Melt Inclusion Faceting: A Chronometer for Magmatic Processes at Mid-Ocean Ridges
2015 OEI Funded Project
When ocean crust is created by magmatism at mid-ocean ridges, mass and heat are transferred from the deep convecting upper mantle to the ocean floor. After their ascent from the mantle, magmas pool in shallow reservoirs where repeated cycles of magma injection, mixing, and crystallization (precipitation of minerals) occur. Understanding the temporal evolution of shallow magma reservoirs – and thereby accretion of the ocean crust – requires quantify the rates at which magmas are injected into these reservoirs from the deep mantle (input) and the rates at which magmas crystallize minerals or erupt onto the ocean floor (outputs). However, conventional geochemical tracers and chronometers provide very little information about magma injection rates because the influence of magma mixing. Here, we propose to develop a new method to constrain the timing of input and output of magma reservoirs. When a melt crystallizes minerals (e.g., olivine) in magma reservoirs, small droplets of that melt become incorporated into the growing olivine. Encapsulated into the core of olivines and thus physically isolated from the mixing process, the composition of those melt droplets will not change prior to eruption. Olivine-hosted melt inclusions are thus capable of recording of injection of compositionally distinct magmas into the reservoir. This becomes especially useful for the chronology of magma processes if the “age” (time of crystallization) of those melt inclusions can be determined.
Our approach takes advantage of the fact that the shape of olivine-hosted melt inclusions is correlated with their age. Olivine-hosted melt inclusions are spherical when they initially form, but develop facets, a planar surface, as they approach textural equilibrium. Therefore, the extent to which facets have developed around a given melt inclusion reflects the amount of time that has passed since it initially formed. We propose to develop faceting of olivine-hosted melt inclusions as a chronometer for determining magma injection rates in reservoirs beneath mid-ocean ridges. We propose to set up a melt inclusion heating stage at WHOI and use it – in combination with the existing x-ray microtomography facility which can provide three-dimensional images of melt inclusions – to determine the faceting rates of olivine-hosted melt inclusions as a function of temperature and inclusion size. Starting materials for the experiments will be olivine-hosted melt inclusions from the WHOI collection of ocean-floor basalts. Experiments will be conducted as follows: spherical melt inclusions of different sizes will be identified and their shapes and volumes quantified using microtomography. The inclusions will then be heated for some amount of time, quenched, and their shapes and volumes determined again by microtomography. This process will be repeated until significant facets have developed around the inclusions. The rates at which faceting occurs will then be compared with known diffusivities for Fe2+, Mg2+, and Si4+ in silicate melt to determine whether it is transport of one of these ions or interface kinetics that controls faceting rate. Once we have developed the calibration, it will form the basis for a full NSF proposal to study magma recharge rates for reservoirs beneath mid-ocean ridges using samples from the WHOI collection. Further, acquisition of a heating stage will provide us with many important new avenues for the study of olivine-hosted melt inclusions.