Petrogenesis of ultramafic rocks and an experimental and natural investigation of non-traditional stable isotope fractionation at high temperatures : implications for the chemical evolution of the earth and planets

Abstract

The fractionation of stable isotopes at high temperatures offers a unique insight into the sources and processes operative during planetary accretion. Through analysis of meteorites and samples that are representative of planetary reservoirs, a coherent picture is built of the isotopic variation present in solar system materials. However, this objective is complicated by the selection of samples appropriate to characterise the isotopic composition of the entire Earth. In minimising the uncertainty in this assumption, ultramafic (olivine-rich) rocks are used, as these are most representative of the Earth's mantle, which constitutes ~ 70% (by mass) of the Earth. Peridotites from the Balmuccia massif and a global selection of komatiites, with care taken to select only the least chemically-modified examples, were collected and characterised. Because it is known that stable isotopes fractionate during magmatic processes, the best estimate of the composition in equilibrium with the primitive mantle is computed by considering the petrogenetic processes that lead to the formation of these rocks. Through the course of this exercise, a new model is posited to explain chemical depletion in peridotites, which arises, not only from melt extraction, but from pressure-solution re-distribution of pyroxenes, two complementary processes. It is proposed that, since the chemical trends observed in peridotites are of global significance, this process is ubiquitous, occurring in-situ in zones of adiabatic decompression mantle melting. Komatiites, as high degree partial melts of the mantle, require high mantle temperatures that can only have been achieved early in Earth history. It is demonstrated that the origin of the two distinct komatiite types, Aluminium-Depleted and Aluminium Undepleted, reflects not only the pressure, but the degree of melting. Over time, their mantle sources incorporate progressive amounts of depleted, upper mantle material, effectively tracking crustal growth. Furthermore, because the magnitude of stable isotope fractionation is proportional to 1/T2, these high temperature samples allow an assessment of mantle composition through time and space. Due to the small variations in isotope composition at these high temperatures, optimised analytical procedures for the measurement of Cu, Fe, Zn and V isotopes are detailed, and document improved precision with respect to previously published methods. Using this technique, new values for the Fe and Zn isotope compositions of the Bulk Silicate Earth are derived. Supplementary analyses of the isotopic composition of Mars, together with already-published analyses of chondritic meteorites (the supposed building-blocks of Earth) and the Moon, shows that the Earth-Moon system has a fractionated isotopic composition. With this information, a new interpretation of the timescales and composition of Earth's accretion is suggested. Briefly, the volatile element budget of the Earth was set early in its accretion, by material more volatile-depleted than any of the known chondrite groups. Accretion then became progressively more oxidising, culminating in the Moon-forming impact, which delivered % of the Earth's complement of iron, and, importantly, in an oxidised form, as observed in the contemporary mantle. These interpretations are underpinned by experimental measurements of expected iron isotope fractionation factors between common igneous phases - silicates, oxides and metal. These determined partition coefficients are also applied to a series of igneous problems, including the generation of basalt, crystallisation of the lunar magma ocean, and the formation of granites.