation period, and then remained fairly constant. Chemical and petrographie studies of the iron‐titanium oxides indicate that the primary phase was an ulvospinel‐magnetite solid solution. Subsolidus oxidation resulted...