摘要
长兴陨石于1964年10月17日降落在上海市长兴岛。本文利用上海天文馆馆藏的第一手样品,在重新界定其分类的基础上,研究了长兴陨石经历的热变质和流体交代作用,并探讨了其中富铝球粒(简称ARC)的成因。长兴陨石是一块H5型普通球粒陨石,金属相丰度高达15.1vol%,橄榄石和辉石的成分均一(Fa_(17.9±0.6),Fs_(16.1±0.8),相对平均偏差PMD<5%),基质经历了明显重结晶。长兴陨石的热变质过程伴随着流体交代作用,导致了斜长石的钠长石化、球粒中微孔结构和氧化物微晶的形成、以及氯磷灰石的形成等。与前人的报道不同,长兴陨石中斜长石的成分并不均一(An_(72-9)),说明H群母体上流体交代反应的条件很可能与L和LL群母体并无显著差异。长兴陨石中共发现12个富铝球粒,平均全岩Al_(2)O_(3)含量为14.8%,平均直径(0.30±0.17mm)与富镁铁质球粒(简称FMC,0.41±0.16mm)接近;全岩SiO_(2)、FeO和CaO含量与普通球粒陨石的II型FMC相似,Ca/Al质量比(0.18~0.58)低于宇宙值(1.10;Ahrens and Von Michaelis,1968)。ARC中橄榄石、辉石的TiO_(2)和Al_(2)O_(3)含量以及斜长石的An指数均明显高于FMC。岩相学、矿物学和全岩化学结果表明长兴陨石中的ARC形成于与FMC相似的高温过程,同一类型球粒陨石中的ARC和FMC很可能形成于原行星盘中的相同或相近区域。ARC的前体物质相对于FMC富集Al、Ti等难熔元素可能是太阳星云中气-固交换反应的结果。
Changxing(meteorite)was observed to fall on Changxing Island,Shanghai,on October 17,1964.As one of the seventy-two Chinese fall meteorites approved by the Meteoritical Society to date,Changxing has high scientific and educational value.However,research on Changxing has been limited.To conduct further study of Changxing,we acquired first-hand sample from Shanghai Astronomy Museum.After verifying its petrologic-chemical classification,we investigated the thermal metamorphism and metasomatism features in Changxing and explored the origin of Al-rich chondrules(ARCs).Changxing is an H5 ordinary chondrite(OC)characterized by abundant(up to 15.1vol%)Fe,Ni-metal and sulfides,chemically homogeneous olivine and pyroxene(Fa _(17.9±0.6);Fs _(16.1±0.8);percent mean deviation<5%),and a distinctly recrystallized matrix.Fluid-assisted metasomatism occurred during thermal metamorphism of Changxing,resulting in the albitization of plagioclase and development of Cl-apatite.Plagioclase is chemically zoned with sodic rims and calcic cores,and the plagioclase in ARCs of Changxing is more sodic(An _(52-72))than that in ARCs from unequilibrated OCs(An_( 66-99);MacPherson and Huss,2005).Numerous micropores and oxide crystallites formed in mesostases of chondrules during the metasomatism.In contrast to previous findings that feldspars in H4-6 chondrites are highly homogeneous(An_(~12) Or _(6);Kovach and Jones,2010),plagioclase in Changxing exhibits unequilibrated compositions(ARCs:An _(52.3-72.3) Or _(0.5-1.0);FMCs:An _(35.1-50.9) Or _(1.4-3.0);matrix:An_(9.2-47.4)Or _(1.1-3.8)).This suggests that there may be no significant differences in the metasomatism conditions between H and L/LL chondrite parent bodies.Twelve ARCs are found in Changxing with an average bulk Al_(2)O_(3)content of 14.8%and an average diameter(0.30±0.17mm)comparable to that of ferromagnesium chondrules(FMCs,0.41±0.16mm).The ARCs have bulk SiO_(2),FeO and CaO contents close to those of type Ⅱ FMCs in OCs,and Ca/Al weight ratios(0.18~0.58)below the cosmic value(1.10;Ahrens and Von Michaelis,1968).Olivine and pyroxene in ARCs have higher contents of TiO_(2)and Al_(2)O_(3)than those in FMCs.Plagioclase is more calcic in ARCs than in FMCs.The petrographic,mineralogic and bulk chemical evidence suggests that ARCs in Changxing are most likely to form through high-temperature processes akin to normal chondrules.The ARCs and FMCs of a same chondritic group might have formed in a common or adjacent region(s)in the protoplanetary disk.The relative enrichment of refractory elements(e.g.,Ti and Al)in ARC precursors probably resulted from gas-solid reactions in the solar nebula.
作者
王英
张欣茹
李明宝
罗业鑫
李少林
杜芝茂
WANG Ying;ZHANG XinRu;LI MingBao;LUO YeXin;LI ShaoLin;DU ZhiMao(CAS Key Laboratory of Planetary Sciences,Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210023,China;School of Astronomy and Space Science,University of Science and Technology of China,Hefei 230026,China;State Key Laboratory of Lunar and Planetary Sciences,Macao University of Science and Technology,Macao,China;Shanghai Astronomy Museum(branch of Shanghai Science&Technology Museum),Shanghai 201306,China)
出处
《岩石学报》
北大核心
2025年第7期2513-2524,共12页
Acta Petrologica Sinica
基金
国家自然科学基金项目(41973065)
上海科技馆“陨石微结构与化学研究创新平台”开放课题和紫金山天文台小行星基金会联合资助.
关键词
长兴陨石
普通球粒陨石
热变质
流体交代
富铝球粒
Changxing(meteorite)
Ordinary chondrite
Thermal metamorphism
Fluid metasomatism
Al-rich chondrules
