The "pressures" of being a ridge

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 Galapagos 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 Galapagos 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 from 100 and 300 MPa, and indicate depths of partial crystallization of ~3-9 km. This suggests that accretion occurs mostly near the base of the crust. However, the range of pressures for some segments is relatively large with maximum calculated values of 500-750 MPa. For example, near the major transform fault at ~85^OW, the calculated maximum pressure is 741 MPa and the average pressure is ~ 300 MPa. We consider it unlikely that the calculated high pressures represent the true pressure of partial crystallization, and suggest that the compositions of some magmas result from processes other than simple crystallization. Correlations between Pressure and MgO, between Na₂O and MgO, P₂O₅ and K₂O, and between Na₈ and longitude suggest that the processes operating beneath this ridge are complex. Near the transform fault for example, MgO vs Pressure shows a negative correlation with an R² value of 0.546. Such trends are inconsistent with magma evolution via crystallization alone and it is suggested that crystallization was accompanied by interaction with pre-existing crust. We hypothesize that modification of magma compositions through assimilation of oceanic crust accounts for the wide range of pressures calculated for samples from some ridge segments. Additional research is underway to test this hypothesis.