火星陨石NWA 8716的岩矿学特征及粗粒橄榄石斑晶的来源

火星陨石可以为研究火星岩浆演化过程提供直接证据并限制其源区特征.通常认为含粗粒橄榄石斑晶辉玻无球粒陨石携带有火星原始地幔的信息,因此选取该类样品Northwest Africa (NWA) 8716为研究对象,进行岩相结构及矿物成分分析. NWA8716由橄榄石、辉石、填隙状熔长石以及其他次要矿物组成.其中橄榄石颗粒有两种级别的粒径,长轴分别约为0.5–1.8 mm和50–400μm.较小橄榄石斑晶内部的熔体包裹体和NWA 8716全岩成分(计算值)均显示明显的轻稀土元素亏损(La/Yb]CI值为0.06–0.1),说明NWA 8716源于一个亏损的火星岩浆池.粗粒橄榄石斑晶的来源对衡量该样品是否能够代表原始熔体成分非常重要.对橄榄石晶体的粒径统计分析发现,粗粒橄榄石斑晶应为堆晶.进一步对铁-镁以及稀土元素分配特征的计算表明NWA 8716并非形成于一个封闭系统,但是计算所得原始熔体成分与全岩成分差异不大,因此粗粒橄榄石斑晶应当来源于与母岩浆成分相似的熔体.总的看来, NWA 8716应当来源于亏损型火星幔源区且演化程度较低.

Petrology, Mineralogy and Origin of Olivine Megacrysts of the Martian Meteorite NWA 8716

Martian meteorites provide direct evidence and criticalconstraints on the evolution of the Martian mantle. Olivine-phyricshergottites are suggested to carry geochemical information of the primitiveMartian mantle. Detailed petrological and mineralogical analyses were madefor the NWA (Northwest Africa) 8716 olivine-phyric shergottite which is composed of largeolivine megacrysts (0.5--1.8 mm) set in a matrix of relatively smallolivine (50--400 $\upmu $m), and pyroxene with interstitialmaskelynite, and other accessory minerals. Both the melt inclusion withinolivine phenocryst and the calculated bulk rock display an LREE (Light Rare Earth Element)-depletedpattern (La/Yb]$_\textCI$ 0.06 to 0.1) which is in concordant with some otherolivine-phyric shergottites. This indicates NWA 8716 is derived from adepleted Martian magma. The origin of olivine megacrysts is a crucial factoron whether NWA 8716 can be counted to represent composition of a primitive,mantle-derived magma. The crystal size distribution analysis indicatesexcesses of large crystals which may be due to the entrainment of cumulates.Calculations of Fe-Mg and rare earth element partition reveal that NWA 8716did not crystallize in a closed system. But estimated compositions of theparent melt are not in large discrepancy with what observed, indicating themegacrysts crystallized from a magma similar to its parent melt. NWA 8716should have derived from a less evolved depleted reservoir.