Pressure dependence on partition coefficients for trace elements between olivine and the coexisting melts

Partition coefficients between olivine and melt at upper mantle conditions, 3 to 14 GPa, have been determined for 27 trace elements (Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr, Y, Zr, Cs, Ba, La and Ce) using secondary-ion mass-spectrometry (SIMS) and electron-probe microanalysis (EPMA). The general pattern of olivine/melt partitioning on Onuma diagrams resembles those reported previously for natural systems. This agreement strongly supports the argument that partitioning is under structural control of olivine even at high pressure. The partition coefficients for mono- and tri-valent cations show significant pressure dependence, both becoming larger with pressure, and are strongly correlated with coupled substitution into cation sites in the olivine structure. The dominant type of trace element substitution for mono- and tri-valent cations into olivine changes gradually from (Si, Mg)↔(Al, Cr) at low pressure to (Si, Mg)↔(Al, Al) and (Mg, Mg)↔(Na, Al) at high pressure. The change in substitution type results in an increase in partition coefficients of Al and Na with pressure. An inverse correlation between the partition coefficients for divalent cations and pressure has been observed, especially for Ni, Co and Fe. The order of decreasing rate of partition coefficient with pressure correlates to strength of crystal field effect of the cation. The pressure dependence of olivine/melt partitioning can be attributed to the compression of cation polyhedra induced by pressure and the compensation of electrostatic valence by cation substitution. Received: March 6, 1997 / Revised, accepted: March 12, 1998     

Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications

Lattice diffusion coefficients have been determined for 19 elements (Li, Be, Na, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Eu, Gd, Lu and Hf) in a single crystal of San Carlos olivine as a function of crystallographic orientation, at 1,300°C, 1 bar and fO₂ = 10^−8.3 bars, by equilibration with a synthetic silicate melt. Results for Li, Na, V, Cr, Fe and Zn are from diffusion of these elements out of the olivine, starting from their indigenous concentrations; those for all other elements are from diffusion into the olivine, from the silicate melt reservoir. Our 25-day experiment produced diffusion profiles 50 to > 700 μm in length, which are sufficiently long that precise analyses could be achieved by scanning laser ablation inductively coupled plasma mass spectrometry, even at concentration levels well below 1 μg g⁻¹. For the divalent cations Ca, Mn, Fe and Ni, profiles were also obtained by electron microprobe analysis. The results of the two methods agree well with each other, and are consistent with divalent cation diffusion coefficients previously determined using different experimental methodologies. Olivine/melt partition coefficients retrieved from the data are also consistent with other published partitioning data, indicating that element incorporation and transport in olivine in our experiment occurred via mechanisms appropriate to natural conditions. Most of the examined trace elements diffuse through olivine at similar rates to the major octahedral cations Fe and Mg, showing that cation charge and radius have little direct influence on diffusion rates. Aluminium and P remain low and constant in the olivine, implying negligible transport at our analytical scale, hence Al and P diffusion rates that are at least two orders of magnitude slower than the other cations studied here. All determined element diffusivities are anisotropic, with diffusion fastest along the [001] axis, except Y and the REEs, which diffuse isotropically. The results suggest that element diffusivity in olivine is largely controlled by cation site preference, charge balance mechanisms and point-defect concentrations. Elements that are present on multiple cation sites in olivine (e.g. Be and Ti) and trivalent elements that are charge-balanced by octahedral site vacancies tend to diffuse at relatively fast rates.     

Trace element composition of silicate minerals in the porphyritic and nonporphyritic chondrules of Elenovka (L5) and Knyahinya (L/Ll5) meteorites

The results of SIMS and EPMA studies on the silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from Elenovka and Knyahinya meteorites are reported. The trace element composition of silicate minerals (olivine, low-Са pyroxene) in equilibrated ordinary chondrites (EOC) has not been affected considerably by thermal metamorphism on the chondritic parent bodies. Therefore, equilibrated chondrites can be used for chondrule-forming processes studies. Low-Са pyroxene in nonporphyritic chondrules contains higher REE, Ba, Sr concentrations than that in porphyritic chondrules at similar trace element concentrations in the olivine of chondrules. The data obtained indicate that the formation of non-porphyritic chondrules was triggered by an increase in the cooling rate of chondrules upon the formation of pyroxene, rather than a difference in the initial conditions of chondrule formation. Higher refractory incompatible element (Nb, LREE) concentrations in the olivine of chondrules than those in the olivine of the matrix and contrasting trace element (Zr, Sr, Cr, REE) concentrations in the low-Са pyroxene of the chondrules and the matrix suggest that the matrix and chondrules of the meteorites formed in one reservoir under different physico-chemical conditions (density, redox state, rotation speed, homogeneity, temperature, shocks, electrical discharge, etc.).     

Sc,Cr, Co and Ni partitioning between minerals from spinel peridotite xenoliths

The partitioning of divalent (Co, Ni) and trivalent (Sc, Cr) trace elements between olivine, ortho- and clinopyroxene and spinel from spinel peridotite xenoliths has been investigated. These peridotites cover a wide range in modal composition from dunite to primitive lherzolites and have equilibrated in the upper mantle between >900° C and <1,200° C.The distribution of Co and Ni shows only minor variation through the whole sequence. In contrast, Sc partitioning between ortho- and clinopyroxene and olivine and clinopyroxene as well as Cr partitioning between olivine and clinopyroxene or olivine and orthopyroxene display high but systematic variations which can be assigned to dependences upon equilibration temperatues. Empirical temperature calibrations are given for Sc-orthopyroxene/clinopyroxene, Sc-olivine/clinopyroxene and Cr-olivine/clinopyroxene which, in principle, may permit to estimate equilibration temperatures not only for lherzolites or harzburgites but for orthopyroxene-free peridotites, too.Sc and Ni partition coefficients between spinel and mantle silicate minerals are primarily dependent upon the major element composition of spinel (e.g. Cr and Al) although a temperature dependence can still be identified. Probably such compositional effects are not observed for trace element partitioning between pyroxenes and olivine or ortho- and clinopyroxene only for the reason that in normal spinel peridotites these minerals show much less variation in major element composition than their coexisting spinels.     

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。     

Trace Elements in Olivines from the Giant Jinchuan Ni-Cu-(PGE) Deposit,NW China,and Its Geological Implication

The giant Jinchuan magmatic sulfide deposit in China is the third largest mining deposits in the world. Although many research have been done, there still exist some debates in the genesis of deposit. This study using the LA-IC-MS to obtain the trace elements concentrations of the olivine in order to discuss the mechanism of influence the element variability and illustrate the process of magmatism and ore-forming. The analytical results show that Ni, Co correlate negatively with Fo in the olivine, Mn/Fe is positively correlate with Fo, while Mn/Zn and Zn/Fe show no obvious correlation with Fo. The primitive mantle olivine-normalized trace element patterns of the Jinchuan olivine show that Jinchuan Ⅰ, Ⅱ intrusions have the same trace elements characteristics, which display negative Cr, V, Ni, Co and Ti anomalies and enrichment of Zr, Y, Ti, Sc and Ca. The multi-element patterns of the Jinchuan olivine imply same parental magma in the intrusion Ⅰ and Ⅱ. The spinel which cocrystallization with the olivine make it display negative Cr, V and Ti anomalies. The contents of Ni and Co in olivine are influenced by the process of sulfide segregation and the reaction between sulfide and olivine. The lower content of Cr and V in olivine of the intrusion Ⅱ compared with the intrusion Ⅰ imply that the parental magma of the intrusion Ⅱ was more evolved. Higher Mn/Zn (>13) ratios and lower Zn/Fe (<11) ratios indicate that the magma of Jinchuan intrusion likely originate from partial melting of peridotite mantle possibly, instead of pyroxene mantle sources.     

Mineral activity–composition relations and petrological calculations involving cation equipartition in multisite minerals: a logical inconsistency

Equipartition is an assumption that preserves the same relative fraction of the cations on each site, originally used just to distribute cations such as Fe and Mg over two or more sites during mineral recalculation. This approximation has been used in almost all thermometry and barometry applications in petrology involving pyroxene, amphibole, chlorite and biotite mica. It has also become the default approach used in deriving activity–composition relations in multisite minerals where details of element partitioning among the sites is unknown and therefore assumed to be random. It is shown that, where a third element, such as Al, resides on just one of the sites, equipartition introduces a serious logical and numerical inconsistency between the enthalpy and entropy of mixing. In particular, application of equipartition is demonstrated to be equivalent to treating the solution as one in which all cations reside on one type of site. For example, in the case of aluminous orthopyroxene the equipartition assumption implies that Fe, Mg and Al are distributed across two identical sites, and that end‐members such as Mg‐tschermak's pyroxene have enthalpies characterized by a totally disordered distribution of Mg and Al over the M2 and M1 sites. Clearly this is unsupportable, and solid solutions should be treated with appropriate thermodynamics for order–disorder. For phases where element partitioning data are unavailable, we offer a simple alternative strategy, using the ordering of Fe and Mg on M1 and M2 sites in biotite as an example.     

Gough Island: Evaluation of a fractional crystallization model

Gough Island is composed of an alkaline olivine basalt-trachyte series. A fractional crystallization model for the development of these rocks has been evaluated by correlating the geochemical trends of major and trace elements. Beginning with an alkali olivine basalt parent the major element abundances were used to determine the varying proportions of crystallizing minerals required to generate the various residual liquids. A least-squares computer model was used for this calculation. The modal proportions of cumulative minerals and trace element distribution coefficients were used to predict the trace element abundances in each rock type.Three significant trace element trends are observed in Gough Island rocks: (1) increasing rare earth (RE) abundance and relative light RE enrichment with increasing major element differentiation, (2) marked Eu, Sr, and Ba depletions in late stage trachytes, (3) Cr and M enrichment in picrite basalt.The trace element abundances predicted by the fractional crystallization model are in good agreement with these observed trends. A fractional crystallization process involving olivine, pyroxene, feldspar, and apatite accounts for all the significant major and trace element trends observed in Gough Island rocks.     

Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis

In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation.     

A plagioclase-olivine-spinel-magnetite inclusion from Maralinga (CK): evidence for sequential condensation and solid-gas exchange

We report a detailed petrography, mineral chemistry, and trace element study of MaTroc, a large calcium-aluminum-rich inclusion (CAI) (5 × 2.5 mm) of irregular triangular shape. The inclusion has a zonal structure: The core consists of a porous plagioclase-olivine-Ca-rich pyroxene intergrowth with subordinate apatite. Its texture is meta-gabbro-like, similar to other plagioclase-olivine inclusions (POIs). The mantle has variable thickness (0.1-1.5 mm) and consists of a compact symplectitic intergrowth of spinel (hercynite) and plagioclase with abundant dispersed magnetite, subordinate Ca-rich pyroxene, and traces of sulfides. The thin (5-50 μm) discontinuous crust of MaTroc consists mainly of plagioclase with some olivine and magnetite.The Mg-Fe phases of MaTroc are Fe-rich: olivine has Fa33.2 and high NiO content, similar to that in the host rock, Ca-rich pyroxene has much lower TiO₂ and Cr₂O₃ contents than that of the host chondrite, and plagioclase is An55-An74. Magnetites have variable compositions, are poorer in Al₂O₃ and Cr₂O₃ and richer in NiO than those in the host. Spinels have also variable compositions, rich in FeO, NiO, and ZnO.Despite their different mineralogy, both core and mantle have bulk trace element abundances similar to those in average group II CAIs. However, the mantle is richer in Nb and U and poorer in Eu, Be, B, Sr, and Li than the core. All minerals have high trace element contents. Minerals in the core show signs of incomplete equilibration of trace elements within and between them. Mantle minerals are far from equilibrium with each other and the bulk system. Spinel and anorthite carry the trace element signature of their precursor melilite (or hibonite), and magnetite contains large amounts of a heterogeneously distributed remnant extremely rich in trace elements (“obscurite”), possibly of a former perovskite.Inclusion MaTroc has a complex history. The POI core probably formed by reaction of an unknown precursor(s) of condensation origin with a vapor to form olivine, plagioclase, clinopyroxene, apatite, and (an) unknown phase(s) that vanished, leaving abundant void space. The spinel-rich mantle is also a secondary mineral assemblage that formed by breakdown of and solid-vapor reactions with a precursor or precursors, possibly melilite (or hibonite). The abundant magnetite formed by reaction of perovskite with an oxidizing vapor and by precipitation from such a vapor. All phases of the inclusion experienced the metasomatic addition of Fe, Ni, and moderately volatile elements such as V, Be, Li, Cr, and Mn—similar to all other constituents of the Maralinga CK chondrite. Phases in MaTroc and in the host rock are close to equilibrium in the distribution of Fe, Mg, Ni, and Mn but far from equilibrium in the distribution of M⁺³ and M⁺⁴ ions. The minor and trace element abundances in the magnetite of the host rock and of MaTroc preclude an origin by oxidation of a metal precursor.     

Silicate mineralogy of martian meteorites

Basalts and basaltic cumulates from Mars (delivered to Earth as meteorites) carry a record of the history of that planet - from accretion to initial differentiation and subsequent volcanism, up to recent times. We provide new microprobe data for plagioclase, olivine, and pyroxene from 19 of the martian meteorites that are representative of the six types of martian rocks. We also provide a comprehensive WDS map dataset for each sample studied, collected at a common magnification for easy comparison of composition and texture. The silicate data shows that plagioclase from each of the rock types shares similar trends in Ca-Na-K, and that K₂O/Na₂O wt% of plagioclase multiplied by the Al content of the bulk rock can be used to determine whether a rock is “enriched” or “depleted” in nature. Olivine data show that meteorite Y 980459 is a primitive melt from the martian mantle as its olivine crystals are in equilibrium with its bulk rock composition; all other olivine-bearing Shergottites have been affected by fractional crystallization. Pyroxene quadrilateral compositions can be used to isolate the type of melt from which the grains crystallized, and minor element concentrations in pyroxene can lend insight into parent melt compositions.In a comparative planetary mineralogy context, plagioclase from Mars is richer in Na than terrestrial and lunar plagioclase. The two most important factors contributing to this are the low activity of Al in martian melts and the resulting delayed nucleation of plagioclase in the crystallizing rock. Olivine from martian rocks shows distinct trends in Ni-Co and Cr systematics compared with olivine from Earth and Moon. The trends are due to several factors including oxygen fugacity, melt compositions and melt structures, properties which show variability among the planets. Finally, Fe-Mn ratios in both olivine and pyroxene can be used as a fingerprint of planetary parentage, where minerals show distinct planetary trends that may have been set at the time of planetary accretion.Although the silicate mineralogical data alone cannot support one specific model of martian magmatism over another, the data does support the basic igneous reservoirs proposed for Mars, and may also be used to constrain some aspects of specific petrogenetic models. Examples include enriched and depleted reservoirs that can be identified by plagioclase K, Na and Al composition, multivalent element partitioning in olivine and pyroxene (V, Cr) elucidates oxygen fugacity conditions of the reservoirs, and minor element concentrations (i.e., Cr in pyx) show that proposed fractional crystallization models linking Y 980459 to QUE 94201 will not work.     

An experimental study of crystalline basalt dissolution from 2 ? pH ? 11 and temperatures from 5 to 75 °C

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ?25 °C but slower at alkaline pH and temperatures ?50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO₂-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study.     

四川攀西地区层状侵入体中堆晶岩成因的矿物学约束

火成岩中可以包含多种晶体群这一发现具有重要意义,使得成因矿物学重新成为揭示岩浆系统演化的基本指导思想。但是,这种重要性在许多文献中都没有得到反映,其典型实例就是镁铁质层状侵入体中堆晶岩的成因。争论在于堆晶矿物是循环晶还是母岩浆的液相线相。因此,本文致力于探讨四川攀西地区镁铁质层状侵入体中堆晶岩的形成过程,重申成因矿物学的重要意义。显微镜观察表明,堆晶单斜辉石富含Fe-Ti氧化物出溶叶片(含叶片辉石),表明其形成环境明显不同于与斜长石呈共结关系的单斜辉石(无叶片辉石);无叶片辉石和斜长石中的橄榄石包裹体呈浑圆状,表明了橄榄石与结晶环境间的热力学不平衡。橄榄石与熔体间Fe-Mg分配关系分析表明,根据母岩浆成分推测的橄榄石Fo值远低于岩体中观测橄榄石化学成分变化范围(Fo₆₁-Fo₈₁)的高限,表明至少部分橄榄石不是寄主侵入体的液相线相。橄榄石的Mg^#值(100×Mg/(Mg+Fe))与微量元素(特别是Ni)的相关关系表明存在多种橄榄石晶体群,它们形成于不同的热力学环境中。晶体沉降过程分析表明,寄主岩浆析出的晶体几乎不可能发生快速重力沉降来形成堆晶岩。所有这些证据都表明,形成堆晶岩的矿物主要来自岩浆系统深部不同的岩浆房中,是被岩浆携带输运到终端岩浆房的循环晶。     

Minor and trace element partitioning between immiscible ocelli-matrix pairs from lamprophyre dikes and sills,Monteregian Hills petrographic province,Quebec

Many lamprophyre dike and sill rocks in the Monteregian Hills petrographic province of southwestern Quebec contain felsic segregations (ocelli) which have been interpreted as globules of immiscible liquid (Philpotts 1976). Ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements. The major element data, when plotted on a Greig diagram, outline a field of possible silicate-liquid immiscibility at higher alumina+alkali content than that previously mapped in iron-rich experimental systems. The trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high-charge density cations are preferentially concentrated in the matrix (mafic) material, a result which is consistent with theoretical and experimental studies. The distribution of minor and trace elements between ocelli and matrix indicates that several factors control the partitioning of these elements between immiscible felsic and mafic liquids. These factors include the difference in relative polymerization (as measured by the Si∶O ratio) of the two liquids, with an increase in this difference favoring partitioning of the high-charge density cations into the mafic liquid; the concentration of P₂O₅ in the mafic liquid which favors the partitioning of high-charge density cations into this liquid; the presence of a CO₂ vapor (?) phase which favors the partitioning of high-charge density cations into the CO₂ enriched phase; and the presence of solid phases at the onset of immiscibility. These observations indicate that the chemical compositions of two possibly immiscible melts should be known if minor and trace element data are to be used as evidence for silicate-liquid immiscibility.     

Minor and trace element partitioning between immiscible ocelli-matrix pairs from lamprophyre dikes and sills,Monteregian Hills petrographic province,Quebec

Many lamprophyre dike and sill rocks in the Monteregian Hills petrographic province of southwestern Quebec contain felsic segregations (ocelli) which have been interpreted as globules of immiscible liquid (Philpotts 1976). Ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements. The major element data, when plotted on a Greig diagram, outline a field of possible silicate-liquid immiscibility at higher alumina+alkali content than that previously mapped in iron-rich experimental systems. The trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high-charge density cations are preferentially concentrated in the matrix (mafic) material, a result which is consistent with theoretical and experimental studies.The distribution of minor and trace elements between ocelli and matrix indicates that several factors control the partitioning of these elements between immiscible felsic and mafic liquids. These factors include the difference in relative polymerization (as measured by the Si∶O ratio) of the two liquids, with an increase in this difference favoring partitioning of the high-charge density cations into the mafic liquid; the concentration of P₂O₅ in the mafic liquid which favors the partitioning of high-charge density cations into this liquid; the presence of a CO₂ vapor (?) phase which favors the partitioning of high-charge density cations into the CO₂ enriched phase; and the presence of solid phases at the onset of immiscibility. These observations indicate that the chemical compositions of two possibly immiscible melts should be known if minor and trace element data are to be used as evidence for silicate-liquid immiscibility.     

Chondrules in the CB/CH-like carbonaceous chondrite Isheyevo: Evidence for various chondrule-forming mechanisms and multiple chondrule generations

The recently discovered metal-rich carbonaceous chondrite Isheyevo consists of Fe, Ni-metal grains, chondrules, heavily hydrated matrix lumps and rare refractory inclusions. It contains several lithologies with mineralogical characteristics intermediate between the CH and CB carbonaceous chondrites; the contacts between the lithologies are often gradual. Here we report the mineralogy and petrography of chondrules in the metal-rich (70 vol%) and metal-poor (20 vol%) lithologies. The chondrules show large variations in textures [cryptocrystalline, skeletal olivine, barred olivine, porphyritic olivine, porphyritic olivine-pyroxene, porphyritic pyroxene], mineralogy and bulk chemistry (magnesian, ferrous, aluminum-rich, silica-rich). The porphyritic magnesian (Type I) and ferrous (Type II) chondrules, as well as silica- and Al-rich plagioclase-bearing chondrules are texturally and mineralogically similar to those in other chondrite groups and probably formed by melting of mineralogically diverse precursor materials. We note, however, that in contrast to porphyritic chondrules in other chondrite groups, those in Isheyevo show little evidence for multiple melting events; e.g., relict grains are rare and igneous rims or independent compound chondrules have not been found. The magnesian cryptocrystalline and skeletal olivine chondrules are chemically and mineralogically similar to those in the CH and CB carbonaceous chondrites Hammadah al Hamra 237, Queen Alexandra Range 94411 (QUE94411) and MacAlpine Hills 02675 (MAC02675), possibly indicating a common origin from a vapor–melt plume produced by a giant impact between planetary embryos; the interchondrule metal grains, many of which are chemically zoned, probably formed during the same event. The magnesian cryptocrystalline chondrules have olivine–pyroxene normative compositions and are generally highly depleted in Ca, Al, Ti, Mn and Na; they occasionally occur inside chemically zoned Fe, Ni-metal grains. The skeletal olivine chondrules consist of skeletal forsteritic olivine grains overgrown by Al-rich (up to 20 wt% Al₂O₃) low-Ca and high-Ca pyroxene, and interstitial anorthite-rich mesostasis. Since chondrules with such characteristics are absent in ordinary, enstatite and other carbonaceous chondrite groups, the impact-related chondrule-forming mechanism could be unique for the CH and CB chondrites. We conclude that Isheyevo and probably other CH chondrites contain chondrules of several generations, which may have formed at different times, places and by different mechanisms, and subsequently accreted together with the heavily hydrated matrix lumps and refractory inclusions into a CH parent body. Short-lived isotope chronology, oxygen isotope and trace element studies of the Isheyevo chondrules can provide a possible test of this hypothesis.     

Reply to comments by Taylor and Neal on “Trace element crystal chemistry of mantle eclogites”

Conclusion In summary, there are no existing clinopyroxene-liquid REE distribution data obtained under appropriate conditions to permit numerical modeling of trace elements to constitute a critical test of the pyroxene fractionation process. There are, however, numerous clinopyroxeneliquid partitioning data in the literature that give qualitatively similar patterns to those observed in grospydites. Further, the high pressure fractionation model accounts for the major-element and exsolution phenomena in a manner directly consistent with experimental petrologic studies, and can qualitatively account for the trace element and isotopic characteristics of the rocks in a manner consistent with the crystal chemistry of the phases. We therefore continue to prefer the high-pressure clinopyroxene fractionation model over a low-pressure plagioclase fractionation model for the origin of the grospydites.     

Clinopyroxene composition in mafic lavas from different tectonic settings

Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na₂O vs MnO vs TiO₂, SiO₂ vs TiO₂ and SiO₂ vs Al₂O₃ provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies.     

Experimentally determined mineral-melt partition coefficients for Sc,Y and REE for olivine,orthopyroxene, pigeonite,magnetite and ilmenite

Our current lack of understanding of the partitioning behavior of Sc, Y and the REE (rare-earth elements) can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set. To address this problem, we conducted a series of experiments using several different natural composition lavas, doped with the elements of interest, as starting compositions. Microprobe analyses of orthopyroxene, pigeonite, olivine, magnetite, ilmenite and co-existing glasses in the experimental charges were used to calculate expressions that describe REE partitioning as a function of a variety of system parameters. Using expressions that represent mineral-melt reactions (versus element ratio distribution coefficients) it is possible to calculate terms that express low-Ca pyroxene-melt partitioning behavior and are independent of both pyroxene and melt composition. Compositional variations suggest that Sc substitution in olivine involves either a paired substitution with Al or, more commonly, with vacancies. The partitioning of Sc is dependent both on melt composition and temperature. Our experimentally determined olivine-melt REE Ds (partition coefficients) are similar to, but slightly higher than those reported by McKay (1986) and support their conclusions that olivines are strongly LREE depleted. Y and REE mineral/melt partition coefficients for magnetite range from 0.003 for La to 0.02 for Lu. Ilmenite partition coefficients range from 0.007 for La to 0.029 for Lu. These experimental values are two orders of magnitude lower than many of the published values determined by phenocryst/matrix separation techniques.     

Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘famous’ region

Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe²⁺ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.The Fe-Mg partitioning between olivine and host rock suggests that the picritic basalts represent olivine (±Cr-spinel) enriched magmas, derived from a less MgO rich parental magma. The partitioning of Fe and Mg between olivine, Cr-spinel and coexisting liquid is used to predict a primary magma composition parental to the picritic basalts. This magma is characterized by relatively high MgO (12.3%) and CaO (12.6%) and low FeO^* (7.96%) and TiO₂ (0.63%).Least squares calculations indicate that the plagioclase phyric basalts are related to the plagioclase-pyroxene basalts by plagioclase and minor clinopyroxene and olivine accumulation. The compositional variations within the olivine basalts can be accounted for by fractionation of plagioclase, clinopyroxene and olivine in an open system, steady state, magma chamber in the average proportions 453223. It is suggested that the most primitive olivine basalts can be derived from a pristine mantle composition by approximately 17% equilibrium partial melting. Although distinguished by its higher Zr/Nb ratio and lower NiO content of phenocryst phases, the magma parental to the picritic basalts can be derived from a similar source composition by approximately 27% equilibrium partial melting. It is suggested that the parental magma to the plagioclase-pyroxene and plagioclase phyric basalts might have been derived from greater depth resulting in the fractionation of the Zr/Nb ratio by equilibration with residual garnet.C.O.B. Contribution No. 722