Chemical and isotopic study of extraterrestrial particles from the ocean floor

Abstract

We report chemical, mineralogic and Rb-Sr data on deep-sea spherules and on particles from an Antarctic Ocean core in which an excess Ir content has been identified. ^(87)Sr/^(86)Sr compositions in the deep-sea spherules are determined to 1–2‰ and are in the range 0.730–0.757. The ^(87)Sr/^(86)Sr compositions and the Sr concentrations are in the range observed for the majority of chondritic meteorites. ^(84)Sr/^(88)Sr ratios are normal to within 1%. Extreme depletion of Rb relative to the chondritic abundance is found in the deep-sea spherules. These data support the inference based on chemical composition and mineralogy that the deep-sea spherules are produced by the ablation or heating of meteoroids in the Earth's atmosphere with substantial loss of Rb by volatilization. Most terrestrial sources for the deep-sea spherules can be excluded, based on the chemical composition and on the Sr isotopic composition. The results on vesicular, Ir-rich particles from the Antarctic Ocean core give ^(87)Sr/^(86)Sr in the range 0.703–0.705 and within the range observed for ocean island basalts but significantly above mid-ocean ridge basalts (MORB). A crystalline basaltic particle from this core shows non-radiogenic ^(87)Sr/^(86)Sr= 0.701 ± 0.001, in the range observed for MORB and basaltic achondrites. The Sr data on the vesicular particles do not provide positive support for an extraterrestrial provenance for these materials. The basaltic particles cannot reasonably be the primary source of the high Ir concentration and some other lithic component remains to be identified.

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