Basalts in the Lancé Carbonaceous Chondrite

Abstract We describe three lithic fragments from the Lancé carbonaceous chondrite which apparently are of igneous origin and of basaltic parentage. They consist of large enstatite grains (pheno-crysts ?) which poikilitically include some olivine and abundant Fe-Ni metal and Fe sulfide blebs. These enstatites are set into a matrix of partly ophitic and partly porphyritic plagioclase (~ an71) and interstitial clinopyroxene and clinopyroxene plus plagioclase intergrowths. Mineral compositions are highly variable and reflect a complex genesis. Olivine compositions range from fo5 to fo60 and sometimes exhibit compositional zoning. Enstatites are mostly of en 98 com-position with some Fe-enrichment towards grain surfaces and cracks. Plagioclases have about an71 but mostly are altered to nonstoichiometric high-alkali compositions (with original Al/Si ratios corresponding to ~ an 70) or are partly replaced by K-free nepheline. Generally two clinopyroxenes are present within the plagiophyric portions of the fragments: One low Fe/Mg augite (~ en53 wo44) rich in Ti, Al, Cr, and Mn (Fe/Mn ≦ 2) and a ferrosalite (~ en15 wo47) poor in Ti, Cr, and Mn (Fe/Mn ~ 100). One fragment also contains spinel (hercynite) which apparently is xenocrystic in nature. Our investigation indicates: (1) The fragments are of igneous origin. (2) The original melt was of basaltic composition, crystallizing the primary mineral association ol + en + plag + low-Fe augite. (3) During crystallization the melt suffered strong reduction which led to segregation of Fe-Ni droplets from the melt and which continuously lowered the Fe/Mn ratio of successively crystallizing silicates from nearly chondritic (~ 100) to 0.6. (4) Probably after incorporation into the Lancé breccia the fragments suffered to different degrees metasomatic alterations which were caused by the addition of Na, K, and Fe to the system. The origin of the pre-Lancé basaltic melt cannot yet be established.