Cumulate Thermobarometry

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The Lesser Antilles volcanic arc provides an unusually complete and accessible case study of an active island arc subduction zone, formed by the subduction of the American plate under the eastern edge of the Caribbean plate since at least the Eocene. The arc offers a worldclass opportunity to explore subduction zone processes in a relatively simple intraoceanic setting. Arc-magmatism processes take place at depth and must be inferred through information available at the surface. A major source of such information is the nature and composition of magmatic rocks, which may either be erupted as lava from the volcanoes, or crystallized within the volcanic plumbing system, carried to the surface by ascending magma, and erupted as xenoliths within it. Both lavas and xenoliths reflect aspects of the processes that take place between the melting of volatile-rich magma in the mantle wedge above the subducting plate, and the eruption of lavas on the surface. An important category of xenoliths, known as cumulates, are formed from crystals that grew in a cooling magma, sank and accumulated at the bottom of the magma chamber, thereby indicating magma storage at discrete locations in the volcanic system. The cumulate sequences that are parental to these xenoliths lie at an unknown depth in the Earth's crust, but must contain extensive information encoded in the compositions of their crystals and captured bubbles of melt. This information includes hints about the source of the magma from which the cumulates crystallized, and its mixing with other magmas during storage and transport. Investigators are currently obliged to make inferences based on the population of cumulate xenoliths, without having a coherent picture of the parental cumulate sequences at depth under the arc, or the relationships of the various cumulate samples to each other. In this study we will attempt to gain a coherent understanding of distribution of buried cumulate sequences, both laterally along the arc and at depth. Previous work by the PI and co-workers has detailed the variation in the mineralogy, textural appearance and composition of cumulate xenoliths erupted along the arc; these are interpreted as showing that, towards the north of the arc, the magma chambers feeding the individual volcanoes tend to lie at shallower levels, and the magmas contain a higher proportion of water. Small but important gaps remain in sampling of cumulate xenoliths along the 750km length of the arc, and we propose to rectify these. A new method is required to derive precise depths and temperatures of cumulate crystallization from compositions of minerals in the xenoliths. We will make use of a large database of experiments with products that approximately mimic the observed cumulate xenoliths; we will supplement it with additional experiments, to ensure that the full observed range of cumulate compositions and mineralogies has been obtained under controlled conditions of pressure, temperature, volatile content and oxygen fugacity. We will then calibrate thermodynamic models against the database, representing each of the observed mineral phases. The models will characterize the variation of composition with the controlled conditions among mineral phases in equilibrium. They will be calibrated with a view to being able to recover the experimental conditions via a rigorous statistical analysis. The finished models will then be applied to the database of natural samples, with the aim of building up an approximate three-dimensional picture of the subsurface magma storage system based on the samples that reach the surface. In principle this information may be combined with and verified against seismic imaging, which uses seismicity to probe crustal structure, and also with radiometric dating to provide a time-dimension to the analysis. The models and interpretations derived from this project will be relevant to a number of other subduction zone settings worldwide.