Magmatism and Crustal Accretion Along the Global Ridge System

Monday, 30 January 2017: 09:00
Sovereign Room (Hobart Function and Conference Centre)
V. Dorsey Wanless, Boise State University, Dept. of Geosciences, Boise, ID, United States and Mark D Behn, Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA, United States
Abstract:
Eruption dynamics and volcanism at the seafloor are intimately linked to petrologic and magmatic processes at depth. At mid-ocean ridges, magma supply, melting systematics, and melt storage and differentiation vary with spreading rate, ultimately resulting in different eruption styles and modes of crustal accretion. Unfortunately, unraveling the petrologic processes occurring in deeper magmatic systems at any spreading rate is complicated by melt homogenization in the shallow crust prior to eruption. However, the combination of lava geochemistry and melt inclusion compositions provides a powerful tool for understanding deeper magmatic systems along the global ridge system.

Here we present volatile (CO2, H2O, F, S, Cl), major, and trace element data from >400 olivine-hosted, naturally glassy, melt inclusions erupted on five mid-ocean ridges that span the range of spreading rates. We use equilibrium CO2-H2O concentrations to determine vapor-saturation pressures for each melt inclusion, which are converted to depths of crystallization. These are combined with crystallization pressures calculated using major element barometers from >10,500 MORB glasses to produce a global model for melt storage, crystallization and accretion. Comparing these new global datasets to ridge thermal models we find that (1) major element and vapor-saturation pressures both increase and become more variable with decreasing spreading rate, (2) crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present, and (3) significant crystallization occurs at the base of the lithosphere at all spreading rates. These observations indicate that while crystallization occurs over a range of pressures, it is enhanced at thermal/rheologic boundaries such as the melt lens and the base of the lithosphere. Furthermore, we show that the range of crystallization pressures in the mantle may relate to cooling and crystallization along the base of the sloping lithosphere as melts are pooled from on-and off-axis. Finally, we suggest that the remarkable similarity in the maximum vapor-saturation pressures (~3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a uniform CO2 content of 50­–85 ppm for the depleted upper mantle feeding the global mid-ocean ridge system.

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