A mineralogical and geochemical study of seven meteorites from Malawi, Namibia and Lesotho

[摘要] English: Seven meteorites from Malawi, Namibia and Lesotho were studied using opticalmicroscopy, scanning electron microscopy with energy dispersive X-rayspectrometry and electron microprobe analysis of mineral phases. Induced coupledplasma spectrometry and X-ray fluorescence techniques were used to obtainchemical information.The Thuathe meteorite from Lesotho is a H4 ordinary chondrite according to majorelement chemistry and mineralogy. The shock features were classified as S2/3 andthe weathering grade as W0. It contains Sb sulphides, which had not beenpreviously reported in any chondritic meteorite. Berthierite and stibnite wasobserved under the SEM and confirmed by electron microprobe analyses. Thechondrite normalised REE pattern is relatively flat with no significant LREE/HREEvariation. Very slight enrichment of Ta and U compared to H-group chondrites arenoted. In this study the Machinga meteorite from Malawi is classified as an E6 ordinarychondrite according to major element chemistry and mineralogy, and the shockfeatures were classified as S4, moderately shocked. Previously it had beenclassified as an L6 chondrite (Graham et al, 1984 and Koeberl et al, 1990). The rareearth element pattern is relatively flat with slight LREE/HREE variation from thenorm. Slight enrichment of W, Pb and U is noted.The Balaka meteorite from Malawi is a L6 ordinary chondrite that is weakly shocked(S2). These classifications were done with major element chemistry as well as themineralogy of the meteorite. The rare earth element pattern is flat with nearly novariation between LREE and HREE. The normalised trace element diagram mirrorsthe trends of the trace element diagram of the L-group chondrites with respect toordinary chondrites. The Chisenga meteorite from Malawi is an IIIAB medium octahedrite according tochemistry and the bandwidth of kamacite (~1.5 mm) and taenite. Kamacite is thedominant Fe/Ni phase and Widmanstätten texture is prominent.The unreported specimens from Asab in Namibia prove to be L6 ordinary chondritesaccording to major element chemistry and mineralogy. The shock features areclassified as moderately shocked (S4) and the weathering grade is W2. The rareearth element spectrum for the sample 1 from Asab shows enrichment of LREE.Enrichment of Ba, Sr, Th and U is shown in this sample. Sample 2 from Asab showsslight La enrichment compared to L-group chondrites. Enrichment in Ba, Sb, Sr andU is shown in the chondrite normalised trace element diagram. Sample 3 from Asabdisplays a flat chondrite normalised REE pattern with nearly no variation betweenLREE and HREE. Chemical variation in these meteorite samples in the chondritenormalised trace element diagram shows enrichment of Ba, Sb and Sr. The generaltrends of the diagrams are similar indicating that the three unknown specimens areof the same fall.

[发布机构] University of the Free State