The Pressures of Partial Crystallization along the Galápagos Spreading Center
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Date
2016-08
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The Ohio State University
Abstract
As part of a larger project aimed at understanding the magma plumbing systems and magmatic
processes responsible for crust formation at divergent plate margins, we have begun a study of the
Galápagos Spreading Center (GSC), an intermediate spreading ridge off the west coast of South
America and connected to the East Pacific Rise. This ridge is of interest because it passes close to
the Galápagos Islands, allowing the effects of a mantle plume on sub-ridge processes and magma
plumbing systems to be examined. In addition, the effects of ridge-ridge intersection, ridge
propagation, and ridge offsets by transform faults on magma evolution can be examined. Published
compositional data for glasses collected along the ridge were used to calculate pressures of partial
crystallization and to examine variations in magma chemistry along the ridge. To aid interpretation
of the results, the ridge was divided into 12 segments based on sample distribution and the
occurrence of ridge offsets. Calculated pressures for most segments range between 100 and 300
MPa, and indicate depths of partial crystallization of ~3–9 km. This suggests that partial
crystallization and hence crustal accretion occurs mostly near the base of the crust. The range of
pressures for some segments is relatively large with maximum calculated values of 500–750 MPa.
Near the major transform fault at ~85OW, the calculated maximum pressure is 741 MPa and the
average pressure is ~300 MPa. It is unlikely that the calculated high pressures represent the true
pressure of partial crystallization. The compositions of some magmas may result from processes
other than simple crystallization. Correlations between pressure and MgO, between Na2O and MgO,
P
2O5 and K2O, and between Na8 and longitude suggest that the processes operating beneath this
ridge are complex. Near the transform fault MgO vs pressure shows a negative correlation with an
R2 value of 0.546. Such trends are inconsistent with magma evolution via crystallization alone
suggesting that crystallization was accompanied by interaction with pre-existing crust. Modification
of magma compositions through assimilation of oceanic crust probably accounts for the wide range
of pressures calculated for samples from some ridge segments.
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Keywords
Partial Crystallization, Galapagos, Spreading Center, mid-ocean ridge, Geology