南极GRV99027火星陨石的地球化学特征

GEOCHEMISTRY OF ANTARCTIC GRV 99027 MARTIAN METEORITE

南极GRV 99027陨石是二辉橄榄岩质辉玻无球粒陨石(L-S)的火星陨石,具有嵌晶、非嵌晶和冲击熔融袋结构。矿物模式组成以橄榄石(55%)、辉石(37.5%)为主,有少量熔长石(6%)、铬铁矿(1.5%)以及微量白磷钙石、陨硫铁等。本文在阐述该陨石的岩石学和矿物化学特征的基础上进一步研究了稀土元素及氢同位素的地球化学特征。该陨石∑REE总量低;橄榄石及辉石以HREE富集为特征;熔长石富LREE,且具高正的δEu;副矿物白磷钙矿含量虽少(约0.2%),但具有极高的∑REE,LREE≈HREE,δEu负异常明显。GRV99027全岩稀土元素分配模式与L-S陨石的模式十分相似,而与SNC其它类型陨石的REE分配模式不同,反映不同类型火星陨石物质源区不同。火星陨石氢同位素以高δD为特征,各类含水矿物对陨石δD的贡献不同。陨石磷酸盐矿物中δD-水含量间一般没有正相关性,间或存有弱的负相关性。根据GRV99027陨石的矿物学模式,REE分配模式,及氢同位素特征,可以确定GRV99027陨石为L-S陨石。为了探讨GRV99027陨石的物质源区及演化历史,我们广泛收集了L-S火星陨石的Sr,Nd,Pb,Os和稀有气体同位素资料。GRV99027与同类L-S陨石的物质来源为原始火星幔源区;约4M a(溅射年龄)经天体强烈碰撞溅射,在宇宙空间经历不同时间的运行(宇宙射线暴露年龄不同),随后进入地球引力范围,先后陨落在南极。

GRV 99027 is a Martian lherzolitic shergottites （L-S）with poikilitic, non-poikilitic and melt pocket textures. The mineral assemblages in GRV 99027 mainly include olivine （55 vol% ） and pyroxene （ 37.5 vol% ）, minor maskelynite （ 6 vol% ） and chromite （ 1.5 vol% ）, trace whitlockite and troilite, and so on. The paper describes the mineralogy and petrology of GRV 99027, and further investigates the geochemical characteristics of the REE and H isotopes in the GRV 99027. The ΣREE in GRV 99027 is relatively low; HREE is rich in olivine and pyroxene grains; LREE is rich in plagioclase with high positive δEu; high ΣREE value is found in rare mineral whitlockite （ less than 0.2 vol % ）, LREE ≈ HREE, and whitlockite has negative δEu abnormity. The REE distribution patterns of the whole -rock of GRV 99027 is similar to that of L-S Martian meteorites, while is different from that of other SNC Martian meteorites, indicating that they came from different location of Mars parent body. GRV 99027 has highδD value. Different water-bearing minerals give different contribution for δD value. The δD of phosphates generally has no correlation with water content, and δD has weak negative correlation with water content. GRV 99027 can be classified as an L-S Martian meteorite based on mineralogical assemblage patterns, REE distribution patterns, and hydrogen isotope of the GRV 99027. The isotope data of Sr, Nd, Pb, Os and REE from other L-S Martian meteorites were collected to discuss the formation history of the GRV 99027. As other L-S Martian meteorites, GRV 99027 originated from part of Mar＇s mantle ; about 4 Ma ago, after one strong impact event, the parent body of the meteorites were ejected from deep mantle into space, and travelled for a different duration in space （indicated by different cosmic exposure time）, and captured by the Earth later in different time, and finally fell on the Antarctica as L-S Martian meteorites.