Extra-terrestrial influx rates of cosmogenic isotopes and platinum group elements: realizable geochemical effects

Abstract

We present estimates of "realizable" influx of selected cosmogenic isotopes, ³He, ¹⁰Be, ²⁶Al, and ¹⁴C, and platinum group elements, Ir, Os, and Re on the earth via influx of meteoroids. We define realizable as the particulate fraction of mass influx which is included in standard geochemical analyses of terrestrial sediments. This is the surviving mass fraction of size < 0.5-1 cm, which gets mixed with terrestrial samples, and is analyzed in normal terrestrial assays of sediments. Larger surviving meteoroid fragments, of the order of cm to m or larger, obviously belong to the non-realizable flux category, since (i) their distribution in terrestrial samples would be very patchy, resulting in a highly variable density matrix, and (ii) they would also generally (except in wide-diameter cores) be excluded from sediment cores. We estimate the realizable influx of meteoritic particles, based on a recent model describing production of smaller size fragments arising during the atmospheric entry of meteoroids. Implicit in this work is the assumption that the secondary fragments are not subject to much heating and therefore most of the initial ³He (and noble gases) would be preserved in the secondary fragments since after break-up they are not subjected to much heating. Under this assumption, production of small size fragments in the ablation process constitutes a "safe" landing mechanism for parts of the meteoroid. In this paper, we show that the meteoritic ablation/fragmentation process produces a significant flux of ³He, platinum group metals, and cosmogenic ²⁶Al. In fact, measurements of these isotopes in ocean sediments should allow a reasonable estimate of temporal variations in the flux of meteoroids of 50 cm to 5 m radii, which produce most of the secondary fragments in the size range < 1 cm. We would like to state here that, as pointed out in our previous work, stratospheric collections would be biased towards collection of the primary incident extra-terrestrial particles, whereas terrestrial accumulations representing large (area × time) accumulations, as in the case of ocean sediments, would efficiently sample the fragmented particles.