Petrogenesis of olivine-phyric shergottite Larkman Nunatak 06319: Implications for enriched components in martian basalts
Petrogenesis of olivine-phyric shergottite Larkman Nunatak 06319: Implications for enriched components in martian basalts
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DOI：
10.1016/j.gca.2009.01.012
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发表时间：
2009-04
影响因子：
5
通讯作者：
A. B. Sarbadhikari;J. Day;Yang Liu;D. Rumble;L. Taylor
中科院分区：
文献类型：
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作者：
A. B. Sarbadhikari;J. Day;Yang Liu;D. Rumble;L. Taylor
我们报告了新发现的橄榄石斑状辉石拉克曼努纳塔克 (LAR) 06319 (LAR) 06319 的岩石学和地球化学。该陨石是斑状的，由橄榄石巨型晶体（长度⩽2.5mm，Fo77–52）和棱柱状辉石晶体组成，其核心为 Wo3En71，至 Wo8-30En23-45。轮辋。基质由较细粒的橄榄石（<0.25mm，Fo62-46）、富铁辉石和变辉石、马氏长石和少量铬铁矿、长尖晶石、磁铁矿、钛铁矿、磷酸盐、硫化物和玻璃组成。来自橄榄石-辉石-尖晶石地质气压计的氧逸度估计表明，LAR 06319 是在比贫化的六角辉石更具氧化性的条件下形成的 (QFM -1.7)。 LAR 06319 的全岩成分还富含相对于贫化六角辉石不相容的微量元素，其微量元素模式与橄榄石状六角辉石 NWA 1068 几乎相同。LAR 06319 的氧同位素组成 (Δ17O = 0.29 ±0.03) 证实了其火星起源。 LAR 06319 中的橄榄石巨晶是斑晶型的，其中镁含量最高的橄榄石巨晶与大块岩石接近平衡。 LAR 06319 的一个显着特征是其橄榄石巨晶颗粒在镁橄榄石核内含有丰富的熔体包裹体。这些早期捕获的熔体包裹体与全岩具有相似的微量元素丰度和模式，为LAR 06319的封闭系统岩浆行为提供了有力的证据。对母体熔体微量元素组成的计算表明，LAR 06319的全岩成分在结晶的最早阶段受鸽子岩和辉石控制，在最后阶段受磷灰石控制。橄榄石的晶体尺寸分布和空间分布模式分析表明至少有两种不同的晶体群。最简单地解释为巨晶橄榄石在岩浆管道中结晶，然后喷发并随后基质橄榄石结晶。 LAR 06319 在矿物和全岩化学方面与橄榄石状六角辉石、NWA 1068 和玄武岩六角辉石 NWA 4468 表现出密切的亲和力。这些陨石的显着特征是，与橄榄石状六角辉石（例如 Y-980459、Dho 019）相比，它们具有相对相似数量的镁铁质矿物，但平坦且升高的稀有矿物地球元素模式与富含轻稀土元素的玄武岩谢尔戈蒂岩（例如，洛杉矶谢尔戈蒂）更加一致。这种关系可以解释为源自富集地幔源的部分熔融和随后的晶体-液体分馏以形成富集的橄榄石-斑岩和玄武岩角辉石，或者是由于不相容元素富集的火星地壳的同化而产生。早期捕获的熔体包裹体和 LAR 06319 的全岩成分的相似性表明，任何地壳同化必定发生在巨晶橄榄石结晶之前，从而将此类过程限制在地壳的更深部分。因此，我们赞成 LAR06319 由“富集”和氧化的地幔储层的部分熔融形成，母体熔体在离开地幔时发生分步结晶。
We report on the petrography and geochemistry of the newly discovered olivine-phyric shergottite Larkman Nunatak (LAR) 06319. The meteorite is porphyritic, consisting of megacrysts of olivine (⩽2.5mm in length, Fo77–52) and prismatic zoned pyroxene crystals with Wo3En71in the cores to Wo8-30En23-45at the rims. The groundmass is composed of finer grained olivine (<0.25mm, Fo62-46), Fe-rich augite and pigeonite, maskelynite and minor quantities of chromite, ulvöspinel, magnetite, ilmenite, phosphates, sulfides and glass. Oxygen fugacity estimates, derived from the olivine–pyroxene-spinel geo-barometer, indicate that LAR 06319 formed under more oxidizing conditions (QFM -1.7) than for depleted shergottites. The whole-rock composition of LAR 06319 is also enriched in incompatible trace elements relative to depleted shergottites, with a trace-element pattern that is nearly identical to that of olivine-phyric shergottite NWA 1068. The oxygen isotope composition of LAR 06319 (Δ17O = 0.29 ±0.03) confirms its martian origin. Olivine megacrysts in LAR 06319 are phenocrystic, with the most Mg-rich megacryst olivine being close to equilibrium with the bulk rock. A notable feature of LAR 06319 is that its olivine megacryst grains contain abundant melt inclusions hosted within the forsterite cores. These early-trapped melt inclusions have similar trace element abundances and patterns to that of the whole-rock, providing powerful evidence for closed-system magmatic behavior for LAR 06319. Calculation of the parental melt trace element composition indicates a whole-rock composition for LAR 06319 that was controlled by pigeonite and augite during the earliest stages of crystallization and by apatite in the latest stages. Crystal size distribution and spatial distribution pattern analyses of olivine indicate at least two different crystal populations. This is most simply interpreted as crystallization of megacryst olivine in magma conduits, followed by eruption and subsequent crystallization of groundmass olivine. LAR 06319 shows close affinity in mineral and whole-rock chemistry to olivine-phyric shergottite, NWA 1068 and the basaltic shergottite NWA 4468. The remarkable features of these meteorites are that they have relatively similar quantities of mafic minerals compared with olivine-phyric shergottites (e.g., Y-980459, Dho 019), but flat and elevated rare earth element patterns more consistent with the LREE-enriched basaltic shergottites (e.g., Shergotty, Los Angeles). This relationship can be interpreted as arising from partial melting of an enriched mantle source and subsequent crystal–liquid fractionation to form the enriched olivine-phyric and basaltic shergottites, or by assimilation of incompatible-element enriched martian crust. The similarity in the composition of early-trapped melt inclusions and the whole-rock for LAR 06319 indicates that any crustal assimilation must have occurred prior to crystallization of megacryst olivine, restricting such processes to the deeper portions of the crust. Thus, we favor LAR06319 forming from partial melting of an “enriched” and oxidized mantle reservoir, with fractional crystallization of the parent melt upon leaving the mantle.