Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results
Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results
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DOI：
10.1111/j.1945-5100.2010.01131.x
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发表时间：
2010-12
影响因子：
2.2
通讯作者：
A. Zhang;W. Hsu;C. Floss;Xian‐Hua Li;Qiu-li Li;Y. Liu;L. Taylor
中科院分区：
文献类型：
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作者：
A. Zhang;W. Hsu;C. Floss;Xian‐Hua Li;Qiu-li Li;Y. Liu;L. Taylor
摘要-西北非洲（NWA）2977是一个橄榄石辉长岩月球陨石，有一个明显不同的岩石结构从其他月球玄武岩。我们用一系列原位分析方法研究了这种岩石。NWA 2977主要由橄榄石和辉石以及少量斜长石组成。它显示了强烈的冲击变质作用的证据，局部高达S6级。熔融脉附近的橄榄石部分转化为林伍德石，富铝、富钛铬铁矿颗粒部分转化为高压多晶型物（可能是CaTi 2 O 4-结构）。NWA 2977中的橄榄石含有两种类型的岩屑包裹体。一种类型是富含Si、Al的熔体夹杂物，由玻璃和（在大多数情况下）树枝状辉石组成。另一种是镁铁质，由相对粗粒的普通辉石和副铬铁矿、稀土镁橄榄石和斜锆石组成。两个富Si、Al的熔融包裹体均为重稀土富集型，而镁铁质包裹体则具有较高的稀土含量和KREEP型。镁铁质包裹体可能是NWA 2977母岩浆上升过程中捕获的残留碎片，而富含Si，Al的包裹体可能代表原始NWA 2977熔体。计算的全岩成分具有类似KREEP的REE模式，表明NWA 2977与KREEP岩石具有亲和力。斜锆石的年轻结晶年龄为3123 ± 7 Ma（2σ），这与以前的全岩和矿物Sm-Nd和Rb-Sr研究结果一致。NWA 2977的岩相学、矿物学、微量元素地球化学和年轻的结晶年龄支持NWA 2977和NWA 773的橄榄石-辉长岩部分之间配对的可能性。
Abstract– Northwest Africa (NWA) 2977 is an olivine‐gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. We studied this rock with a series of in situ analytical methods. NWA 2977 consists mainly of olivine and pyroxene with minor plagioclase. It shows evidence of intense shock metamorphism, locally as high as shock‐stage S6. Olivine adjacent to a melt vein has been partially transformed into ringwoodite and Al,Ti‐rich chromite grains have partially transformed into their high‐pressure polymorph (possibly CaTi2O4‐structure). Olivine in NWA 2977 contains two types of lithic inclusions. One type is present as Si,Al‐rich melt inclusions that are composed of glass and, in most cases, dendritic pyroxene. The other type is mafic and composed of relatively coarse‐grained augite with accessory chromite, RE‐merrillite, and baddeleyite. Two Si,Al‐rich melt inclusions are heavy rare earth elements (REE) enriched, whereas the mafic inclusion has high REE concentrations and a KREEP‐like pattern. The mafic inclusion could be a relict fragment captured during the ascent of the parent magma of NWA 2977, whereas the Si,Al‐rich inclusions may represent the original NWA 2977 melt. The calculated whole‐rock composition has a KREEP‐like REE pattern, suggesting that NWA 2977 has an affinity to KREEP rocks. Baddeleyite has recorded a young crystallization age of 3123 ± 7 Ma (2σ), which is consistent with results from previous whole‐rock and mineral Sm‐Nd and Rb‐Sr studies. The petrography, mineralogy, trace element geochemistry, and young crystallization age of NWA 2977 support the possibility of pairing between NWA 2977 and the olivine‐gabbro portion of NWA 773.