Experimental Constraints on Crystal Liquid Ni and Mn Partitioning in Mafic and Ultramafic Systems

California Institute Of Technology, Pasadena CA

Investigators

Abstract

A fundamental goal of igneous petrology is to understand the present chemical composition of the Earth's crust and mantle (two of the major structural components of the planet; the third component is the Earth's core) based on a study of rocks that crystallized from molten or partially molten material. Although samples of the Earth's interior are occasionally carried up in volcanic eruptions, much of the information concerning the chemical composition of the mantle must be inferred from partial melts that have risen and been erupted on the Earth's surface (these rocks are called basalts). Two of the best 'windows' into the mantle are basalts from mid-ocean ridges (MOR) and basalts from ocean islands (e.g., Hawaii). While petrologists have a good zeroth-order understanding of the origin of mid-ocean ridge basalts (e.g., the composition of the parcels of mantle that melt to produce these rocks), the same cannot be said for ocean island basalts and several hypotheses are currently being debated in the literature. The experimental study proposed here is designed to help distinguish between these competing hypotheses, which largely revolve around the question of how different the mantle sources of ocean island basalt source are to those of MOR basalts. Answering this question has important implications for understanding the chemical heterogeneity of the Earth's mantle and processes that produce rocks that are seen on the surface of the Earth. The mineral olivine is commonly found in basalts from MORs, ocean islands, and continental flood basalt provinces and analyses of a large number of these olivines show that their NiO and FeO/MnO concentrations are positively correlated (olivines with low NiO also have low FeO/MnO, while those with high NiO tend to have high FeO/MnO). The low NiO-FeO/MnO end of the array is defined by olivines from MORB and Icelandic rocks whereas the high NiO-FeO/MnO end is largely defined by olivines from Hawaiian, Réunion, and Karoo rocks, which can have NiO contents up to ~0.6 wt%. Olivines from spinel lherzolites (the rock type thought to represent the bulk of the upper mantle and the source of MOR basalts) lie approximately in the middle of the array with NiO values of 0.35-0.40 wt%. Much of the effort to explain this global array has focused on the high NiO-FeO/MnO portion, since it is difficult to generate partial melts of spinel lherzolite with normal mantle concentrations of NiO that will crystallize olivine with >0.4 wt% NiO using any of the common 1-atm-based expressions for the partitioning of NiO between olivine and a basaltic melt (designated as DNi). Models to date include melting of an olivine-free source (a hybrid mixture of spinel lherzolite plus silica-rich melt of oceanic crust that has been subducted back into the mantle), addition of small amounts of outer core (rich in Ni and Fe) to the sources of the lavas that display these high NiO-Fe/Mn olivines, and the idea that DNi varies with pressure and temperature. Efforts to calculate quantitatively the observed NiO and FeO/MnO contents of partial melts of spinel lherzolite or mantle rocks with little or no olivine as well as the olivines that crystal from these melts on ascent are hampered by a lack of high-pressure and temperature Ds with which to calculate NiO and MnO contents of liquids and coexisting crystalline phases. The goal of this project is to determine a set of NiO and MnO partition coefficients between olivine and basaltic melt over a range of pressures, temperatures, and bulk compositions and to, then, use these experimentally determined values to evaluate the different hypotheses that have been put forth to explain the global NiO-FeO/MnO array.