Chromium-bearing olivine in the St. Mesmin chondrite

Olivine phenocrysts in microporphyritic xenoliths in the St. Mesmin chondrite (LL-breccia) show parallel rimward variations of FeO (10 to > -30wt.%), CaO (0·1–0·4%), MnO (0·2–0·8%) and Cr₂O₃ (0·2–0·7%). Aluminum is near background levels and does not vary systematically with iron. Titanium, sodium and nickel are below the limit of detection. Covariation of Fe and Cr in this olivine distinguishes it from the olivines in lunar mare basalts, in which Cr varies inversely with Fe.Transmission electron microscopy of the St. Mesmin olivine suggests that it is free of submicroscopic inclusions and exsolution lamellae and that the chromium present occurs in solid solution in the olivine. Charge balance and ionic radius considerations suggest that it occurs as Cr²⁺, whose effective ionic radius is nearly identical to that of Mn²⁺.The different Fe-Cr relationships observed in the lunar basaltic and St. Mesmin olivines reflect different crystallization sequences. Chromian spinel coprecipitated with olivine in the lunar basalts, reducing the activity of chromium in the melt and leading to the observed anticorrelation of Fe and Cr in olivine (butler, 1972). By contrast, olivine precipitated first in the St. Mesmin microporphyry and was the only solid phase present until more than half of the rock had crystallized. Parallel variation of Fe and Cr reflects crystallization from a melt in which the activity of chromium was increasing.     

The petrology and geochemistry of Miller Range 05035: A new lunar gabbroic meteorite

Miller Range (MIL) 05035 is a lunar gabbroic meteorite. The mineralogy, Fe/Mn ratios in olivine and pyroxene, bulk-rock chemical composition and the bulk oxygen isotope values (δ¹⁷O = 2.86-2.97‰ and δ¹⁸O = 5.47-5.71‰) are similar to those of other mare basalts, and are taken as supporting evidence for a lunar origin for this meteorite. The sample is dominated by pyroxene grains (54-61% by area mode of thin section) along with large plagioclase feldspar (25-36% by mode) and accessory quartz, ilmenite, spinel, apatite and troilite. The bulk-rock major element composition of MIL 05035 indicates that the sample has a very low-Ti (VLT) to low-Ti lunar heritage (we measure bulk TiO₂ to be 0.9 Wt.%) and has low bulk incompatible trace element (ITE) concentrations, akin to samples from the VLT mare basalt suite. To account for these geochemical characteristics we hypothesize that MIL 05035’s parental melt was derived from a mantle region dominated by early cumulates of the magma ocean (comprised principally of olivine and orthopyroxene). MIL 05035 is likely launch paired with the Asuka-881757 and Yamato-793169 basaltic lunar meteorites and the basaltic regolith breccia MET 01210. This group of meteorites (Y/A/M/M) therefore may be a part of a stratigraphic column consisting of an upper regolith environment underlain by a coarsening downwards basalt lava flow.     

月海玄武岩与月球演化

月海玄武岩主要产于月球近边的盆地中,覆盖面积为月球表面的l％,其形成年龄多在39～31亿年之间,是各类月岩中最年轻的。与地球玄武岩相似,月海玄武岩由斜长石、辉石和橄榄石组成,但它们比地球玄武岩具有更低的Mg#、A1：0,、K和Na含量．高的FeO含量（大于16％）和变化范围大的TiO2含量（小于l％到大于13％）。根据TiO2含量的变化,月海玄武岩分成高Ti（〉6％）,低Ti（1．5％〈TiO：〈6％）以及极低Ti（〈1．5％）三类。所有月海玄武岩都具有Eu负异常,并亏损挥发性元素和亲铁元素。月海玄武岩的同位素特征指示其至少为三个组分混合的产物：（1）高：238U／204Pb、高87Sr／86Sr和负εNd组分,可能是岩浆海分异的残余岩浆即KREEP;（2）低：238U／204Pb、低87Sr／86sr和正εNd组分,来源于原始月幔,其熔融产物为低Ⅱ玄武岩;（3）中等87Sr／86Sr和εNd组分,位于月幔的顶部,经历了岩浆海（洋）过程中形成的堆晶物质的再熔融,还可能受到了陨击事件的影响,其熔融产物是高Ti玄武岩。月海玄武岩的元素和同位素地球化学性质支持岩浆海的假说,其源区的形成与岩浆海的分异密切相关,并经历了三个阶段：（a）岩浆海阶段,通过岩浆海的结晶分异形成顶部为斜长岩月壳,中间为高Ⅱ、富钛铁矿层,底部为巨厚的硅酸盐低Ti层的三层壳幔结构;（b）富钛铁矿堆晶岩（携带少量残余熔体）因密度大而下沉至下部的硅酸盐月幔（400km以下）;（C）月幔中这些不同源区的岩石发生减压熔融。早期由较浅的低熔点组分熔融形成低K高Ti玄武岩,之后形成来源较深的高Ti玄武岩和低Ti玄武岩。     

Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ¹⁸O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO₂, Al₂O₃). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ⁵⁶Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ¹⁸O and δ⁵⁶Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ⁵⁶Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ¹⁸O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ¹⁸O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ¹⁸O and δ⁵⁶Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).     

The lunar crust: a product of heterogeneous accretion or differentiation of a homogeneous Moon?

The outer portion of the Moon including the Al-rich crust and the source regions of mare basalts was either accreted heterogeneously or was the product of widespread differentiation of an originally homogeneous source. A number of constraints make the heterogeneous accretion model unlikely; the differentiation model appears more plausible.If the differentiation model is correct, a series of cumulate rocks complimentary to the Al-rich crustal rocks must exist. The mare basalts may have been derived from such a complimentary cumulate for several reasons. For example, Philpottset al. (1973) on the basis of REE studies, suggest that Apollo 11 and 17 mare basalts were formed by partial melting of a cumulate rich in a phase(s) containing high Ti and heavy REE. The high Ti of Apollo 11 and 17 basalts is not readily explained in terms of partial melting of an undifferentiated mantle, but is consistent with partial melting of a pyroxene cumulate enriched in Fe, Ti oxides. The characteristic Fe-rich nature of mare basalts would be partly a consequence of melting of oxide cumulate minerals. It is postulated that the plagioclase-poor source region of mare basalts was enriched in an intercumulus residual liquid. During the partial melting that produced mare basalts, this material was largely incorporated into the melt, thus explaining the ancient model ages observed in most mare basalts. If the cumulate model is correct, then samples derived from the true (undifferentiated) lunar mantle have not been identified.     

Comparative petrology, geochemistry, and petrogenesis of evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica

New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo_64-2) and pyroxene (Fs₃₂Wo₈En₆₀ to Fs_84-86Wo₁₅En_2-0) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (∼2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon.     

Valence of titanium and vanadium in pyroxene in refractory inclusion interiors and rims

The clinopyroxene in coarse-grained refractory inclusions contains significant amounts of Ti and V, two elements that are multivalent over the range of temperatures and oxygen fugacities under which the inclusions formed. The Ti³⁺/Ti⁴⁺ ratios and the valence of V of these pyroxenes are valuable recorders of nebular conditions. The former can be calculated stoichiometrically from electron probe analyses, but only for relatively Ti-rich grains (i.e., >∼4 wt% ). For Ti-poor pyroxene, and for measurement of V valence, another technique is needed. We have, for the first time, applied K-edge X-ray absorption near edge structure (XANES) spectroscopy to the measurement of Ti and V valence in meteoritic clinopyroxene in refractory inclusions. Use of MicroXANES, a microbeam technique with high (μm-scale) spatial resolution, allowed measurement of Ti and V valence along traverses across (a) Ti-, V-rich “spikes” in pyroxene in Type B1 inclusions; (b) typical grains in a Type B2 inclusion; and (c) the pyroxene layer of the Wark-Lovering rim sequence on the outsides of two inclusions. Measurements of Ti³⁺/(Ti³⁺ + Ti⁴⁺), or Ti³⁺/Ti^tot, by XANES agree with values calculated from electron probe analyses to within ∼0.1, or ∼2σ. The results show that Ti³⁺/Ti^tot increases sharply at the spikes, from 0.46 ± 0.03 to 0.86 ± 0.06, but the V valence, or V²⁺/(V²⁺ + V³⁺), does not change, with V²⁺ ≈ V³⁺. We found that pyroxene in both Types B1 and B2 inclusions has Ti³⁺/Ti^tot and V²⁺/V^tot ratios between 0.4 and 0.7, except for the spikes. These values indicate, to first order, formation at similar, highly reducing oxygen fugacities that are consistent with a solar gas. The pyroxene in the rim on an Allende fluffy Type A coarse-grained refractory inclusion, TS24, has an average Ti³⁺/Ti^tot of 0.51 ± 0.08 and an average V²⁺/V^tot of 0.61 ± 0.06, determined by XANES. These values are within the range of those of pyroxene in the interiors of inclusions, indicating that the rims also formed under highly reducing conditions. Measurements of Ti³⁺/Ti^tot of pyroxene in the rim of a Leoville compact Type A inclusion, 144A, by both XANES and electron probe give a wide range of results. Of our 72 XANES analyses of this rim, 66% have Ti³⁺/Ti^tot of 0.40-0.71, and the remaining analyses range from 0 to 0.38. In data from Simon et al. [Simon J. I., Young E. D., Russell S. S., Tonui E. K., Dyl K. A., and Manning C. E. (2005) A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution ²⁶Al-²⁶Mg dating of CAI rims. Earth Planet. Sci. Lett.238, 272-283.] for this sample, 7 electron probe analyses yield calculated Ti³⁺/Ti^tot values that are positive and 15 do not. In the probe analyses that have no calculated Ti³⁺, Ca contents are anticorrelated and Al contents directly correlated with the total cations per 6 oxygens, and the data fall along trends calculated for addition of 1-7% spinel to pyroxene. It appears likely that electron probe analyses of pure pyroxene spots have Ti³⁺/Ti^tot values that are typical of refractory inclusions, in agreement with the majority of the XANES results. The average of the XANES data for 144A, 0.41 ± 0.14, is within error of that for TS24. The rim of 144A probably formed under reducing conditions like those expected for a solar gas, and was later heterogeneously altered, resulting in an uneven distribution of secondary, FeO-, Ti-bearing alteration products in the rim, and accounting for the measurements with low Ti³⁺/Ti^tot values.     

Review of melting experiments on carbonated eclogite and peridotite: insights into mantle metasomatism

Experimental studies on the partial melting of eclogite and peridotite provide important clues on mantle metasomatism. Here, we review results from some of the recent experiments and show that melting of carbonated eclogite and peridotite can produce carbonatitic to carbonated silicate melt, in which carbonates melt preferentially before Ti oxides and silicates. Low-degree melting results in carbonatitic melt coexisting with Ti oxides and silicates. This process also leads to the fractionation between some high-field strength elements (Nb, Ta, Zr, Hf, and HREE) and highly incompatible elements (U and Th) in the melt. When Ti oxides are nearly exhausted in eclogite, extremely high TiO₂ contents (e.g. 19 wt.%) are present in the melt with marked concentration of Nb and Ta. These results help to explain the features of carbonatitic metasomatism and the Nb–Ta spike in oceanic island basalts as identified in experimental studies. These studies also explain the reducing conditions that stabilize diamond in the deep mantle (>150 km) as well as the occurrence of diamond at different depths reported in various studies. Melting in such a reduced mantle can happen through redox reaction between diamond, pyroxene, and olivine, in which the initial liquid is a carbonated silicate melt. However, the theoretical oxygen fugacity (fO₂) in the asthenosphere is much lower than that predicted by the reaction and requires elevated fO₂, which can be caused by the addition of relatively oxidized materials from the lower mantle, deep asthenospheric material, and various recycled components. A combination of these processes generates locally oxidized domains in the deep mantle.     

Petrology and geochemistry of LaPaz Icefield 02205: A new unique low-Ti mare-basalt meteorite

LaPaz Icefield 02205 (LAP 02205) is a new low-Ti mare-basalt meteorite that was discovered in the LaPaz Ice Field in Antarctica. This is the first crystalline lunar basalt in the US Antarctic collection and the only 5th unbrecciated mare-basalt meteorite to be discovered to date. The rock has a typical basaltic texture with tabular and elongated pyroxene and plagioclase crystals, and minor olivine grains commonly rimmed by pyroxenes. Core- to rim-zoning in terms of Fe and Mg is present in almost all pyroxene grains. Accessory minerals include ilmenite, chromite, ulvöspinel, troilite, and FeNi metal. This rock is highly enriched in late-stage mesostasis. Free silica is also abundant. In terms of texture and mineralogy, LAP 02205 displays features of low-Ti mare basalts, with similarities to some low-Ti Apollo 12 and Apollo 15 basalts. Whole-rock major- and trace-element compositions confirm the highly fractionated nature of this basalt. The whole-rock REE contents of the meteorite are the highest among all known low-Ti mare basalts. The platinum group element (PGE) contents in LAP are also enriched suggesting the possibility of endogenously enriched source regions or the PGEs generally behaved as incompatible elements during crystal fractionation under low fO₂ conditions. Trace-element contents of mineral grains in LAP 02205 display wide variations, suggesting extensive non-equilibrium crystallization. The REE concentrations in the earliest-formed minerals provide constraints on the composition of the parental liquid, which is similar to the measured whole-rock composition. Crystallization modeling of the LAP 02205 bulk composition yields a reasonable fit between predicted and observed mineral phases and compositions, except for the high-Mg olivine cores, which are observed in the rock but not predicted by the modeling. An isochron age of 2929 ± 150 Ma for phosphate minerals makes this rock one of the youngest lunar basalts known to date. The young age and specific geochemical characteristics of LAP distinguish it from those of most other low-Ti mare basalts. However, the low-Ti mare basalt meteorite, NWA 032, has a similar young age, and the two meteorites also appear to be closely related from some geochemical perspectives and might have originated from similar source regions on the Moon.     

On the elusive isotopic composition of lunar Pb

Highly radiogenic Pb isotope compositions determined for volcanic glass beads from the Apollo 14 soil sample 14163 are similar to those commonly determined for mare basalts and are correlated with chemical variations observed in the beads. This indicates that Pb unsupported by in-situ U decay has a similar origin in both glass beads and mare basalt samples and is likely to reflect variations of ²³⁸U/²⁰⁴Pb (μ) in the lunar mantle. An alternative explanation that this Pb is a result of late equilibration with the radiogenic Pb present in soil is less likely as it would imply that all other characteristics of glass beads such as their chemistry must also be a consequence of equilibration near the lunar surface. Regardless of the origin of unsupported Pb, observed variations of Pb isotope compositions in the glass beads and mare basalts appear to be a result of two component mixing between a primitive reservoir with a μ-value similar to the Earth’s mantle and KREEP with a μ-value in excess of several thousand. This range cannot be explained by the fractionation of major rock forming minerals from the crystallising Lunar Magma Ocean and instead requires substantial extraction of sulphide late in the crystallisation sequence. The proportion of sulphide required to produce the inferred range places limits on the starting μ of the Moon prior to differentiation, demanding a relatively high value of about 100-200. Low μ indicated by several basalt samples and previously analysed volcanic glass beads can be explained by the preservation of an early (but post Ferroan Anorthosite) sulphide rich reservoir in the lunar mantle, while a complete range of Pb isotope compositions observed in the glass beads and mare basalts can be interpreted as mixing between this sulphide rich reservoir and KREEP.     

Quantitative Untersuchungen mit der Elektronen-Mikrosonde an Pyroxenen aus Basalten und Peridotit-Einschlüssen

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content.The alkali olivine basalts undersatured in SiO₂ and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize.The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.     

Moon and Earth : compositional differences inferred from siderophiles,volatiles, and alkalis in basalts

We have compared RNAA analyses of 18 trace elements in 25 low-Ti lunar and 10 terrestrial oceanic basalts. According to Ringwood and Kesson, the abundance ratio in basalts for most of these elements approximates the ratio in the two planets.Volatiles (Ag, Bi, Br, Cd, In, Sb, Sn, Tl, Zn) are depleted in lunar basalts by a nearly constant factor of 0.026 ± 0.013, relative to terrestrial basalts. Given the differences in volatility among these elements, this constancy is not consistent with models that derive the Moon's volatiles from partial recondensation of the Earth's mantle or from partial degassing of a captured body. It is consistent with models that derive planetary volatiles from a thin veneer (or a residuum) of C-chondrite material; apparently the Moon received only 2.6% of the Earth's endowment of such material per unit mass.Chalcogens (Se and Te) have virtually constant and identical abundances in lunar and terrestrial basalts, probably reflecting saturation with Fe(S, Se, Te) in the source regions.Siderophiles show diverse trends. Ni is relatively abundant in lunar basalts (4 × 10^?3 × Cl-chondrites), whereas Ir, Re, Ge, Au are depleted to 10^?4?10^?5× Cl. Except for Ir, these elements are consistently enriched in terrestrial basalts: Ni 3 × , Re 370 ×, Ge 330 × , Au 9 × . This difference apparently reflects the presence of nickel-iron phase in the lunar mantle, which sequesters these metals. On Earth, where such metal is absent, these elements partition into the crust to a greater degree. Though no lunar mantle rock is known, an analogue is provided by the siderophile-rich dunite 72417 (~0.1% metal) and the complementary, siderophile-poor troctolite 76535. The implied metal-siderophile distribution coefficients range from 10⁴ to 10⁶, and are consistent with available laboratory data.The evidence does not support the alternative explanation advanced by Ringwood—that Re was volatilized during the Moon's formation, and is an incompatible element (like La or W⁴⁺) in igneous processes. Re is much more depleted than elements of far greater volatility: (Re/U)_Cl～- 4 × 10^?6 vs (T1/U)_Cl = 1.3 × 10^?4, and Re does not correlate with La or other incompatibles.Heavy alkalis (K, Rb, Cs) show increasing depletion with atomic number. Cs/Rb ratios in lunar basalts, eucrites, and shergottites are 0.44, 0.36, and 0.65 × Cl, whereas the value for the bulk Earth is 0.15–0.26. These ratios fall within the range observed in LL and E6 chondrites. supporting the suggestion that the alkali depletion in planets, as in chondrites, was caused by localized remelting of nebular dust (= chondrule formation). Indeed, the small fractionation of K, Rb and Cs, despite their great differences in volatility, suggests that the planets, like the chondrites, formed from a mixture of depleted and undepleted material, not from a single, partially devolatilized material.     

Water in Mantle-derived Xenoliths in the Hannuoba Basalt

Minerals of various mantle-derived xenoliths from the Hannuoba basalt in Hebei Province have been studied by means of IR spectroscopy. The results show that all xenoliths from the mantle at depths <75 km contain trace amounts of water (0.45%-11.6×10-2 % H2O). The data of about 0.1% H2O contained in primary pyrolite estimated by earlier studies may be on the high side. The water might enter the frameworks of olivine, pyroxene and garnet earlier than it entered those of amphibole and phlogopite. The presence of water in amphibole and phlogopite may be a local phenomenon of water enrichment, which is related to relatively small-scale magmatic or metasomatic events although they can contain a hundred times more water than pyroxene contains. There is a little more water (1.11%-3.01×10-2 % of H2O mostly) in xenoliths from the Hannuoba basalt than in those from mid-ocean ridge basalt and kimberlites of South Africa (less than 1×10-2 % of H2O mostly). This indicates the heterogeneity of water in time and spa     

Fractional crystallization and the formation of thick Fe–Ti–V oxide layers in the Baima layered intrusion,SW China

The Baima layered intrusion is located in the central part of the Emeishan Large Igneous Province (ELIP). The N–S striking intrusion is ~ 24 km long and ~ 2 km thick and dips to the west. Based on variations in modal proportions and cumulus mineral assemblages, the intrusion from the base to the top is simply subdivided into a lower zone (LZ) with most of the economic magnetite layers, and an upper zone (UZ) with apatite-bearing troctolite and gabbro. The rock textures suggest crystallization of the Fe–Ti oxide slightly later than plagioclase (An_67-54) but relatively earlier than olivine (Fo_74-55), followed by clinopyroxene and finally apatite.Relatively low olivine forsterite content and abundant ilmenite exsolution lamellae in clinopyroxene indicate that the Baima parental magma is a highly evolved Fe–Ti-rich magma. Via MELTS model, it demonstrates that under a closed oxygen system, extensive silicate mineral fractionation of a picritic magma might lead to Fe and Ti enrichment and oxygen fugacity elevation in the residual magma. When such Fe–Ti-rich magma ascends to the shallower Baima intrusion, the Fe–Ti oxides may become an early liquidus phase. Well-matched olivine and plagioclase microprobe data with the results of MELTS calculation, combined with relatively low CaO content in olivine (0.02–0.08 wt.%) indicate that wall-rock contamination probably plays a weak role on oxygen fugacity elevation and the early crystallization of Fe–Ti oxides. Several reversals in whole-rock chromium and plagioclase anorthite contents illustrate that multiple recharges of such Fe–Ti-rich magma mainly occurred along the lower part of the Baima magma chamber. Frequent Fe–Ti-rich magma replenishment and gravitational sorting and settling are crucial for the development of thick Fe–Ti oxide layers at the base of the Baima layered intrusion.     

Early crustal building processes on the moon: Models for the petrogenesis of the magnesian suite

The plutonic rocks of the magnesian suite (Mg-suite) represent the period of lunar basaltic magmatism and crustal growth (∼4.46 to 4.1 Ga) that immediately followed the initial differentiation of the Moon by magma ocean (LMO) formation and crystallization. The volume and distribution of the Mg-suite and its petrogenetic relationship to latter stages of lunar magmatism (mare basalts) remains obscure. These plutonic rocks exhibit a range of compositions and include ultramafics, troctolites, spinel troctolites, norites, and gabbronorites. A distinguishing characteristic of this suite is that they contain some of the most magnesium-rich phases (Fo_95-90) that had crystallized from lunar magmas, yet they also are significantly enriched in an incompatible element component referred to as KREEP (a late-stage product of LMO crystallization containing abundant potassium (K), rare earth elements (REE), phosphorous (P), uranium, and thorium). Ion microprobe analyses of individual mineral phases (olivine, pyroxene, and plagioclase) from the Mg-suite have shown some very unexpected characteristics that have profound implications on the origin of these basaltic magmas. Although the Mg-suite lithologies are typified by silicates with relatively high Mg′, early liquidus phases such as olivine are fairly low in Ni, Co, and Cr relative to more iron-rich olivines in the younger mare basalts. The high Y and Ti/Y in early phases such as olivine and orthopyroxene indicate that the parental basaltic melts were high in incompatible elements and contained an “ilmenite fractionation” signature. However, the Y in olivine from many of the troctolites and ultramafic lithologies are only slightly greater than that of the olivine in the mare basalts whereas olivine in the norites, gabbronorites, and Apollo 14 troctolites are exceedingly high. The KREEP component may have been added to the Mg-suite parent magmas by assimilation or mixing into the mantle source. The volume of KREEP required to be added to the parental magmas of the Mg-suite tends to favor the latter mechanism for KREEP incorporation. The extremely high abundances of KREEP in the norites and gabbronorites are a product of substantial crystallization (40% to 70%) of KREEP-enriched Mg-suite parental magmas. Basaltic magmatism associated with KREEP extended for over 1.5 billion years and appears to have changed over time. The early stages of this style of lunar magmatism (Mg-suite) appear to represent melting of early LMO cumulates with low abundances of Ni, Co, Cr, and V. Later stages of KREEP-rich basaltic magmatism seemed to clearly involve melting of a variety of LMO cumulate assemblages with higher incompatible element enrichment. It appears that the heat derived from the KREEP component was instrumental in at least initiating melting of the lunar mantle over this period of time.     

The oxygen isotope composition, petrology and geochemistry of mare basalts: Evidence for large-scale compositional variation in the lunar mantle

To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K - potassium, REE - Rare Earth Element, and P - phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock δ¹⁸O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock δ¹⁸O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock δ¹⁸O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt δ¹⁸O bulk-rock values. Differences between the low- and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk δ¹⁸O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples.Consistent with the results of previous studies, our data reveal no detectable difference between the Δ¹⁷O of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.     

A Fe K-edge XAS study of amethyst

An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO₂) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe²⁺ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell:  = 1.78(2) Å, the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe³⁺ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe⁴⁺ is stabilised by the favourable local structural arrangement.     

金川超大型Ni-Cu-(PGE)矿床橄榄石微量元素特征及地质意义

我国金川超大型铜镍硫化物矿床是世界上第三大在采岩浆硫化物矿床,Ni开采量仅次于俄罗斯Noril'sk-Talnakh和加拿大Sudbury矿床,其成因研究备受关注。利用激光—等离子体质谱(LA-ICP-MS)原位分析了金川岩体中橄榄石微量元素含量,并探讨了影响元素含量变化的因素,进而阐述成岩及成矿过程。分析结果显示橄榄石中元素Ni,Cr与Fo呈负相关,Mn/Fe与Fo呈正相关,而Mn/Zn,Zn/Fe与Fo无相关性。在原始地幔橄榄石多元素标准化图中,金川Ⅰ号和Ⅱ号岩体橄榄石具相同的配分模式,均显示Cr,V,Ni,Co和Ti的亏损,富集不相容元素Zr,Y,Ti,Sc和Ca的特征。元素变化特征暗示Ⅰ号和Ⅱ号岩体具相同的母岩浆成分;与铬尖晶石的共结使橄榄石亏损Cr,V和Ti元素,而熔离的硫化物及其与橄榄石的相互反应共同影响着橄榄石中Ni和Co元素的含量。Ⅱ号岩体橄榄石较Ⅰ号岩体具较低的Cr和V含量,暗示Ⅱ号岩体母岩浆较Ⅰ号岩体经历了更高程度的演化。橄榄石高的Mn/Zn值(>13)和低的Zn/Fe值(<11)指示金川岩体岩浆可能起源于橄榄岩地幔的部分熔融,而非辉石岩地幔源区。     

Sulfur K- and L-edge X-ray absorption spectroscopy of sphalerite,chalcopyrite and stannite

Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and L-edge XANES of sphalerite (ZnS), chalcopyrite (CuFeS₂) and stannite (Cu₂FeSnS₄) have been recorded using synchrotron radiation. The K- and L-edge XANES features are interpreted using a qualitative MO/energy band structure model. The densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The SK- and L-edge XANES also indicate that, for sphalerite, the Fe²⁺ 3d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu⁺ 3d and Fe³⁺ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu⁺ 3d band strongly hybridizes with S 3p and S 3s orbitals, but the Fe²⁺ 3d band does not. The post-edge XANES features (15–50 eV above the edge) of sphalerite, chalcopyrite and stannite are similar. These features are related to the tetrahedral coordination of sulfur in all these structures, and interpreted by a multiple scattering model. The resonance energies from both the K-edge and L-edge XANES for these minerals are well correlated with reciprocal interatomic distances and lattice spaces. Sulfur K-edge EXAFS analyses using Fourier transform and curve fitting procedures are presented. Comparison of the structural parameters from EXAFS with x-ray structure data shows that the first shell bond distances (BD) from EXAFS are usually accurate to ±0.02 Å, and that coordination numbers (CN) are generally accurate to ±20 percent. For sphalerite, EXAFS analysis yields the structure parameters for the first three neighbour shells around a sulfur atom; the BD and CN even for the third shell are in close agreement with the x-ray structure, and the Debye-Waller term decreases from the first shell to the third shell. It is shown that sphalerite (ZnS) is a good model compound for EXAFS analysis of sulfur in chalcogenide glasses and metalloproteins.