Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.410n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 410
Mathematics
Logic
Accretion, Chondrites, Core Formation, Siderophile Elements, Volatile Elements
Scientific paper
The use of siderophile-element depletions as probes of planetary accretion and core formation depends on a quantitative knowledge of the initial abundances of the siderophile elements and normalizing elements, such as the REE, Si, and Mg. The siderophile-element abundances are surprisingly uniform in primitive meteorites, especially compared to the large depletions of many siderophile elements in differentiated planetary bodies. Thus, for most siderophile elements the uncertainties in initial abundances do not effect the conclusions of quantitative core formation models. Fractionation processes in the solar nebula that did affect siderophile- element abundances include incomplete condensation of volatile siderophile elements and the physical segregation of different mineralogical components. Although small, these effects can be very important for interpreting the depletion of the slightly siderophile elements such as V, Cr, and Mn in the Earth and Moon. Ratios of refractory elements are remarkably constant in carbonaceous chondrite groups. For example, the mean Mo/Ir ratio for CI, CM, and C3V chondrites is 1.95 +/- 0.03, while the ratio for C3O meteorites is only 15% lower [1]. Mo/Ce ratios [2] are also quite constant as follows: 1.49 (CI), 1.63 (CV), 1.79 (CM), 1.86 (CO), 1.21 (LL), 1.44 (L), and 2.05 (H). Compared to the CI value, Mo/Ce ratios range from -19% to +38%. These factors are relatively insignificant compared to the depletions of the Mo/Ce ratios in differentiated planetary bodies, which range from a factor of 35 for the Earth to a factor of 1150 for the Moon. Unfortunately, data for Mo and W are not available for EH and EL chondrites. The results for W are similar to Mo; the range of both W/La and W/Ba ratios is from -30% to +28% of the CI ratio. Molybdenum and W exhibit some complications in their distributions in primitive meteorites. In carbonaceous chondrites Mo and W are associated with a refractory component, including Re, Os, and Ir, but in the ordinary and enstatite chondrites Mo and W are found primarily in the Fe-Ni metal component [3]. Molybdenum and W are also depleted in some calcium-aluminum-rich inclusions due to volatility under oxidizing conditions [4], but Mo/Ce ratios in carbonaceous and ordinary chondrites, as discussed above, vary by less than a factor of 2. Another potential problem for initial abundances of siderophile elements is the fractionation of the refractory and volatile siderophile elements relative to Fe, Ni, and Co [3]. However, for the moderately siderophile elements the effects are relatively small; Ni/Mo ratios in chondrites vary from the CI ratio down to a ratio only 45% lower for CV chondrites [2]. Relative to CI chondrites, other primitive meteorite groups exhibit a depletion of volatile elements. Similar depletion patterns for volatile lithophile elements are observed in differentiated planetary bodies. The volatile siderophile elements are depleted in these bodies both by core formation processes and volatile depletion. The depletions due to core formation are obtained by correcting for the volatile depletion through the use of the volatile lithophile elements. This leads to the question of whether the initial abundances of the volatile siderophile elements in the primitive meteorite groups, other than CI, are consistent with volatile depletion only. The answer is yes, even considering uncertainties in the condensation temperatures, as well as analytical and sampling uncertainties. For CM, and probably CO and CV chondrites, the data [5] indicate that the uncertainties increase with increasing volatility, such that Au, P, As, Mn, and Cu have a range of +/-10%, Ag, Ga, and Sb a range of +/-15%, and Ge, Sn, Se, and Zn a range of +/-20%. Surprisingly, Au, P, and As are enriched by an unknown reason in EH chondrites by as much as 50% relative to the volatile lithophile elements [5]. This work was funded by the Institute of Meteoritics and NSF EAR 9209641. References: [1] Palme and Rammensee (1981) EPSL, 55, 356-362. [2] Wasson and Kallemeyn (1988) Phil. Trans. R. Soc., 325, 535-544. [3] Larimer and Wasson (1988) In Meteorites and the Early Solar System, 416-435. [4] Fegley and Palme (1985) EPSL, 72, 311-326. [5] Palme et al. (1988) In Meteorites and the Early Solar System, 436-461.
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