Magma plumbing system under the fast spreading Galapagos Ridge
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Denman Undergraduate Research Forum. 2008
The Galapagos Ridge System is one of the fastest spreading and most unique ridges on the Earth. Typical ridges spread at a rate of about 25-35 mm/yr, while the Galapagos spreads at between 47-63 mm/yr. Fast spreading rates are associated with increased magma output and with relatively thin crust beneath the ridge axis, as observed along the Galapagos Spreading Center (GSC). Spreading rate may also influence the depth of partial crystallization and the structure of magma plumbing systems beneath ridges. To constrain the depth at which partial crystallization of magmas occurs beneath the GSC, we have used a method to calculate the pressure of crystallization that is more accurate and reliable than similar methods such as that described by Claude Hertzberg (2004, Journal of Petrology). The method involves calculating the pressure at which a liquid, represented by volcanic glass, is chemically in equilibrium with olivine, plagioclase, and augite. Our data set is comprised of analyses of volcanic glass collected on scientific cruises along the Galapagos ridge. These analyses were downloaded from the RIDGE data base maintained by Lamont Earth Observatory. Filtering of the analyses was necessary to exclude glasses that were not in equilibrium with olivine, plagioclase, and augite. The 1,110 remaining glasses were then divided into twelve groups based on longitude. Results thus far indicate that Galapagos magmas crystallize over a range of pressure from 0.001- 10 kBar, equivalent to 0 to 30 km depth, but that most crystallization occurred between 2 and 6 kBar, or 8-18 km depth. This range of depth suggests that the magma plumbing system is complex and is likely composed of multiple, stacked chambers that are interconnected by dikes. Some of these chambers are probably located beneath the crust in the uppermost mantle. Further work is underway to determine whether there is any relation between magma chamber depth and other geochemical indicators of crustal thickness, such as Na concentrations (normalized to 8.0 wt % MgO). Emily Klein and co-workers have shown that Na8 is a measure of crustal thickness with low values indicating higher mantle temperatures and greater degrees of melting. Implications for the interpretation of the Galapagos magma plumbing system include the possible influence of the Galapagos hotspot and transform faults near the western end of the ridge on magma plumbing systems.