Fractionation of volatile elements in the early solar system: evidence from heating experiments on primitive meteorites

Abstract

Compared to the average composition of the solar nebula (as determined by solar photospheric abundances), most meteorites and planets are depleted in volatile elements. The only exceptions are CI chondrites and perhaps interstellar dust particles and comets. The depletion of volatile elements is one of the most important fractionation processes that has affected solid matter in the early solar system. Loss of volatile elements may have occurred either by evaporation during heating of material of solar composition or by incomplete condensation in the early solar nebula. To test the evaporation hypothesis heating experiments on chunks of the Allende CV3 chondrite were performed under controlled oxygen fugacities ( fO 2), from air to log fO 2 = −16.5, and at temperatures ranging from 1050 to 1300°C. After removal from the furnace, residues were analyzed by instrumental neutron activation analysis, spark source mass spectrometry and with a C,S analyzer. Experiments in closed quartz vials, with solid fO 2 buffers (FeFeO, etc.), were found to be problematic because of strong preference of quartz for alkalis and Ga. Most experments were therefore made in an open system with fO 2 controlled by COCO 2 gas mixing. The volatile element pattern of heated samples from the Murchison CM meteorite is very similar to that of heated Allende. Since Murchison and Allende are different in chemical composition (in particular in volatile element contents), texture and mineralogy, the results of the heating experiments do not strongly depend upon these parameters. Most experiments were therefore confined to Allende. The results show for most elements a strong dependence of apparent volatilities on fO 2. The elements Os, Re, Au and As are stable at reducing (low fO 2) and volatile under oxidizing conditions (high fO 2). The transition occurs for Re, Au and As between the FeFeO and NiNiO buffers. Volatilization of Os requires oxygen fugacities several orders of magnitude above NiNiO. Alkali elements (Na, K), Cu, Ga and Zn show the apposite behavior, losses in experiments at reducing and retention at oxidizing conditions. The element Mn shows no depletion, even in the 1300°C experiments, while the major fraction of Se is lost in all experiments, reflecting loss of sulfide, which is confirmed by the results of S analyses. Lead was found to be similarly volatile to Se and S. Since the host phases of most of the volatile elements are similar in CM, CV and ordinary chondrites, the results obtained in the present study should be applicable to carbonaceous and ordinary chondrites. Metamorphism at temperatures above 1050°C and at oxygen fugacities above the FeFeO buffer would thus lead to large losses of Au, As and Re, which is observed neither in carbonaceous nor in ordinary chondrites. Ratios of volatile to non-volatile metals, e.g. Au Ir , As Ir and Re Ir ratios, are “normal” in these meteorites (i.e. minor depletion in Au and As and chondritic Re Ir ratios). Apparently, carbonaceous and ordinary chondrites were never exposed to such conditions. The strong depletion of Ga and Zn in experiments at low oxygen fugacities excludes metamorphism of chondritic meteorites under these conditions. This is confirmed by earlier results of Matza, S. D. and Lipschutz, M. E. ( Proc. 8th Lunar Sci. Conf., pp. 161–176, 1977), who found that in reducing conditions losses of Zn are observed at temperatures as low as 700°C and Ga losses at 900°C. A detailed comparison of the volatile element patterns in residues of heating experiments with volatile element patterns of CV3 meteorites shows that samples produced during heating at the IW buffer (iron-wüstite buffer) have volatile element patterns approximately resembling the CV3 pattern, although there are still significant differences, making the origin of such patterns by thermal metamorphism of CI material unlikely. Additional arguments are presented suggesting that heating of meteoritic material to produce fractionated volatile element patterns is unlikely. The observed element patterns are better explained by incomplete condensation, whereby solar gas is dissipated during condensation of volatile elements.