Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975
Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975
复制标题
DOI：
10.1111/maps.12571
复制
发表时间：
2015-11
影响因子：
2.2
通讯作者：
Qi He;Long Xiao;J. Balta;I. Baziotis;W. Hsu;Y. Guan
中科院分区：
文献类型：
--
作者：
Qi He;Long Xiao;J. Balta;I. Baziotis;W. Hsu;Y. Guan
我们提出了一个研究的玄武岩shergottite西北非洲2975（NWA 2975）的岩石学和地球化学。NWA 2975是一种中等粒度的玄武岩，具有亚蛇绿到粒状结构。电子探针（EMP）分析表明，两个不同的辉石成分的趋势和斑片状的组成分区模式不同于其他陨石，如Shergotty或QUE 94201。由于我们无法获得完整岩石测量的大块样品，我们通过二次离子质谱仪（西姆斯）测量和激光消融电感耦合等离子体光谱（LA‐ICP‐MS）分析来表征熔融地壳及其可变性，作为大块岩石成分的最佳替代。熔融壳的主要元素组成与其他富集的玄武质shergottites的整体组成相当，将NWA 2975放在该样品组中。CI标准化的稀土元素（REE）模式是平坦的，也平行于其他富集的玄武质shergottites。镁线石是主要的稀土载体，具有平坦的稀土配分模式，轻微亏损的Eu，平行于其他玄武质shergottites镁线石的稀土配分模式。由Fe-Ti氧化物对计算的NWA 2975的氧化态为NNO-1.86，接近QFM缓冲液。NWA 2975代表了来自氧化和富集的shergottite组的样品，我们对其起源的测量和限制与两种不同的火星地幔储层的假设一致：一种还原的LREE贫化储层和一种氧化的LREE富集储层。在陨石中还发现了斯季绍维特（Stishovite），可能是硅酸盐岩（seifertite）和致密的二氧化硅玻璃，这使我们能够推断NWA 2975经历了约30 GPa的真实冲击压力。
We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium‐grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA‐ICP‐MS) analyses as a best‐available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI‐normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe‐Ti oxide pairs is NNO‐1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE‐depleted reservoir and an oxidized, LREE‐enriched reservoir. Stishovite, possibly seifertite, and dense SiO2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of ~30 GPa.