Pressures of Partial Crystallization along the Galapagos Spreading Center

As part of a larger project aimed at understanding the magma plumbing systems and magmatic \nprocesses responsible for crust formation at divergent plate margins, we have begun a study of the \nGalápagos Spreading Center (GSC), an intermediate spreading ridge off the west coast of South \nAmerica and connected to the East Pacific Rise. This ridge is of interest because it passes close to \nthe Galápagos Islands, allowing the effects of a mantle plume on sub-ridge processes and magma \nplumbing systems to be examined. In addition, the effects of ridge-ridge intersection, ridge \npropagation, and ridge offsets by transform faults on magma evolution can be examined. Published \ncompositional data for glasses collected along the ridge were used to calculate pressures of partial \ncrystallization and to examine variations in magma chemistry along the ridge. To aid interpretation \nof the results, the ridge was divided into 12 segments based on sample distribution and the \noccurrence of ridge offsets. Calculated pressures for most segments range between 100 and 300 \nMPa, and indicate depths of partial crystallization of ~3–9 km. This suggests that partial \ncrystallization and hence crustal accretion occurs mostly near the base of the crust. The range of \npressures for some segments is relatively large with maximum calculated values of 500–750 MPa. \nNear the major transform fault at ~85OW, the calculated maximum pressure is 741 MPa and the \naverage pressure is ~300 MPa. It is unlikely that the calculated high pressures represent the true \npressure of partial crystallization. The compositions of some magmas may result from processes \nother than simple crystallization. Correlations between pressure and MgO, between Na2O and MgO, \nP \n2O5 and K2O, and between Na8 and longitude suggest that the processes operating beneath this \nridge are complex. Near the transform fault MgO vs pressure shows a negative correlation with an \nR2 value of 0.546. Such trends are inconsistent with magma evolution via crystallization alone \nsuggesting that crystallization was accompanied by interaction with pre-existing crust. Modification \nof magma compositions through assimilation of oceanic crust probably accounts for the wide range \nof pressures calculated for samples from some ridge segments.