An experimental study of anorthosite dissolution in lunar picritic magmas: Implications for crustal assimilation processes

Experiments characterizing the kinetics of anorthosite dissolution in lunar picritic magmas (very low-Ti, low-Ti, and high-Ti picritic glasses) were conducted at 0.6 GPa and 1250-1400 °C using the dissolution couple method. Reaction between the anorthosite and lunar picritic magmas at 1250-1300 °C produced a spinel + melt layer. Reaction between the anorthosite and an olivine-saturated low-Ti magma at 1250-1300 °C produced a crystal-free region between the spinel + melt layer and the olivine-saturated magma. The anorthosite dissolution experiments conducted at 1400 °C simply dissolved anorthosite and did not result in a crystal-bearing region. The rate of anorthosite dissolution strongly depends on temperature and composition of the reacting melt. Concentration profiles that develop during anorthosite dissolution are nonlinear and extend from the picritic glass compositions to anorthite. These profiles feature a large and continuous variation in melt density and viscosity from the anorthosite-melt interface to the initial picritic magmas. In both the low-Ti and high-Ti magmas the diffusive fluxes of TiO₂, Al₂O₃, and SiO₂ are strongly coupled to the concentration gradients of CaO and FeO. Anorthosite dissolution may play an important role in producing the chemical variability of the lunar picritic magmas, the origin of spinel in the lunar basalts and picritic glasses, and the petrogenesis of the high-Al basalts.     

Isotopic and elemental abundances of copper and zinc in lunar samples, Zagami, Pele’s hairs, and a terrestrial basalt

We used ICP–MS to measure the elemental concentrations and isotopic abundances of Cu and Zn in: nine Ti-rich lunar basalts (10017, 10022, 10024, 10057, 70215, 71055, 74255, 75055, and 75075); size-separated samples prepared by sieving of pyroclastic black glass 74001, orange glass 74022, and the lunar soils 15021, 15231, 70181, and 79221; a basalt from the Piton des Neiges volcano, Reunion Island; two samples of Pele’s hairs from the Nyiragongo volcano, Democratic Republic of Congo, and the martian meteorite Zagami.The isotopic fractionation of zinc in lunar basalts and Zagami is mass dependent relative to a terrestrial standard (JMC 400882B). These and published results imply that lunar, terrestrial, meteoritic, and perhaps martian zinc all come from one or more reservoirs linked by mass-dependent fractionation processes. Relative to terrestrial basalts, Ti-rich lunar basalts are enriched in the heavier isotopes of Cu and Zn: we find for Ti-rich lunar basalts the following ranges and averages ±1 − σ (‰): δ⁶⁵Cu/⁶³Cu ≡ δ⁶⁵Cu, 0.1–1.4, 0.5 ± 0.1‰ (N = 7); δ⁶⁶Zn/⁶⁴Zn ≡ δ⁶⁶Zn = 0.2–1.9, 1.2 ± 0.2‰ (N = 8; 10017 excluded). For two terrestrial samples, we find δ⁶⁶Zn  +0.3‰ and δ⁶⁵Cu  0‰, which are consistent with published values. The differences between the lunar basalts and terrestrial basalts could reflect minor, planetary-scale vaporization or igneous processes on the Moon.Data for size separates of the pyroclastic glasses 74001 and 74220 confirm the well-known surface correlation of Cu and Zn, but modeling calculations reveal no sharp differences between either the elemental ratios or the isotopic composition of grain interiors and exteriors. The absence of such differences indicates that the isotopic compositions for bulk samples are dominated by a light-isotope-rich surface component.Data for size separates of lunar soils also confirm the surface correlation of Cu and Zn, but an enrichment of heavy rather than light isotopes. Averages for bulk lunar soils from this work and the literature are (‰): δ⁶⁵Cu, from 1.4 to 4.1, average 3.0 ± 0.3 (N = 9); δ⁶⁶Zn, from 2.2 to 6.4, average 4.0 ± 0.3 (N = 14). As with the glasses, in all but soil 15231 our data show no strong differences between the isotopic composition of soil sub-samples with small and large grains.The size of the isotopic fractionation inferred for the surface component in the soils is 3× smaller than predicted by a published model of sputtering primarily by solar particles. At the same time, the observed fractionation is larger than predicted by calculations based on a model of micrometeorite impact heating and hydrodynamic quenching. Because impact heating appears unable to explain the observations, we conclude that sputtering must be important even though samples with very large isotopic fractionation of Cu and Zn have not yet been found.     

Basaltic magmatism on the Moon: A perspective from volcanic picritic glass beads

It is widely accepted that basaltic magmas are products of partial fusion of periodotite within planetary mantles. As such, they provide valuable insights into the composition, structure, and processes of planetary interiors. Those compositions which approach primary melt compositions provide the most direct information about planetary interiors and serve as a starting point to understand basaltic evolution. Within the collection of lunar samples returned by the Apollo and Luna missions are homogeneous, picritic glass beads of volcanic origin. These picritic glasses are our closest approximations to primary magmas. As such, these glass beads provide a unique perspective concerning the origin of mare basalts, the characteristics of the lunar interior, and processes in the early differentiation of the Moon. We have obtained trace element data for these picritic glasses using SIMS techniques. These data and literature isotopic and experimental data on the picritic glasses are placed within the framework of mare basaltic magmatism.The volcanic glasses are very diverse in their trace element characteristics, for example, they have a wide range of REE pattern shapes and concentrations. Like the crystalline mare basalts, all picritic glasses have a negative Eu anomaly. Unlike the crystalline mare basalts, there is little correlation between the size of the Eu anomaly and overall REE concentrations. Trace element differences among the various glasses suggests that a KREEP component was incorporated into their mantle source. This implies large scale mixing of the “Lunar Magma Ocean”-derived cumulate pile. Subtle differences among glasses suggest that local mixing of sources may also have been an important process. Preservation of subtle chemical differences in the picritic glasses and crystalline basalts may be interpreted as indicating that they were produced by small to moderate degrees of partial melting and that the lunar mantle did not experience extensive melting during episodes of mare volcanism.Several lines of evidence are consistent with the view that the picritic glasses were derived from mantle sources that were compositionally distinct from the sources for crystalline mare basalts. These are parallel, but no common, liquid lines of descent; chemical differences between picritic glasses and the more primitive crystalline mare basalts; experimental studies indicating that the picritic glasses are multiply saturated at depths greater than that of the mare basalts; differences in lead isotopic data; and the mode of eruption (i.e., fire fountaining for glass beads). These data also provide circumstantial evidence that suggests that the picritic glasses were derived from a source somewhat more volatile-rich than that of the mare basalts.Several petrogenetic models are suggested by the trace element characteristics of the picritic glasses:

• 1.(1) Partial melting of heterogeneous lunar mantle at depths greater than 300 km to produce the parental magmas (picritic) for both the mare basalts and picritic glasses. Picritic magmas represented by glass beads were erupted to the surface with small degrees of fractional crystallization while mare basalts were produced by larger degrees of fractional crystallization (15–30%) of similar (but not identical) picritic magmas.
• 2.(2) Picritic magmas represented by the glass beads were generated at depths greater than 400 km in a volatile-enriched (relative to the mare basalt source) heterogeneous mantle while mare basalts are fractional crystallization products of picritic magmas generated at depths of less than 400 km.
• 3.(3) The picritic magmas represented by the glass beads represent polybaric melting that initiated at depths of at least 1000 km. A primitive mantle component or less processed cumulate mantle components may have been involved in the generation of the picritic glasses in any of these models.

     

Iron-Titanium Oxide Geothermometry and Petrogenesis of Lava Flows and Dykes from Mandla Lobe of the Eastern Deccan Volcanic Province, India

Compositional studies on different forms of magnetite, ulvospinel, ilmenite and hematite mineral phases occurring in 37 lava flows and 6 dykes of the Mandla lobe are presented in this paper. Ilmenite (0001) in equilibrium with titanomanetite show high values of temperature of equilibration, ranging from 1172–974°C, for high alumina quartz normative tholeiitic lava flows of Chemical Type - A; 1129–1229°C for low alumina quartz normative tholeiitic lava flows of Chemical Type - B; 1283–1124°C for tholeiitic lava flows of Chemical Type - F and 1243°C and 99O°C for two diopside olivine normative tholeiite flows of Chemical Type D. High olivine normative flows of Chemical Type - G and H show 1095°C and 1092°C respectively. Whereas, high hypersthene normative tholeiite flow of Chemical me C shows temperature of 1187°C. Data plots disposition over iron-titanium oxide equilibration temperature vs – logfo₂, diagram for Mandla lava flows and other parts of the Deccan (Igatpuri, Mahabaleshwer, Nagpur and Sagar areas) revealed that tholeiitic (evolved) basalt of the eastern Deccan volcanic province formed at high temperatures whereas, picritic (primitive) lavas of Igatpuri and tholeiitic basalt of Mahabaleshwar areas were formed at low temperatures. Mahabaleshwer basalts follow FMQ (fayalite-magnetite-quartz) buffer curve but, plots of the Mandla basalts lie above this curve indicating higher temperatures of crystallisation of ilmenite-titanomagnetite than that of the lava flows from other parts of Deccan 'Raps. The eastern Deccan Traps are most evolved types of lava as characterised by its low Mg-number and Ni content whereas, Igatpuri lava flows are picritic (primitive), having high Mg-number and Ni contents. Temperature vs FeO + Fe₂O₃ / FeO + Fe₂O₃ + MgO ratio data plots for Mandla and other Deccan lava flows and liquidus data for Hawaiian tholeiites, indicated that Igatpuri basalts lie parallel to the liquidus line of Hawaiian tholeiite but at lower temperatures. Large data plots of Mandla lava flows lie along the liquidus line of the Hawaiian lava. The highly vesicular nature of compound lava flows having large amount of volatile is responsible for low temperature values whereas, lava flows represented by high temperatures show high modal values of glass and opaque minerals.     

Fluorine and chlorine abundances in lunar apatite: Implications for heterogeneous distributions of magmatic volatiles in the lunar interior

Apatite has been analyzed from mare basalts, the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks using an electron probe microanalysis routine developed specifically for apatite. We determined that all the lunar apatite grains analyzed are predominantly fluorine rich; however, they also contain varying concentrations of chlorine and a missing structural component that, after ruling out other possibilities, we attribute to OH. Apatite grains from mare basalts are compositionally distinct from the apatite grains in the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks, which all had similar apatite compositions. Apatite grains in mare basalts are depleted in chlorine, and many of the analyzed grains have stoichiometry that suggests a significant OH component (i.e., >0.08 structural formula units), whereas apatite grains in the magnesian suite, alkali suite, and KREEP-rich impact melts are enriched in chlorine and do not typically have a missing structural component that could be attributed to OH⁻ (within the detection limit of 0.08 sfu). From these data, we infer that residual liquids in the mare basalts were enriched in H₂O and fluorine relative to chlorine at the time of apatite crystallization, whereas residual liquids in magnesian-suite, alkali-suite, and KREEP-rich impact melts were enriched in chlorine relative to H₂O and fluorine at the time of apatite crystallization. The relative volatile abundance that we determined for the mare basalts is identical to the previously determined relative volatile abundance for the lunar picritic glasses. This result indicates that the observed relative volatile abundance signature of the picritic glass source is the same as that in the mare basalt source regions. The magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks likely reflect a volatile source with different volatile abundances than the sources of mare volcanics. Moreover, the magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks may reveal the relative volatile abundance of urKREEP, the residual melt of the magma ocean. This difference in relative magmatic volatile abundance among the lithologic groups investigated cannot be explained by degassing of a single source composition (relative to magmatic volatiles). The most reasonable explanation for the compositional disparity is a difference in the relative volatile abundances in the magmatic source regions of the Moon. Therefore, we conclude that the Moon has a heterogeneous distribution of magmatic volatiles within its interior, with a chemical divide (with respect to magmatic volatiles) existing between magmas that arise by partial melting of the lunar mantle and magmas that have seen significant contamination by a KREEP component.     

Fluid regime and diamond formation in the reduced mantle: Experimental constraints

The composition and potential diamond productivity of C–O–H fluids that could exist in the reduced regions of the Earth’s upper mantle and in the mantles of Uranus and Neptune were studied in experiments at 6.3 GPa and 1400–1600 °C and durations of 15–48 h. Hydrogen fugacity in the fluid phase was controlled by the Mo–MoO₂ or Fe–FeO buffers, using a specially modified double-capsule method. The oxygen fugacity in the samples was controlled by adding different amounts of water, stearic acid, anthracene, and docosane to a graphite charge. At high P–T conditions, the degree of decomposition of the heavy hydrocarbons added to the charge was 99.9%. The composition of the fluids coexisting with graphite/diamond in the buffered experiments varied from H₂O  H₂ > CH₄ (at fO₂ somewhat lower than the “water maximum”) to H₂ > CH₄ > (C₂H₄ + C₂H₆)>C₃H₈ (in C–H system). In the C–H system the maximum concentrations of major species in the synthesized fluid were: H₂ = 79 mol.% and CH₄ = 21 mol.%. The composition of the H₂-rich fluids, which were synthesized at 6.3 GPa and 1400–1600 °C for the first time, differs considerably from that of the ultra-reduced CH₄-rich fluids stable at 2.0–3.5 GPa and 1000–1300 °C. Thermodynamic calculations of the reduced C–O–H fluids at the P–T conditions of the experiments revealed CH₄-rich compositions (CH₄  H₂ > (C₂H₄ + C₂H₆)>C₃H₈), which however drastically differed from the synthesized compositions. The rates of diamond nucleation and growth in the experiments depended on the fluid composition. Diamond crystallization had a maximum intensity in the pure aqueous fluids, while in the H₂-rich fluids no diamond formation was observed. Only metastable graphite precipitated from the ultra-reduced fluids. The type of the initial hydrocarbon used for the fluid generation did not affect this process.     

Compressibility of molten Apollo 17 orange glass and implications for density crossovers in the lunar mantle

We performed density measurements on a synthetic equivalent of lunar Apollo 17 74,220 “orange glass”, containing 9.1 wt% TiO₂, at superliquidus conditions in the pressure range 0.5-8.5 GPa and temperature range 1723-2223 K using the sink/float technique. In the lunar pressure range, two experiments containing pure forsterite (Fo₁₀₀) spheres at 1.0 GPa and 1727 K, and at 1.3 GPa-1739 K, showed neutral buoyancies, indicating that the density of molten orange glass was equal to the density of Fo₁₀₀ at these conditions (3.09 ± 0.02 g cm⁻³). A third tight sink/float bracket using Fo₉₀ spheres corresponds to a melt density of 3.25 ± 0.02 g cm⁻³ at ∼2.8 GPa and ∼1838 K.Our data predict a density crossover for the molten orange glass composition with equilibrium orthopyroxene at ∼2.8 GPa, equivalent to a depth of ∼600 km in the lunar mantle, and a density of ∼3.25 g cm⁻³. This crossover depth is close to the orange glass multiple saturation point, representing its minimum formation depth, at the appropriate oxygen fugacity (2.8-2.9 GPa). A density crossover with equilibrium olivine is predicted to fall outside the lunar pressure range (>4.7 GPa), indicating that molten orange glass is always less dense than its equilibrium olivines in the Moon. Our data therefore suggest that that lunar liquids with orange glass composition are buoyant with respect to their source region at P < ∼2.8 GPa, enabling their initial rise to the surface without the need for additional external driving forces.Fitting the density data to a Birch-Murnaghan equation of state at 2173 K leads to an array of acceptable solutions ranging between 16.1 and 20.3 GPa for the isothermal bulk modulus K₂₁₇₃ and 3.6-8 for its pressure derivative K′, with best-fit values K₂₁₇₃ = 18.8 GPa and K′ = 4.4 when assuming a model 1 bar density value of 2.86 g cm⁻³. When assuming a slightly lower 1 bar density value of 2.84 g cm⁻³ we find a range for K₂₁₇₃ of 14.4-18.0 and K′ 3.7-8.7, with best-fit values of 17.2 GPa and 4.5, respectively.     

Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland Greenstone Belt, northwestern Tanzania

The late Archaean volcanic rocks of the Rwamagaza area in the Sukumaland Greenstone Belt consists of basalts and basaltic andesites associated with volumetrically minor rhyodacites and rhyolites. Most basalts and basaltic andesites yield nearly flat patterns (La/Sm_CN = 0.89–1.34) indicating derivation by partial melting of the mantle at relatively low pressure outside the garnet stability field. On primitive mantle normalized trace element diagrams, the basalts and basaltic andesites can be subdivided into two groups. The first group is characterised by moderately negative Nb anomalies (Nb/La_pm = 0.51–0.73, mean = 0.61 ± 0.08) with slight enrichment of LREE relative to both Th and HREE. The second group is characterised by nearly flat patterns with no Nb anomalies (Nb/La_pm = 0.77 ± 0.39). The observed Nb and Th anomalies in the Rwamagaza basalts and basaltic andesites, cannot be explained by alteration, crustal contamination or melt–solid equilibria. Rather, the anomalies are interpreted, on the basis of Nb–Th–La–Ce systematics, as having formed by partial melting of a heterogeneous mantle consisting of variable mixtures of components derived from two distinct sources. These sources are depleted mantle similar to that generating modern MORB and an LREE-enriched and HFSE-depleted source similar to that feeding volcanism along modern convergent margins.The rhyolites are characterised by high Na₂O/K₂O ratios (>1) and Al₂O₃ (>15 wt.%), low HREE contents (Yb = 0.24–0.68 ppm) leading to highly fractionated REE patterns (La/Yb_CN = 18.4–54.7) and large negative Nb anomalies (Nb/La_pm = 0.11–0.20), characteristics that are typical of Cenozoic adakites and Archaean TTG which form by partial melting of the hydrated basaltic crust at pressures high enough to stabilize garnet ± amphibole. The Rwamagaza basalts and basaltic andesites are geochemically analogous to the Phanerozoic Mariana Trough Back Arc Basin Basalts and the overall geochemical diversity of Rwamagaza volcanic rocks is interpreted in terms of a geodynamic model involving the interaction of a depleted mantle, a melting subducting oceanic slab in a back arc setting.     

Picritic porphyrites and associated basalts from the remnant Comei Large Igneous Province in SE Tibet: records of mantle‐plume activity

Picritic porphyrites and associated basalts are found in Lower Cretaceous slates within the recently identified remnant Comei Large Igneous Province, SE Tibet. They provide an opportunity to explore the mantle potential temperature of this large igneous province and the genetic correlation between the picritic porphyrites and associated basalts. The high levels of MgO and CaO in olivine phenocrysts from the picritic porphyrites indicate that these olivines crystallized from high‐Mg magmas. By deducting the accumulated olivines, we determined a parental magma MgO content for the picritic porphyrites of c. 20% MgO, corresponding to a mantle potential temperature of >1550 °C for the parental magma. Such a high temperature provides solid evidence for a mantle‐plume origin of the remnant Comei Large Igneous Province. Parallel trace‐element patterns and similar ε_Nd (T) and (¹⁷⁷Os/¹⁷⁸Os)_T values in the picritic porphyrites and associated basalts suggest a common hot mantle source region for their generation.     

Isotopic composition of zinc, copper, and iron in lunar samples

We determined by ICP-MS the concentrations and isotopic ratios of Fe, Cu, and Zn in the Ti-rich lunar basalt 74275, in the lunar orange glass 74220, and in up to 10 lunar soils, namely, 14163, 15231, 64501, 66041, 68841, 69941, 70011, 72501, 75081, and 76501. Two analyses of zinc in lunar basalt 74275 give δ⁶⁶Zn = 0.17‰ and 0.75‰, values within the range of those measured in terrestrial basalts; copper in lunar basalt 74275 has δ⁶⁵Cu  +1.4‰, which is isotopically heavier than values observed in terrestrial basalts. In the orange glass, we measured δ⁵⁶Fe = −0.24‰, δ⁶⁵Cu = −0.42‰, and δ⁶⁶Zn  −3.6‰. These values of δ are more negative than those obtained for 74275 and for typical lunar basalts, but for Cu, comparable to those observed in terrestrial sulfides and meteorites. In lunar soils we found 0.11‰  δ⁵⁶Fe  0.51‰, 2.6‰  δ⁶⁵Cu  4.5‰, and 2.2‰  δ⁶⁶Zn  6.4‰. Insofar as we can generalize from a small sample set, S, Fe, Cu, Zn, and Cd show similar trends in isotopic fractionation on the Moon. Lunar basalts have nearly terrestrial isotopic ratios. Relative to the lunar basalt 74275, the pyroclastic glass 74220 is enriched in the lighter isotopes of Fe, Cu, and Zn, and the soils are enriched in the heavier isotopes of Fe, Cu, and Zn. The patterns in the basalts are likely inherited from the source material; the light-isotope enrichments seen in the orange glass originated during lava fountaining or, less probably, during partial condensation of vapor; and the heavy-isotope enrichments in the lunar soils were likely created by a combination of processes that included micrometeorite vaporization and sputtering. In the orange glass, the light-isotope enrichments (relative to lunar basalts) of Zn are larger than those of Cu. If these enrichments reflect accurately the isotopic composition of the gas, they suggest that Cu is more volatile than Zn in the liquid from which the gas derived. A simple model built on the known flux of micrometeorites to the lunar surface and a published estimate that micrometeorites generate 10 times their own mass of vapor, predicts heavy-isotope enrichments comparable to those observed in soils but only if the regolith gardening rate is set at about one twentieth of the generally accepted value of 1 cm/My. This discrepancy may reflect the difference in the time constants for micrometeorite milling and decimeter-scale gardening, or the importance of sputtering.     

Experimental constraints on Pt, Pd and Au partitioning and fractionation in silicate melt–sulfide–oxide–aqueous fluid systems at 800 °C, 150 MPa and variable sulfur fugacity

We have performed experiments to constrain the effect of sulfur fugacity (fS₂) and sulfide saturation on the fractionation and partitioning behavior of Pt, Pd and Au in a silicate melt–sulfide crystal/melt–oxide–supercritical aqueous fluid phase–Pt–Pd–Au system. Experiments were performed at 800 °C, 150 MPa, with oxygen fugacity (fO₂) fixed at approximately the nickel–nickel oxide buffer (NNO). Sulfur fugacity in the experiments was varied five orders of magnitude from approximately log fS₂ = 0 to log fS₂ = −5 by using two different sulfide phase assemblages. Assemblage one consisted initially of chalcopyrite plus pyrrhotite and assemblage two was loaded with chalcopyrite plus bornite. At run conditions pyrrhotite transformed compositionally to monosulfide solid solution (mss), chalcopyrite to intermediate solid solution (iss), and in assemblage two chalcopyrite and bornite formed a sulfide melt. Run-product silicate glass (i.e., quenched silicate melt) and crystalline materials were analyzed by using both electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The measured concentrations of Pt, Pd and Au in quenched silicate melt in runs with log fS₂ values ranging from approximately 0.0 to −5.0 do not exhibit any apparent dependence on fS₂. The measured Pt, Pd and Au concentrations in mss do vary as a function of fS₂. The measured Pt, Pd and Au concentrations in iss do not appear dependent on fS₂. The data suggest that fS₂, working in concert with fO₂, via the determinant role that these variables play in controlling the magmatic sulfide phase assemblage and the solubility of Pt, Pd and Au as lattice bound components in magmatic sulfide phases, is a controlling factor on the budgets of Pt, Pd and Au during the evolution of magmatic systems.     

The origin of refractory minerals in comet 81P/Wild 2

Refractory Ti-bearing minerals in the calcium-, aluminum-rich inclusion (CAI) Inti, recovered from the comet 81P/Wild 2 sample, were examined using analytical (scanning) transmission electron microscopy (STEM) methods including imaging, nanodiffraction, energy-dispersive spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Inti fassaite (Ca(Mg,Ti,Al)(Si,Al)₂O₆) was found to have a Ti³⁺/Ti⁴⁺ ratio of 2.0 ± 0.2, consistent with fassaite in other solar system CAIs. The oxygen fugacity (logf_O2) of formation estimated from this ratio, assuming equilibration among phases at 1509 K, is −19.4 ± 1.3. This value is near the canonical solar nebula value (−18.1 ± 0.3) and in close agreement with that reported for fassaite-bearing Allende CAIs (−19.8 ± 0.9) by other researchers using the same assumptions. Nanocrystals of osbornite (Ti(V)N), 2–40 nm in diameter, are embedded as inclusions within gehlenite, spinel and diopside in Inti. Vanadium is heterogeneously distributed within some osbornite crystals. Compositions range from pure TiN to Ti_0.36V_0.64N. The possible presence of oxide and carbide in solid solution with the osbornite was evaluated. The osbornite may contain O, but C is not present at detectable levels. The presence of osbornite, likely a refractory early condensate, together with the other refractory minerals in Inti, indicates that the parent comet contains solids that condensed closer to the proto-sun than the distance at which the parent comet itself accreted. The estimated oxygen fugacity and the reported isotopic and chemical compositions are consistent with Inti originating in the inner solar system like other meteoritic CAIs. These results provide insight for evaluating the validity of models of radial mass transport dynamics in the early solar system. The oxidation environments inferred for the Inti mineral assemblage are inconsistent with an X-wind formation scenario. In contrast, radial mixing models that allow accretion of components from different heliocentric distances can satisfy the observations from the cometary CAI Inti.     

Neoproterozoic hydrothermally altered basaltic rocks from Rajasthan, northwest India: Implications for late Precambrian tectonic evolution of the Aravalli Craton

The isolated volcano-sedimentary sequences of the Punagarh and Sindreth Groups occur along the western flank of the Delhi Fold Belt in northwest India, and include mafic rocks (pillow basalts and dolerite dykes) that are dominantly olivine tholeiites with minor quartz-normative and alkali basalts. Sindreth samples appear to have higher primary TiO₂ and P₂O₅ abundances relative to those from Punagarh. Both suites of mafic rocks show variable, but profound hydrothermal alteration effects, with loss on ignition (LOI) values up to 10.3 wt.%, and extensive secondary minerals including albite, sericite, chlorite and calcite. Despite this, there is excellent preservation of magmatic textures, but there has been extensive albitization of plagioclase phenocrysts, a hallmark of hydrothermal alteration processes in oceanic crust. Supporting evidence for such hydrothermal alteration comes from correlations of LOI abundances with CaO/Na₂O, and evidence for U mobility is apparent on diagrams of Nb/Th vs. Nb/U. Felsic volcanic rocks (rhyolite, dacite) interlayered with the Sindreth basalts yield U–Pb zircon ages (TIMS method) between 761 ± 16 and 767 ± 3 Ma, which we interpret as representing the time of primary magmatic activity. We infer that the volcano-sedimentary rocks of the Punagarh also formed at this time, on the basis of similarities in lithology, stratigraphy, field relations and geochemistry. Intermediate granitoid rocks yield older U–Pb ages of 800 ± 2 and 873 ± 3 Ma, which we correlate with the post-Delhi Supergroup Erinpura Granites. Taken together, the features of the Punagarh and Sindreth Groups are consistent with their formation in a back-arc basin setting. Their coevality with other magmatic systems in NW India (Malani Igneous Suite), the Seychelles and Madagascar, for which a continental arc setting has also been proposed, supports the notion of an extensive convergent margin in western Rodinia at 750–770 Ma.     

Isothermal decompression history in the “Western Granulite” terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapelites

The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl–Qtz ± Fe–Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP granulite-facies assemblages record P–T conditions of 13–14 kbar at 760–800 °C. Retrogression and the release of fluids from crystallizing melts occurred at 7 kbar and 650–700 °C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO₂, as well as H₂O–NaCl and H₂O–CO₂ mixtures. We suggest that a immiscible CO₂-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H₂O–NaCl or nearly pure CO₂ represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

A periclase-dolomite-calcite carbonatite from the Oka complex,Quebec, and its calculated volatile composition

The eutectic mineral assemblage calcite-dolomite-periclase-apatite-forsterite-magnesioferrite-pyrrhotite-alabandite in a carbonatite dike within the Oka complex, Quebec, buffers the fugacities (and partial pressures) of all gas species in C-O-H-S-F, assuming vapor saturation. At the inferred eutectic (640° C, 1 kbar), the most important gas species and their partial pressures (bars) were: H₂O, 882; CO₂, 110; H₂, 4.6; H₂S, 2.7; CO, 0.5; and CH₄, 0.1. Oxygen fugacity was near the QFM buffer, logf(O₂)=–18.6, and sulfur fugacity was near the QFM-pyrrhotite buffer, logf(S₂)=–5.9. Fluorine fugacity was low, logf(F₂)=–43.9, consistent with the absence of fluoride minerals other than apatite. Presence of a water-rich gas phase is consistent with experiments on synthetic carbonatite systems (e.g. Fanelli et al. 1981), although compositions of the gas phase in published experiments cannot be determined exactly.Contribution no. 390 from the Mineralogical Laboratory, The University of Michigan     

The effect of experimental deformation on the graphitization of Pennsylvania anthracite

“Hard” carbon-based Pennsylvania anthracite was deformed in the steady-state at high temperatures and pressures in a series of coaxial and simple shear experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T 700°C, with average bireflectance values (%) increasing from 1.68 at 700°C to 6.36 at 900°C. R_omin is lowest and bireflectance is highest in zones of high strain (e.g., kink bands) at all temperatures.In anthracite samples deformed in simple shear over the 600°–900°C range at 1.0 GPa, average R_omax (%) values increase up to 11.9, whereas average bireflectance (%) values increase up to 10.7. Bireflectance increases with increasing shear strain and locally exceeds 12.5%. Graphitization occurs in several anthracite sample deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy confirms the presence of graphite with d₀₀₂=0.3363 nm. These data strongly suggest that shear strain is the dominant factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 1600°C required for graphitization of other hard carbons in earlier hydrostatic heating experiments at 0.5 GPa pressure.     

Low-density CO2-rich fluid inclusions from charnockites of southwestern Madurai Granulite Block, southern India; implications on graphite mineralization

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO₂ inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ¹³C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO₂ formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ¹³C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO₂ fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block.     

Geochemistry and geochronology of the bimodal volcanic rocks of the Suguti area in the southern part of the Musoma-Mara Greenstone Belt, Northern Tanzania

The Suguti volcanic rocks of the southern Musoma-Mara greenstone belt in northern Tanzania comprise mainly of a bimodal suite of tholeiitic basalts-basaltic andesites and calc-alkaline rhyolites with a subordinate amount of intermediate rocks. Zircon U–Pb and whole rock Sm–Nd geochronology suggests that the two suites are cogenetic and were emplaced at 2755 ± 1 Ma with a common initial _Nd value of 2.1.The tholeiitic basalts are characterised by relatively flat chondrite-normalised REE patterns with La/Yb_CN ratios of 0.8–1.6 (mean = 1.0). The basalts also exhibit negative Ti and Nb anomalies in primitive mantle-normalised multi-element diagrams. The flat REE patterns, the presence of prominent negative Nb anomalies and the positive initial _Nd value of 2.1 suggest that the basalts were formed by low pressure melting of a mantle wedge in an active continental margin setting.Compared to the tholeiitic basalts, the calc-alkaline rhyolites are characterised by low abundances of the transition elements (Cr < 20 ppm, Ni < 20 ppm) and moderately high HFSE (e.g. Zr = 111–250 ppm) abundances. The rhyolites display strongly fractionated, slightly concave upward chondrite normalised REE patterns that are characterised by a slight depletion of the MREE relative to the HREE and minor to large negative Eu anomalies (Eu/Eu* = 0.3–0.9) and their epsilon Nd values range from +2.05 to +2.33. The depletion of the MREE relative to the HREE is an indication of fractionation of clinopyroxene and hornblende during petrogenesis whereas the negative Eu anomalies indicate plagioclase fractionation. The rhyolites are interpreted to have formed from the parental magma of the basalts by fractional crystallization and/or partial melting of a relatively young basaltic crust.     

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.