Petrogenesis of olivine-phyric shergottite Yamato 980459, revisited

Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84-86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe-Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized.We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80-85) exhibit minor and trace element (Mn-Ni-Co-Cr-V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (∼IW + 1).Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3-7 °C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir.Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0-2 wt% of H₂O and 0-5 wt% of CO₂. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<∼6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition.     

On the nature and origin of isolated olivine grains in carbonaceous chondrites

Many carbonaceous chondrites contain discrete olivine fragments that have been considered to be primitive material, i.e. direct condensates from the solar nebula or pre-solar system material. Olivine occurring in chondrules and as isolated grains in C3(0) chondrites has been characterized chemically and petrographically. Type I chondrules contain homogeneous forsterite grains that exhibit a negative correlation between FeO and CaO. Type II chondrules contain zoned fayalite olivines in which FeO is positively correlated with CaO and MnO. The isolated olivines in C3(0) chondrites form two compositional populations identical to olivines in the two types of porphyritic olivine chondrules in the same meteorites. Isolated olivines contain trapped melt inclusions similar in composition to glassy mesostasis between olivines in chondrules. Such glasses can be produced by fractional crystallization of olivine and minor spinel in the parent chondrule melts if plagioclase does not nucleate. The isolated olivine grains are apparently clastic fragments of chondrules. Some similarities between olivines in C3(0), C2, and Cl chondrites may suggest that olivine grains in all these meteorites crystallized from chondrule melts.     

Oxygen isotope heterogeneity and disequilibria of olivine crystals in large volume Holocene basalts from Iceland: Evidence for magmatic digestion and erosion of Pleistocene hyaloclastites

This work considers petrogenesis of the largest Holocene basaltic fissure eruptions of Iceland, which are also the largest in the world: Laki (1783-84 AD, 15 km³), Eldgjá (934 AD, 18 km³), Veidivötn (900, 1480 AD, multiple eruptions, >2 km³), Núpahraun (ca. 4000 BP, >1 km³) and Thjórsárhraun (ca 8000 BP, >20 km³). We present oxygen isotope laser fluorination analyses of 55 individual and bulk olivine crystals, coexisting individual and bulk plagioclase phenocrysts, and their host basaltic glasses with average precision of better than 0.1‰ (1SD). We also report O isotope analyses of cores and rims of 61 olivine crystals by SIMS with average precision on single spots of 0.24‰ (1SD) in 13 samples coupled with electron microprobe data for major and trace elements in these olivines. Within each individual sample, we have found that basaltic glass is relatively homogeneous with respect to oxygen isotopes, plagioclase phenocrysts exhibit crystal to crystal variability, while individual olivines span from the values in equilibrium with the low-δ¹⁸O matrix glass to those being three permil higher in δ¹⁸O than the equilibrium. Olivine cores with maximum value of 5.2‰ are found in many of these basalts and suggest that the initial magma was equilibrated with normal-δ¹⁸O mantle. No olivines or their intracrystalline domains are found with bulk or spot value higher than those found in MORB olivines. The δ¹⁸O variability of 0.3-3‰ exists for olivine grains from different lavas, and variable core-to-rim oxygen isotopic zoning is present in selected olivine grains. Many olivines in the same sample are not zoned, while a few grains are zoned with respect to oxygen isotopes and exhibit small core-to-core variations in Fe-Mg, Ni, Mn, Ca. Grains that are zoned in both Mg# and δ¹⁸O exhibit positive correlation of these two parameters. Electron microprobe analysis shows that most olivines equilibrated with the transporting melt, and thin Fe-richer rim is present around many grains, regardless of the degree of olivine-melt oxygen isotope disequilibrium.The preservation of isotopic and compositional zoning in selected grains, and subtle to severe Δ¹⁸O (melt-olivine) and Δ¹⁸O (plagioclase-olivine) disequilibria suggests rather short crystal residence times of years to centuries. Synglacially-altered upper crustal, tufaceous hyaloclastites of Pleistocene age serve as a viable source for low-δ¹⁸O values in Holocene basalts through assimilation, mechanical and thermal erosion, and devolatilization of stoped blocks. Cumulates formed in response to cooling during assimilation, and xenocrysts derived from hyaloclastites, contribute to the diverse δ¹⁸O crystalline cargo. The magma plumbing systems under each fissure are likely to include a network of interconnected dikes and sills with high magma flow rates that contribute to the efficacy of magmatic erosion of large quantities (10-60% mass) of hyaloclastites required by isotopic mass balance.Olivine diversity and the pervasive lack of phenocryst-melt oxygen isotopic equilibrium suggest that a common approach of analyzing bulk olivine for oxygen isotopes, as a proxy for the basaltic melt or to infer mantle δ¹⁸O value, needs to proceed with caution. The best approach is to analyze olivine crystals individually and demonstrate their equilibrium with matrix.     

峨眉山大火成岩省平川苦橄岩地幔源区性质——来自橄榄石微量元素的制约

橄榄石是基性岩浆中最早期结晶的硅酸盐矿物之一,其主量、微量元素特征可以反映出岩浆演化环境、岩浆源区岩性和再循环组分性质等重要信息.本次研究通过对峨眉山大火成岩省平川苦橄岩中橄榄石主量和微量元素分析,以及橄榄石内尖晶石包裹体分析,并与大理苦橄岩中橄榄石和尖晶石成分进行对比,来探讨不同苦橄岩母岩浆氧逸度及源区性质的异同.橄...     

Chemical evolution of basalts from 23°N along the mid-Atlantic ridge: evidence from melt inclusions

The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe²⁺), 1.01–0.68 wt% TiO₂) and are comparable to other proposed parental magmas except in having higher Al₂O₃ and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An_78.3-An_83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo₈₉-Fo₉₁) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo₈₉ but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine, plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts and in the cores of zoned plagioclases, any model must account for all phases present.     

蚀变洋壳和俯冲带变质流体的Fe-Mg同位素组成

贫碳酸盐的蚀变洋壳具有与新鲜洋中脊玄武岩一致的Mg同位素组成,说明低温和高温洋壳蚀变不会导致Mg同位素分馏.大别山港河和花凉亭的早期变质脉比榴辉岩具有偏高的δ56Fe-δ26Mg值,而且早期到晚期变质脉的δ56Fe-δ26Mg值逐渐降低.这些结果说明,在流体-岩石反应和流体演化过程中,Fe-Mg同位素发生了显著的分馏,且矿物溶解-再沉淀是同位素分馏的控制因素.相比洋中脊玄武岩,蚀变洋壳和变质脉具有相似或偏高的δ56Fe-δ26Mg值,说明蚀变洋壳脱水产生的流体富集重Fe-Mg同位素,不能解释弧岩浆岩的轻Fe/重Mg同位素组成.因此,弧岩浆岩异常的Fe-Mg同位素组成是熔体提取和富集54Fe-26Mg的蛇纹岩流体交代地幔楔两个过程共同作用的结果.      

Supra-subduction affinity in the Neoproterozoic serpentinites in the Eastern Desert, Egypt: Evidence from mineral composition

Serpentinites in the Eastern Desert (ED) of Egypt represent integral components of the ophiolites. Metamorphic textures of the serpentinites preserve the complex mineralogical evolution from primary peridotite through metamorphism, and late-stage hydrothermal alteration. Two textural types are distinguished in the olivines of the present serpentinized peridotites, namely (a) highly-strained olivine grains with kink bands, as in the deformed mantle tectonites from ophiolites, and (b) non-strained grains. The latter may represent recrystallized crystals during later thermal metamorphic events due to the intrusion of granite. On the basis of X-ray diffraction analysis, antigorite is the main serpentine minerals with lesser chrysotile and lizardite which indicates that serpentinites were formed under prograde metamorphism. Relict primary minerals of the serpentinites are Cr-spinel, olivine and pyroxene. Chrome spinel relicts have high Cr# (0.60–0.80), whereas primary olivines are Mg-rich nature (Fo = 89–96). Geochemical compositions of serpentinites indicate that they formed not at mid-ocean ridges but at spreading centers associated with subduction zones and this could have happened in a supra-subduction zone either in the fore-arc or back-arc environments. Mineral compositions of primary chrome spinels and olivines are similar to those of modern fore-arcs. High Cr# in the relict chrome spinels and Fo in the primary olivines of serpentinites indicate that they are residual after extensive partial melting and originated by sea-floor spreading during subduction initiation.     

Thermodynamics of water in cordierite and some petrologic consequences of cordierite as a hydrous phase

Olivine melilitites from Namaqualand, South Africa are characterized by a broad range in olivine compositions on the scale of individual hand specimens. It is possible to distinguish four petrographically and chemically distinct olivine populations in both the northern and southern pipe clusters studied: (a) Scarce anhedral or subhedral olivines that display marked disequilibrium features with the surrounding matrix, and which are characterized by having high iron and extremely low nickel contents (referred to as HILN olivines) relative to the other olivines in the same rock, (b) A dominant population of euhedral and often skeletal (hopper) olivines that are richer in Mg and Ni than the HILN olivines in the same rock. There are in addition unusual hopper olivines that are petrographically similar to the skeletal olivines, but show aberrant zonation patterns. Hopper and HILN type olivines contain fluid and carbonate inclusions which apparently record the loss of a vapour phase and an immiscible carbonate liquid during magma ascent, (c) A third population consists of large rounded olivines (megacrysts), up to 40 mm in greatest diameter. Individuals are chemically homogeneous, but megacrysts from the same pipe collectively define a trend of decreasing Mg and Ni (Fo₉₂, 0.36% Ni to Fo₇₅, 0.17% Ni). The most fayalitic megacrysts are depleted in Mg and Ni relative to the hopper olivines in the same rock, (d) Scarce magnesium-rich (Fo₉₁) anhedral olivines which show strained extinction are believed to be xenocrysts.It is suggested that the HILN-type olivines crystallized from primitive carbonate-rich magmas under conditions of low oxygen fugacity, intermediate between the Ni-NiO and Fe-FeO buffers. Mineral-melt partition coefficients for the transition elements determined in basaltic systems are considered to be inappropriate to such carbonate-rich melts. Loss of volatiles and an immiscible carbonate liquid during magma ascent resulted in an increase in oxygen activity, a decrease in the Fe-Mg distribution coefficient (K _D ) for olivine and liquid and an increase in liquidus temperatures. These effects led to the rapid crystallization of Mg- and Ni-enriched skeletal hopper olivines. The unusual hoppers crystallized later than the HILN olivines but prior to the normal hoppers, under conditions chracterized by rapid and independent changes in oxygen activity and partition coefficients associated with the loss of volatiles and an immiscible carbonate liquid. The range in chemistry which characterizes the megacryst-olivine suite is believed to record physico-chemical changes to the magmas subsequent to separation from a mantle source area, but prior to crystallization of the HILN olivines. Most important of these changes was an increasing degree of polymerization of the liquid structure and a progressive decrease in oxygen activity as the molar ratio (CO ₃ ^2- /(CO ₃ ^2- + CO₂)) in the magma increased with decreasing pressure. Increasing polymerization of the liquid resulted in an increase in olivine-liquid partition coefficients for transition elements.Olivines in kimberlites show compositional characteristics and zonation patterns similar to those recognized in the olivine melilitites which, coupled with ilmenite compositions, suggests that the two magma types initially evolved along similar physico-chemical paths.     

The pattern of Ni and Co abundances in lunar olivines

Near liquidus experiments on peridotite and other olivine normative compositions from 1.7 to 6 GPa confirm the applicability of exchange-based empirical models of Ni and Co partitioning between olivine and silicate liquids with compositions close to the liquidus of peridotite. Given that most estimates of lunar bulk composition are peridotitic, the partitioning models thus lend themselves to calculation of olivine compositions produced during the early stages of magma ocean crystallization. Calculation of olivine compositions produced by fractional crystallization of a model lunar magma ocean, initially 700 km deep, reveals a prominent maximum in Ni concentration versus fraction crystallized or Mg’ (molar MgO/(MgO + FeO)), but a pattern of monotonically increasing Co concentration. These patterns qualitatively match the puzzling patterns of Ni and Co concentrations observed in lunar rocks in which forsteritic olivines in magnesian suite cumulates have lower Ni and Co abundances than do less magnesian olivines from low-Ti mare basalts, and olivines from the ferroan anorthosite suite (FAS) have lower Ni, but similar Co to mare basalt olivines.The Ni and Co abundances in olivines from the magnesian suite cumulates can be reconciled in terms of fractional crystallization of a deep magma ocean which initially produces a basal dunite comprised of the hottest and most magnesian olivine overlain by an olivine-orthopyroxene (harzburgite) layer that is in turn overlain by an upper zone of plagioclase-bearing cumulates. The ultramafic portion of the cumulate pile overturns sending the denser harzburgite layer, which later becomes a portion of the green glass source region, to the bottom of the cumulate pile with Ni- and Co-rich olivine. Meanwhile, the less dense, but hottest, most magnesian olivines with much lower Ni and Co abundances are transported upward to the base of the plagioclase-bearing cumulates where subsequent heat transfer leads to melting of mixtures of primary dunite, norite, and gabbronorite with KREEP (a K-REE-P enriched component widely believed to be derived from the very latest stage magma ocean liquid). These hybrid melts have Al₂O₃, Ni, and Co abundances and Mg’ appropriate for parent magmas of the magnesian suite. Ni and Co abundances in the FAS are consistent with either direct crystallization from the magma ocean or crystallization of melts of primary dunite-norite mixtures without KREEP.     

Zoning patterns in orthopyroxene and garnet in granulites: implications for geothermometry

Compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched-in. The extent of late Fe-Mg exchange was controlled by neighbouring minerals, with negligible Fe-Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe-Mg minerals. Cores of grains in contact with exchangeable Fe-Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high-temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe-Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe-Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d? C; (2) Fe-Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature-sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d? C higher than maximum Fe-Mg exchange temperatures, even in grains with flat Fe/(Fe - Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe-Mg exchange thermometry.     

Chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in CV chondrites: Constraints on the evolution of nebular dust

A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca-Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and ¹⁶O-rich olivine (Fa_1-40 and Δ¹⁷O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and ¹⁶O-depleted (Fa_15-50 and Δ¹⁷O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and ¹⁶O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and ¹⁶O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ¹⁷O values. However, not all fine-grained olivines are FeO-rich and ¹⁶O-poor; some small (<1 μm in Leoville and 5-10 μm in Vigarano and Allende) ferrous (Fa_>20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in ¹⁶O (Δ¹⁷O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an ¹⁶O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as ¹⁶O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an ¹⁶O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of ¹⁶O-rich forsterites with grain overgrowths or newly formed grains of ¹⁶O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of ¹⁶O-rich forsterite and subsequently exchanged Fe-Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).     

The significance of unusual zoning in olivines from FAMOUS area basalt 527-1-1

Major element, Ni, Mn and Ca electron microprobe analyses of olivine phenocrysts in one of the most primitive basalts from the FAMOUS area of the Mid-Atlantic Ridge, 527-1-1, reveal two different olivine populations, distinguished by their zoning characteristics. The often skeletal Group I olivines have zoning profiles with high forsterite, high Ni, low Mn, and low Ca cores. These profiles can be explained by low pressure crystallization from the 527-1-1 magma on cooling. The equant, often megacrystic Group II olivines also have high forsterite, low Mn cores, but the cores have low Ni compared to rims. Thus they are normally zoned with respect to Mg, Fe, and Mn but reversely zoned with respect to Ni. For example, Ni ranges from 1,700 ppm at Fo_90.5 in the core to 2,100 ppm at Fo_89.5 at the rim. In view of the published whole rock and mineral data of le Roex et al. (1981), the most likely explanation for these data is that the Group II olivines are xenocrysts assimilated from solidified plagioclase-pyroxene basalts through which the 527-1-1 basalt ascended. The diffusion rate of Ni and size of the xenocrystic olivines are used to calculate the residence time of the assimilated olivines in the magma. An alternative hypothesis would be a high pressure origin for the Group II olivines. This would be possible if future experiments show that with increasing pressure the partition coefficient for Ni decreases relative to the partition coefficient for Mg for a given bulk composition. Current evidence suggests this is unlikely. The data from 527-1-1 and other samples from the FAMOUS area require magmas with distinct Ni-MgO-FeO characteristics. In general, MORB from different ridge segments fall on distinct trends on plots of MgOvs. Ni as well as MgOvs. FeO. Calculation of the MgO and Ni contents of primary magmas suggests a mechanism by which such distinct trends could come about.     

Effect of Temperature upon Fe-Mg Composition of Garnet in Pelitic System

Garnet is an essential phase in a wide range of metamorphic grades. Ratios of Fe and Mg concentrations in garnet have been widely used as a geothermometer for the crystallization, cooling rate and tectonic uplift rate because garnets in natural rocks always show a variety of Fe-Mg compositional partition. Normally, Fe-Mg concentrations of natural garnets vary widely and the Fe-Mg exchange reactions usually occur in a complex chemical system. Effect of pressure and temperature on Fe-Mg compositions of garnet in natural pelitic system was studied experimentally at temperatures of 700 to 780 ℃ and pressures from 21 to 29.4 kbar. The concentrations of FeO and MgO of garnet in the run products showed a linear relation with experimental temperatures. The result provided experimental evidence of Fe-Mg partition trend with temperature in a complex natural rock and therefore improved our understanding in the determinations of metamorphic temperature calculated by garnet geothermometer. Although the accurate geological application of our results requires the knowledge of the effects of other components upon [D(Mg)] and [D(Fe)] which is largely unknown currently, the data provided in this study are useful to build the relation between FeO-MgO contents and temperature, which is useful in the metamorphic temperatures determine of rocks with pelitic composition.     

橄榄石标型特征及其找矿意义

通过国内２０个含铬铁矿、铜镍矿和饥钛磁铁矿的基性、超基性岩体中６８个橄榄石化学成分、产出状态、地球化学特征、有序度和岩浆包裹体橄榄石子矿物、温度、压力等标型特征研究显示，产于不同矿化基性、超基性岩中橄榄石的标型特征明显不同，为找寻铬铁矿、铜镍矿和钒钛磁铁矿矿产提供重要标志。     

Zoning of phosphorus in igneous olivine

We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P₂O₅ contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2–0.4 wt% over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15–30°C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma’s history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.     

Glasses in the D'Orbigny angrite

The angrites are a small and heterogeneous group of achondritic meteorites with highly unusual chemical and mineralogical features. The abundant presence of glasses in D'Orbigny makes this rock a unique member of the angrite group. Glasses fill open spaces, form pockets, and occur as inclusions in olivines. Their physical settings exclude an incorporation from an external source. Major and trace element (rare earth elements [REE], Li, B, Be, transition elements, N and C) contents of these glasses and host olivines were measured combining laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), Nuclear Reaction Analysis (NRA), and EMP techniques. Based on the major element composition, glasses filling voids could represent either a melt formed by melting an angritic rock or a melt from which angrites could have crystallized. Trace element contents of these glasses strongly indicate a direct link to the D'Orbigny bulk meteorite. They are incompatible with the formation of the glasses by partial melting of a chondritic source rock or by shock melting. The refractory elements (e.g., Al, Ti, Ca) have about 10 × CI abundances with CaO/TiO₂ and FeO/MnO ratios being approximately chondritic. Trace element abundances in the glasses appear to be governed by volatility and suggest that the refractory elements in the source had chondritic relative abundances. Although the glasses (and the whole rock) lack volatile elements such as Na and K, they are rich in some moderately volatile elements such as B, V, Mn, Fe (all with close to CI abundances), and Li (about 3-5 × CI). These elements likely were added to the glass in a sub-solidus metasomatic elemental exchange event. We have identified a novel mechanism for alteration of glass and rock compositions based on an exchange of Al and Sc for Fe and other moderately volatile elements in addition to the well-known metasomatic exchange reactions (e.g., Ca-Na and Mg-Fe).Because glass inclusions in olivine were partly shielded from the metasomatic events by the host crystal, their chemical composition is believed to be closer to the original composition than that of any other glasses. The relative trace element abundances in glasses of glass inclusions in olivine and glass pockets are also unfractionated and at the 10 to 20 × CI level. These glasses are chemically similar to the common void-filling glasses but show a much wider compositional variation. Inclusion glasses demonstrate that at least olivine grew with the help of a liquid. In analogy to olivines in carbonaceous chondrites, initial formation could also have been a vapor-liquid-solid condensation process. At that time, the glass had a purely refractory composition. This composition, however, was severely altered by the metasomatic addition of large amounts of FeO and other moderately volatile elements. The presence of volatile elements such as carbon and nitrogen in glasses of glass inclusions is another feature that appears to give these glasses a link with those hosted by olivines of carbonaceous chondrites. All these features point to an origin from a vapor with relative abundances of condensable elements similar to those in the solar nebula.     

一种判别原始岩浆的方法--以苦橄岩和碱性玄武岩为例

在前人工作基础上，利用橄榄石和熔浆的Fe—Mg分配乐数，重新制作了MgO—Fo—FeO图解，该图解的优点在于其适用范围更宽，既可用于超基性的苦橄岩，也可用于一般的玄武岩或橄榄玄粗岩系。同时介绍了该图解运用的方法，并且以峨眉山大火成岩省中苦橄岩和黑龙江镜泊湖地区全新世碱性玄武岩为例，说明如何应用该图解来恢复原始岩浆以及解释岩石的成因。研究结果表明，峨眉山大火成岩省中的苦橄岩部分代表了原始岩浆，部分有过剩橄榄石的加入，部分为演化的岩浆形成。与苦橄岩密切共生的辉斑玄武岩是苦橄质岩浆通过橄榄石的分离结晶作用形成的，而在峨眉山大火成岩省中占绝对优势的温流玄武岩则不是苦橄质岩浆简单的分离结晶作用形成。镜泊湖地区的碱性玄武岩均为演化的岩浆，而碧玄岩演化程度较低，碱玄岩则是原始岩浆高度演化的产物。     

地幔橄榄岩中橄榄石的指示意义

橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素（如Mg和Fe）组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明：地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#（〉92）,而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#（〈91）。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明：该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。     

Chloritoid-bearing pelitic rocks of the Horsethief Creek Group,Southeastern British Columbia

Aluminous pelitic rocks of the Late Precambrian Horsethief Creek Group of southeastern British Columbia contain the assemblage chloritoidmuscovite-paragonite-quartz-chlorite (biotite zone). Additional members of the assemblage may include graphite, Fe-Mg carbonate, rutile, ilmenite and pyrite. No albite was detected. Lower grade pelitic rocks (chlorite zone) contain muscovite-chlorite and rare paragonite.Chloritoids from carbonate-free assemblages show a narrow range of composition (85±5 mol % Fe-chtd) and most porphyroblasts are zoned with higher Mn in cores and higher Mg in rims. For eight chloritoid-chlorite pairs, K _D = (Mg/Fe chtd/Mg/Fe chl) = 0.188±0.0234.Correlation of these mineral assemblages with experimental and computed phase equilibria and oxygen isotope temperatures suggest a minimum pressure near 4.5 Kbar, a minimum temperature near 335 ° C and an upper limit on temperature near 460 ° C. Variation in X _{CO 2} content of fluids attending metamorphism is inferred from the alternate appearance of either Fe-Mg carbonate + rutile or ilmenite-bearing assemblages. The assemblage paragonite-chloritoid-quartz-Fe-Mg carbonate-rutile is inferred to be stable at a T near 360 ° C, an X _{CO 2} near 0.9 and P near 5 Kbar.     

Olivine phenocrysts in some Cameroonian Basalts—implications for primary magma composition

Summary The chemical composition of olivine phenocrysts was determined for 13 basaltic samples taken from the central part of the Cameroon Volcanic line to estimate the compositions of the primary olivine phenocrysts and the primary magmas. The Mg/(Mg + Fe) ratios of the olivines attain 0.91, though many of the magnesian olivines are identified on a textural basis as xenocrysts from peridotite inclusions. Most magnesian olivine phenocrysts have Mg/(Mg + Fe) ratios of 0.87–0.88 and NiO contents of 0.32–0.35 wt %. The NiO versus Mg/(Mg + Fe) ratios of the olivines can be used to estimate the compositions of the primary phenocrysts in a range of Mg/(Mg + Fe) = 0.88–0.90. Assuming the Fe(3)/(total Fe) ratios of the magmas are 0.17–0.25, which is estimated from the Fe-Mg partitioning between plagioclase and groundmass, the compositions of the primary magmas were obtained from the Fe²⁺-Mg partition equilibrium with the primary olivine phenocrysts. The primary magmas of the Cameroonian basalts have 13.5 ± 4.0 wt MgO, which is similar to the compositions of basalts with highest MgO contents reported so far from the Cameroon volcanic line.

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Olivin Kristalle in einigen Basalten aus Kamerun: Hinweise auf die Zusammensetzung des primitiven Magmas

Zusammenfassung Die chemische Zusammensetzung von Olivin Phenokristallen wurde in 13 Basalt-Proben, die vom Zentralteil .des Kamerun-Vulkangürtels stammen, bestimmt, um Hinweise auf die Zusammensetzung der primären Olivineinsprenglinge and des primären Magmas zu erlangen. Die Mg/(Mg + Fe) Verhältnisse der Olivine reichen bis 0.91, obwohl viefle dieser Mg-Olivine auf Grund textureller Kriterien als aus Peridotit-Einschlüssen stammende Xenokristalle zu identifizieren sind. Die Mg/(Mg + Fe) Verhältnsse der Hauptmasse der Olivin-Phenokristalle liegen im Bereich von 0.87 bis 0.88, die Ni0 Gehalte zwischen 0.32 und 0.35 Gew %. Die Ni0 gegen Mg/(Mg + Fe) Verhältnisse eignen sich zur Abschätzung der Zusammensetzung der primären Phenokristalle, solange die Mg/(Mg + Fe) Verhältnisse im Bereich von 0.88 bis 0.90 liegen. Die Zusammensetzung des Primärmagmas wurde mittels der Fe²⁺-Mg-Verteilung zwischen dem Magma und den sich mit dem Magma im Gleichgewicht befindlichen primären Olivinkristallen unter der Annahme eines Fe³⁺/Fe total Verhältnisses von 0.17–0.25, welches sich aus der Mg-Fe Verteilung zwischen Plagioklas und Grundmasse ergibt, bestimmt. Demnach ergibt sich für das primäre Magma der Kamerun Basalte ein MgO Gehalt von 13.5 + 4.0 Gew%. Dieser Wert ist mit der Zusammensetzung von Basalten mit höchsten MgO-Gehalten, wie sie bisher vom Kamerun-Vulkangürtel berichtet worden sind, vergleichbar.

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With 6 Figures