Tektites: A post-Apollo view

Prior to the receipt of the lunar samples, it was the scientific consensus that tektites were melted and splashed material formed during large cometary or meteorite impact events. Whether the impact took place on the Earth or the Moon was the topic of a long-standing scientific debate, which raged with particular intensity during the decade previous to the lunar landings.Four definite and separate tektite-strewn fields are known: bediasites (North America, 34 m.y.); moldavites (Czechoslovakia, 14 m.y.); Ivory Coast (1.3 m.y.); and Southeast Asian and Australian fields (0.7 m.y.). A fifth possible occurrence, of high-Na australites, possibly 3–4 m.y. old, remains to be substantiated. The age of infall of the australites is not agreed upon. Radiometric and fission track dates agree with the magnetic stratigraphy for deep-sea core microtektite occurrences at about 0.7 m.y. Terrestrial stratigraphic evidence favours a recent (30,000 years) date.The chemistry of tektites appears to reflect that of the parent material, and losses during fusion appear to be restricted to elements and compounds more volatile than cesium. Terrestrial impact glasses provide small-scale analogues of tektite-forming events, and indicate that only the most volatile components are lost during fusion.The Apollo lunar missions provide critical evidence which refutes the hypothesis of lunar origin of tektites. Tektite chemistry is totally distinct from that observed in lunar maria basalts. These possess Cr contents which are two orders of magnitude higher than tektites, distinctive REE patterns with large Eu depletions, high Fe and low SiO₂ contents, low K/U ratios and many other diagnostic features, none of which are observed in the chemistry of tektites. The lunar uplands compositions, as shown by Apollo 14, 15 and 16 samples and the μ-ray and XRF orbiter data, are high-Al, low-SiO₂ compositions totally dissimilar to those of tektites. The composition of lunar rock 12013 shows typical lunar features and is distinct from that of tektites. The small amounts of lunar K-rich granitic material found in the soils have K/Mg and K/Na ratios 10–50 times those of tektites.The ages of the lunar maria (3.2–3.8 aeons) and uplands (> 4.0 aeons) are an order of magnitude older than the parent material of the Southeast Asian and Australian tektites, which yield Rb-Sr isochrons indicating ages of the order of 100–300 m.y. The lunar lead isotopic compositions are highly radiogenic whereas tektites have terrestrial Pb isotopic ratios. Lunar δ¹⁸ O values are low (< 7 per mil) compared with values of +9.6 to +11.5 per mil for tektites. In summary, a lunar impact origin for tektites is not compatible with the chemistry, age or isotopic composition of the lunar samples. A lunar volcanic origin, recently revived by O'Keefe (1970) encounters most of the same problems. Recent lunar volcanism (< 50 m.y.), if the source of tektites, should contribute tektite glass to the upper layers of the regolith. None has been found. The presence of meteoritic components in tektites, and the high pressure phase coesite, are more readily interpreted as evidence of impact.The element abundances and inter-element variations in tektites do not resemble those in terrestrial igneous rocks, but show a close similarity to terrestrial sandstones. The composition of the Southeast Asian tektites, australites and moldavites resembles that of micaceous sandstones or subgreywackes, the Ivory Coast tektite composition is similar to that of greywacke, and the bediasite chemistry is analogous to that of arkose.No suitable terrestrial impact site has been identified for the bediasites, Southeast Asian tektites and australites. It is suggested that a search for the source of these latter strewnfields be made using satellite photographs to look for wide shallow craters produced by super-Tunguska type events on areas of Mesozoic sandstones. The moldavites were possibly formed during the Ries Crater event but, if so, the precise source of the material remains to be identified. The Ivory Coast tektites are linked by chemistry, isotope and age evidence to the Bosumtwi Crater, Ghana. The overall evidence now supports the origin of tektites by cometary (or meteorite) impact on terrestrial sedimentary rocks.     

Enigmatic cathodoluminescent objects in the Dhofar 025 lunar meteorite: Origin and sources

Dhofar 025 is a lunar highland breccia consisting mainly of anorthositic, with less common noritic, gabbronoritic, and troctolitic material. Rare fragments of low-Ti basalts are present as well, but no KREEP (component enriched in incompatible elements) was found in the meteorite. The cathodoluminescence study of this meteorite showed that its impact–melt matrix contains unusual cathodoluminescent (CL) objects of feldspathic composition, which frequently contain microlites of Fe-Mg spinel (pleonaste). They were presumably formed by impact mixing and melting of olivine and plagioclase with subsequent rapid quenching of the impact melts. Such mixing could happen either during assimilation of anorthosites by picritic/troctolitic magmas or during impact melting of troctolites. The enrichment of CL objects of Dhofar 025 in incompatible trace elements suggests that the mafic component of the impact mixture may be related to the high-magnesium suite rocks, which are frequently enriched in KREEP component. The depletion of CL objects in alkalis indicates their possible relation with residual glasses formed by evaporation. However, the presence of FeO in most objects points to the insignificant extent of evaporation. Thus, evaporation cannot explain the enrichment of the CL objects in Al₂O₃ and other refractory components, although this process definitely took place in their formation. Their similarity to the lunar pink spinel anorthosites, whose existence was predicted from orbital data, serves as an argument in support of the possible formation of the latters by impact mixing.     

The Bovedy meteorite; mineral chemistry and origin of its Ca-rich glass inclusions

The Bovedy meteorite fell on 25 April 1969 in Northern Ireland; the main mass of 4·94 kg was found at Bovedy (54°57′N, 06°37′W). It is an L3 chondrite with abundant chondrules clearly visible in hand specimen. Bulk chemical analyses are presented, the total Fe content being 22·5%. The olivines are homogeneous (Fa₂₄) but the pyroxenes are not equilibrated (Fs_8–28). Brown glass is common within chondrules but a clear glass of composition An₈₅ is present interstitially in a few orthopyroxene-rich (Fs_17–28) chondrules. A bleb, 2 mm across, of clear glass, again of composition An₈₅ was found in one stone of the meteorite and in the glass five REE (rare earth elements), (La, Sm, Eu, Yb, Lu) were determined. The low REE abundances coupled with a large positive Eu anomaly are characteristic of plagioclase, but the finer details of the pattern suggest that this glass has a closer affinity to the lunar anorthosites than to plagioclases from lunar mare basalts or eucritic meteorites. There is also evidence that the magnitude of the Eu anomaly for plagioclases and anorthosites from extra-terrestrial sources is inversely related to trivalent REE content. The existence of anorthositic material and, as a consequence, a differentiated planetary body prior to the formation of the Bovedy meteorite is suggested.     

嫦娥三号着陆区月海玄武岩的年龄、成因及地质意义

月海玄武岩主要分布在月海中,由斜长石、辉石和橄榄石组成,与地球玄武岩相比,富铁而贫钠和钾。月海玄武岩的成因,年代和成分研究是理解月岩形成与演化的基础。月球返回的岩石样品数量和覆盖面积有限,并且CE-3号着陆区没有月球样品返回,所以地球上的实验室样品分析方法不能应用到CE-3号着陆区玄武岩研究。本文对CE-3号着陆区月表和下伏玄武岩的组分、来源、分布、年代和层序进行反演和分析。主要使用的研究方法主要包括:玄武岩单元的撞击坑频率-分布函数定年,基于撞击坑的月壤下伏玄武岩单元识别、划分及厚度反演,基于遥感数据的元素含量和矿物成分分析等。结果表明:(1)CE-3号着陆区至少出现了6次较大规模的岩浆充填事件,由新到老分别为EIm、EIm_1、EIm_2、Im、Im_1和Im_2,其中EIm单元年龄约为3.17Ga,Im单元年龄为3.48Ga;(2)研究区玄武岩单元铁元素成分变化不大,而钛元素含量有较大变化。并且玄武岩越年轻,铁和钛元素更加富集。对指示月表硅酸盐矿物的克里斯蒂安参数(CF)和弯曲指数(CI)进行计算,发现研究区没有高硅物质的分布;(3)研究区月海玄武岩充填活动具有多期次性,每期月海玄武岩的充填流动大体上保持由南向北方向,并且活动规模逐步减小;(4)下伏玄武岩单元EIm_1、EIm_2、Im、Im_1和Im_2的平均厚度分别为68.3m,68.6m,81.8m,59.1m和52.1m,其中EIm_1单元的厚度最大为150m,从西到东逐渐减小;Im_1最大深度为224m,位于研究区的北部,向东西两侧依次减小。     

Petrology, geochemistry, and age of low-Ti mare-basalt meteorite Northeast Africa 003-A: A possible member of the Apollo 15 mare basaltic suite

Northeast Africa 003 (NEA 003) is a lunar meteorite found as a two paired stones (6 and 118 g) in Libya, 2000 and 2001. The main portion (∼75 vol%) of the 118 g meteorite, used for this study, (NEA 003-A) consists of mare-basalt and a smaller adjacent portion (∼25 vol%) is a basaltic breccia (NEA 003-B). NEA 003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase. The late-stage mineral association is composed mainly of elongated plagioclase, ilmenite, troilite, fayalite, Si-K-rich glass, apatite, and a rare SiO₂ phase. Other accessory minerals include ulvöspinel, chromite, and trace Fe-Ni metal. Olivine and pyroxene contain shock-induced fractures, and plagioclase is completely converted into maskelynite.The Fe/Mn values of the whole rock, olivines and pyroxenes, and the bulk-rock oxygen isotopic composition provide evidence for the lunar origin of NEA 003-A meteorite. This is further supported by the presence of Fe-Ni metal and the anhydrous mineral association.NEA 003-A is geochemically and petrographically distinct from previously described mare-basalt meteorites and is not paired with any of them. The petrography and major element composition of NEA 003-A is similar to the composition of low-Ti olivine mare basalts from Apollo 12 and olivine-normative basalts from Apollo 15. The NEA 003-A meteorite shows obvious geochemical similarities in trace elements contents with Apollo 15 olivine-normative basalts and could represent a yet unknown geochemically primitive member of the olivine-normative basalt series. The meteorite is depleted in rare earth elements (REE) and incompatible trace elements indicating a primitive character of the parental magma. The bulk-rock chemical composition demonstrates that the parent melt of NEA 003-A was not contaminated with KREEP components as a result of magma mixing or assimilation processes. Results of crystallization modelling and low minimum cooling rate estimates (∼0.07 °C/h) suggest that the parent melt of NEA 003-A crystallized in the lower part of a lava flow containing cumulate olivine (∼10%) and was probably derived from more primitive picritic magma by fractional crystallization processes.Sm-Nd dating yields an age of 3.09 ± 0.06 Ga which corresponds to the period of lower Eratosthenian lunar volcanic activity, and the near-chondritic ε_Nd value of −0.4 ± 0.3 indicates that the meteorite could be derived from a slightly enriched mantle source similar to the Apollo 15 green glasses. Ar-Ar step release results are inconsistent with Sm-Nd ages suggesting that NEA 003-A was exposed to one or more impact events. The most extensive event took place at 1.8 Ga and the shock intensity was likely between 28 and 45 GPa. The absence of solar Ar suggests that NEA 003-A has not been directly exposed at the lunar surface but the cosmic ray exposure age of 209 ± 6 Ma suggests that NEA 003-A resided in the upper regolith for part of its history.     

The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite

The two textural varieties of olivine-rich Allende inclusions (rimmed and unrimmed olivine aggregates) consist primarily of a porous, fine-grained mafic constituent (inclusion matrix) that differs from the opaque meteorite matrix of CV3 chondrites by being relatively depleted in sulfides, metal grains, and (perhaps) carbonaceous material. Olivine is the most abundant mineral in Allende inclusion matrix; clinopyroxene, nepheline, sodalite, and Ti-Al-pyroxene occur in lesser amounts. Olivine in unrimmed olivine aggregates (Type 1A inclusions) is ferrous and has a narrow compositional range (Fo_50–65). Olivine in rimmed olivine aggregates (Type 1B inclusions) is, on average, more magnesian, with a wider compositional range (Fo_53–96). Olivine grains in the granular rims of Type 1B inclusions are zoned, with magnesian cores (Fo_>80) and ferrous rinds (Fo_<70). Ferrous olivines (Fo_<65) in both varieties of inclusions commonly contain significant amounts of Al₂O₃ (as much as ~0.7 wt%), CaO (as much as ~0.4 wt%), and TiO₂ (as much as ~0.2 wt%), refractory elements that probably occur in submicroscopic inclusions of Ca,Al,Ti-rich glass (rather than in the olivine crystal structure). Defocussed beam analyses of Allende matrix materials demonstrate that: (1) inclusion matrix in Type 1A inclusions is more enriched in olivine and FeO than inclusion matrix in the cores of Type 1B inclusions; (2) opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix; (3) the bulk compositions of Type 1A and Type 1B inclusions overlap; and (4) excluding sulfides and metal, the bulk compositions of Allende matrix materials cluster in a complementary pattern around the bulk composition of C1 chondrites.Inclusion matrix and meteorite matrix in Allende and other CV3 chondrites are probably relatively primitive nebular material, but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition. Allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing, non-equilibrium conditions from supercooled, supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula; CV3 meteorite matrix contains, in addition, a proportion of interstellar material that was heated (but not vaporized) in the nebula. Granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula. The mineral chemistry of matrix olivine in Allende seems to have been established by three different processes: non-equilibrium vapor → solid condensation; recrystallization and partial melting in the nebula; and $\text{Fe}\text{Mg}$ equilibration (without textural homogenization) in the meteorite parent body.     

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。     

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.     

Plagioclase-aqueous solution equilibrium: Concentration dependence

The plagioclase-(NaCl + CaCl₂) exchange equilibrium was examined experimentally at 700°C, 0.5 GPa in aqueous solutions with salt concentrations from 1 to 64 m. The Ca/(Ca + Na) distribution between plagioclase and solution (salt melt) is illustrated in five diagrams constructed for concentrations of 1, 4, 8, 16, and 64 m. The elevated bulk salinity of the fluid at a constant Ca/(Ca + Na) ratio results in plagioclase albitization, with this effect reaching a maximum in relatively dilute solutions (1–4 m). In concentrated solutions (salt melts), the shift in the plagioclase composition with variations in the salinity is relatively insignificant. The simple hydration of basic rocks (purely metamorphic reaction) is associated with the albitization of plagioclase, and calculations suggest a possible shift from anorthite to oligoclase. This is also applicable to chemically more complex mineral associations: an increase in the overall salinity of the fluid should result in an increase in the activity of monovalent cations relative to that of bivalent ones and, correspondingly, stimulate reactions in which alkali earth cations (Ca + Mg + Fe) are substituted for alkalis (Na + K + Li). Although our experiments were carried out at temperatures 50°C lower than the melting point of albite under a pure water pressure (0.5 GPa), the addition of CaCl₂ solution to albite (i.e., plagioclase anorthitization and a decrease in the water activity in the salt solutions) induced the appearance of melt because of quartz formation by the reaction 2Ab + CaCl₂ → An + 2NaCl + 4Qtz and the eutectic phase proportions in the Ab + Qtz system.     

Chemical anomalies of lunar plagioclase,described by substitution vectors and their relation to optical and structural properties

Lunar plagioclase posed an enigma because optical properties indicate slow cooling while structural properties suggest rapid quenching when compared directly to terrestrial plagioclase. If, however, the An-content is inferred from the Al/Si ratio rather than from the Ca/Na ratio, lunar plagioclase corresponds more closely to normal volcanic plagioclase. The differences between An=4(Al/(Al+Si))-1 and An=Ca/(Ca+Na+K) is caused by substitutions which appear so far to be unique to lunar feldspars. A vector representation of chemical composition makes it possible to single out several substitutions which account for the chemical peculiarities of lunar plagioclase. It permits to assign sites in the structure to various minor elements simply from the chemical analysis without having to rely on sophisticated spectroscopic techniques or an X-ray structure determination. 1. The deficiency in Al+Si (up to 0.06 per formula unit) is compensated for by Fe, Mg, and small amounts of Na or Ca occupying tetrahedral sites. The Ca/Na ratio is increased accordingly to balance the electric charge. This substitution accounts for most of the differences. 2. The vacancycoupled substitution Ca+□ → 2 Na (Weill et al., 1970) is another cause for the increased Ca/Na ratio. 3. The amount of alkali evaporation from plagioclase, if it occurred, is probably small. These unusual substitutions are an expression of special conditions on the moon during the crystallization of plagioclase. Crystallization at high temperatures, rapid cooling, and peculiar magma composition seem to be important factors, but experimental work and detailed study of terrestrial volcanic plagioclase are necessary to interpret the chemical composition quantitatively.     

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.     

Origin of organic matter in early solar system—V. Further studies of meteoritic hydrocarbons and a discussion of their origin

The Murchison meteorite contains aliphatic and aromatic hydrocarbons similar to those made in static Fischer-Tropsch-type syntheses. Principal compound classes above C₈ are n-alkanes, mono- and dimethylalkanes, alkenes, alkylbenzenes and -naphthalenes. Below C₈, n-alkanes are virtually absent; instead, benzene, toluene, branched alkanes dominate. The CH₄/C₂H₆ ratio is greater than 30, possibly greater than 700. Isoprenoids from C₁₇ to C₂₀ occur in a surface rinse but not in subsequent extracts and appear to be terrestrial contaminants. Thiophenes, porphyrin-like pigments and chlorobenzenes were also found; the latter appear to be contaminants. In the Allende meteorite, only methane, benzene, toluene and an aromatic polymer seem to be indigenous.     

Details of the gabbro‐to‐eclogite transition determined from microtextures and calculated chemical potential relationships

Permian‐aged metagabbros from the eclogite type‐locality in the eastern European Alps were partially to completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro‐to‐eclogite transition, allowing the details of the eclogitization process to be investigated. Original, anorthite‐rich igneous plagioclase is pervasively replaced by fine‐grained intergrowths of clinozoisite, kyanite and Na‐rich plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100–200 μm thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet. Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds. Given that the igneous plagioclase is pervasively replaced by clinozoisite, kyanite and Na‐rich plagioclase, whereas kyanite–diopside symplectites are confined to narrow rim zones, we suggest that the development of these textures was controlled by the (im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of anhydrous plagioclase indicates that H₂O diffusivity occurred on a mm‐scale. By contrast, the size of the anhydrous diopside–kyanite and diopside–garnet symplectites indicate that Fe–Mg–Ca–Na diffusivity was limited to a 10s of μm scale. Chemical potential relations calculated in the idealized NCASH chemical system show that the clinozoisite–kyanite–albite intergrowths formed due to an increase of μH₂O to plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid conditions indicated by this texture span from virtually dry conditions (0.15) to H₂O‐saturation, and therefore does not imply that the rocks were ever fluid‐saturated. Calculations in the CMAS and NCFMAS systems show that the gabbro‐to‐eclogite transition is characterized by the growth of garnet, diopsidic clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg (+ Fe) along a μCaO (+ Na₂O)–μMgO (+ FeO) chemical potential gradient developed between orthopyroxene and plagioclase compositional domains. The anhydrous nature of the textures indicate that the gabbro‐to‐eclogite transition is not driven by hydration; however, increased μH₂O acts as a catalyst that increases diffusivity of all elements and rates of dissolution–precipitation, allowing the overstepped metamorphic reactions to occur. Our results show that crustal eclogite formation requires low H₂O content, confirming that true eclogites are dry rocks.     

Experimental investigation of K and Na partitioning between miscible liquids

Model silicate melts with variable Al₂O₃ and SiO₂ contents were experimentally saturated with alkalis at a total pressure of 1 atm and temperatures of 1300–1470°C, using the crucible supported loop technique. It was shown that Al₂O₃ content has little influence on the degree of silicate melt saturation with K and Na. In contrast, SiO₂ content strongly affects the solubility of alkalis in silicate melts. Model calculations were performed to evaluate the behavior of alkalis during the contamination/mixing of basic and silicic magmas.     

Amoeboid olivine aggregates in the Allende meteorite

Greyish-brown, irregularly-shaped aggregates composed predominantly of olivine make up ~2% of the Allende meteorite by volume. Many of the aggregates are constructed of subspherical lumps of micron-sized crystals of olivine, pyroxene, nepheline and sodalite surrounded by coarsergrained olivine. Rarely, anorthite, spinel and perovskite are also present. The olivine ranges in composition from Fo64 to Fo99. Pyroxenes range from aluminous diopside to hedenbergite to very Al-rich and Ti-Al-rich varieties. The nepheline contains 1.6–2.4% K₂O and 1.6–5.2% CaO but the sodalite is significantly poorer in these elements. The spinel contains 2.1–13.4% FeO. Textural information and oxygen isotopic data suggest that the aggregates are composed of primary, solid condensates from the solar nebula. The perovskite. spinel and Ti-Al-rich pyroxenes are the remains of high-temperature condensates but the olivine compositions and the presence of feldspathoids indicate that some of the grains continued to react with the solar nebular vapor in the temperature range 500–900°K.     

Crystallization of oxidized, moderately hydrous arc basalt at mid- to lower-crustal pressures: implications for andesite genesis

This study focuses on the production of convergent margin calc-alkaline andesites by crystallization–differentiation of basaltic magmas in the lower to middle crust. Previous experimental studies show that dry, reduced, subalkaline basalts differentiate to tholeiitic (high Fe/Mg) daughter liquids, but the influences of H₂O and oxidation on differentiation are less well established. Accordingly, we performed crystallization experiments at controlled oxidized fO₂ (Re–ReO₂ ≈ ΔNi–NiO + 2) on a relatively magnesian basalt (8.7 wt% MgO) typical of mafic magmas erupted in the Cascades near Mount Rainier, Washington. The basalt was synthesized with 2 wt% H₂O and run at 900, 700, and 400 MPa and 1,200 to 950 °C. A broadly clinopyroxenitic crystallization interval dominates near the liquidus at 900 and 700 MPa, consisting of augite + olivine + orthopyroxene + Cr-spinel (in decreasing abundance). With decreasing temperature, plagioclase crystallizes, Fe–Ti-oxide replaces spinel, olivine dissolves, and finally amphibole appears, producing gabbroic and then amphibole gabbroic crystallization stages. Enhanced plagioclase stability at lower pressure narrows the clinopyroxenitic interval and brings the gabbroic interval toward the liquidus. Liquids at 900 MPa track along Miyashiro’s (Am J Sci 274(4):321–355, 1974) tholeiitic versus calc-alkaline boundary, whereas those at 700 and 400 MPa become calc-alkaline at silica contents ≥56 wt%. This difference is chiefly due to higher temperature appearance of magnetite (versus spinel) at lower pressures. Although the evolved liquids are similar in many respects to common calc-alkaline andesites, the 900 and 700 MPa liquids differ in having low CaO concentrations due to early and abundant crystallization of augite, with the result that those liquids become peraluminous (ASI: molar Al/(Na + K + 2Ca) > 1) at ≥61 wt% SiO₂, similar to liquids reported in other studies of the high-pressure crystallization of hydrous basalts (Müntener and Ulmer in Geophys Res Lett 33(21):L21308, 2006). The lower-pressure liquids (400 MPa) have this same trait, but to a lesser extent due to more abundant near-liquidus plagioclase crystallization. A compilation of >6,500 analyses of igneous rocks from the Cascades and the Sierra Nevada batholith, representative of convergent margin (arc) magmas, shows that ASI increases continuously and linearly with SiO₂ from basalts to rhyolites or granites and that arc magmas are not commonly peraluminous until SiO₂ exceeds 69 wt%. These relations are consistent with plagioclase accompanying mafic silicates over nearly all the range of crystallization (or remelting). The scarcity of natural peraluminous andesites shows that progressive crystallization–differentiation of primitive basalts in the deep crust, producing early clinopyroxenitic cumulates and evolved liquids, does not dominate the creation of intermediate arc magmas or of the continental crust. Instead, mid- to upper-crustal differentiation and/or open-system processes are critical to the production of intermediate arc magmas. Primary among the open-system processes may be extraction of highly evolved (granitic, rhyolitic) liquids at advanced degrees of basalt solidification (or incipient partial melting of predecessor gabbroic intrusions) and mixing of such liquids into replenishing basalts. Furthermore, if the andesitic-composition continents derived from basaltic sources, the arc ASI–SiO₂ relation shows that the mafic component returned to the mantle was gabbroic in composition, not pyroxenitic.     

Distribution of Mg2+ between olivine and silicate melt,and its implications regarding melt structure

Consideration of experimental data on the distribution of Mg²⁺ between olivine and silicate liquid clearly demonstrates that the distribution coefficient (K_Mg) is dependent upon variations in temperature, pressure and melt composition, largely because these variables control the solubility of Mg²⁺ in the melt phase. Attempts to minimize composition dependence of K_Mg, utilizing various activity-composition models for silicate melts, have been partially successful. Composition-related effects do not appear to be large, however, for melts of restricted range in composition (e.g., tholeiitic or lunar basalts) as long as the contents of alkalis and the alkali/alumina ratio are relatively small (on a molar basis). For such melts, K_Mg may be used as a reliable geothermometer. By analogy, these conclusions can be extended to the distribution of other divalent cation such as Fe²⁺, Mn²⁺, Ni²⁺ and Co²⁺.     

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.     

Tectonic significance of Triassic mafic rocks in the June Complex,Sanandaj–Sirjan zone,Iran

This study concentrates on the petrological and geochemical investigation of mafic rocks embedded within the voluminous Triassic June Complex of the central Sanandaj–Sirjan zone (Iran), which are crucial to reconstruct the geodynamics of the Neotethyan passive margin. The Triassic mafic rocks are alkaline to sub-alkaline basalts, containing 43.36–49.09 wt% SiO₂, 5.19–20.61 wt% MgO and 0.66–4.59 wt% total alkalis. Based on MgO concentrations, the mafic rocks fall into two groups: cumulates (Mg# = 51.61–58.94) and isotropic basaltic liquids (Mg# = 24.54–42.66). In all samples, the chondrite-normalized REE patterns show enrichment of light REEs with variable (La/Yb)_N ratios ranging from 2.48 to 9.00, which confirm their amalgamated OIB-like and E-MORB-like signatures. Enrichment in large-ion lithophile elements and depletion in high field strength elements (HFSE) relative to the primitive mantle further support this interpretation. No samples point to crustal contamination, all having undergone fractionation of olivine + clinopyroxene + plagioclase. Nevertheless, elemental data suggest that the substantial variations in (La/Sm)_PM and Zr/Nb ratios can be explained by variable degrees of partial melting rather than fractional crystallization from a common parental magma. The high (Nb/Yb)_PM ratio in the alkaline mafic rocks points to the mixing of magmas from enriched and depleted mantle sources. Abundant OIB alkaline basalts and rare E-MORB appear to be linked to the drifting stage on the northern passive margin of the Neotethys Ocean.