Molybdenum Isotopic Composition of the Archean Mantle As Inferred from Studies of Komatiites

Abstract:

Molybdenum isotopic composition has been shown to be a powerful tool in studies of planetary processes, e.g. estimating core formation temperatures [1,2]. However, Mo isotope compositions of terrestrial reservoirs are not well constrained. In order to better constrain the Mo isotopic composition of the early Earth’s mantle, komatiites from four locations were analyzed for their Mo concentrations and isotopic compositions. Komatiites are particularly appropriate for this type of study because they formed by high degrees of partial melting of the mantle leading to a complete base metal sulfide removal from the residual mantle and the production of sulfur-undersaturated melts and thus a quantitative removal of Mo from the source into the melt. All samples, except for two strongly altered specimens specifically chosen to study the effects of secondary alteration, are very fresh having preserved most of their primary mineralogy.

The Mo concentrations in komatiites range from 10 to 120 ng/g. Fresh komatiites have lighter δ⁹⁸Mo (NIST SRM 3134 = 0.25‰, [3]) than altered samples. The estimated primary Mo isotope compositions of the studied komatiite melts range from 0.02 ± 0.16‰ to 0.19 ± 0.14‰ and are therefore indistinguishable within analytical uncertainty (2SD) from published values for chondritic meteorites (0.09 ± 0.04 ‰; 2SD; [2]) and lighter than the proposed average for Earth’s continental crust (0.3 to 0.4‰ [4]). All data combined, although overlapping in errors, show a consistent trend of lighter δ⁹⁸Mo and lower Mo concentrations in more melt-depleted mantle sources, indicating incompatible behaviour of Mo and preferential mobilization of heavy Mo isotopes during mantle melting.

[1] Hin et al. (2013) EPSL, 379 [2] Burkhardt et al. (2014) EPSL, 391 [3] Nägler, et al. (2014) GGR, 38. [4] Voegelin et al. (2014) Lithos, 190-191.