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.
Unlike chemistry and physics, geology is fundamentally a historical science—a science whose goal is to understand both the current workings of the Earth and the Earth’s evolution. For example, the current state of a mountain belt reflects a long series of processes and events that have occurred in the past. By studying the mountain belt, geologists try to unravel this history based both on an understanding of chemistry and physics as well as on detailed observations and measurements. Like the other geological sub-disciplines, advances in igneous petrology require observations made in the present to explain past events—for example: How are lavas that we see erupted on the surface generated deep within the Earth? Using observations and measurements made on lavas, igneous petrologists work to understand how chemical composition and temperature varies within the Earth and how these variations have evolved through time. This is fundamental to our understanding of the Earth since these compositional and temperature differences are the drivers of partial melting within the planet. The total amount of volcanic rock that is produced in a particular area over some specified time period (for example, the thickness of the volcanic crust that makes up the island of Iceland) also implies a compositional and temperature structure—higher temperatures produce more magma while, at the same temperature, some types of rock within the Earth will melt to a greater extent than others. The major goals of this project are to use elemental abundances found in volcanic rocks as well as the amount of lava that is erupted over time in a particular area to place constraints on (1) the chemical composition of the material that melted and (2) the temperature during melting. Nickel (Ni) is an element that is present in minor amounts (a few tenths of a weight percent) in the Earth’s mantle (that region of the Earth between the crust, i.e., the outer most surface layer of the planet, and the iron-rich core). Understanding the extent to which Ni is transferred from solid rock to a partial melt of that same rock can be used to help make estimates of the temperature and composition of the material that is melting. One of the major goals of this product has been to characterize experimentally this partitioning of Ni between a silicate melt and an extremely abundant mineral in the mantle called olivine (gem quality olivine is known as peridot). We have used the experimentally determined temperature and compositional controls on this partitioning to interrogate two different explanations of the high Ni contents found in Hawaiian lavas: (1) that the material that is melting within the mantle to produce lavas on the Big Island of Hawaii is rich in olivine or (2) that the material has little or no olivine. Our conclusion is that (1) is consistent with the Ni contents in Hawaiian lavas. Note that this conclusion has broader implications for the sorts and abundances of compositional heterogeneities that exist beneath Hawaii and other oceanic islands like Reunion and Iceland. As mentioned in the preceding paragraph, it is well known that the Earth’s mantle is compositionally heterogeneous—veins of material compositionally different from the enclosing matrix in kilogram-sized pieces of the mantle carried to the surface during volcanic eruptions show this. It is also well known, in a qualitative sense, that at the same temperature and pressure within the mantle, these compositional heterogeneities will melt to different extents. Another major goal of this project has been to generate, using experimental data, a mathematical expression that describes quantitatively how much these different compositional heterogeneities melt when they are held at the same temperature and pressure. Since the total amount of magma produced in the mantle beneath a volcanic region (e.g., Iceland) can be equated with the thickness of volcanic crust at that locality, our expression can be used to place constraints on both the types and abundances of such heterogeneities as well as the temperature in the mantle beneath regions like Iceland. Applying our expression to Iceland suggests that the mantle directly beneath the island is more than 100°C hotter than mantle at the same depth that is far removed from Iceland. These results have important implications for temperature variations in the Earth’s mantle that in turn help drive the flow of material within the mantle. This largely temperature-driven flow of mantle material is responsible for melting within the mantle that produces much of the lava erupted on the Earth’s surface.
