Pressures of Partial Crystallization along the East Pacific Rise: Preliminary Results and Evidence for Deep Crustal Chambers?

The fast spreading East Pacific Rise (EPR) between 8°N and 14°N is characterized by ~20 ridge axis discontinuities. Previous work has suggested that magmas erupted at some of these discontinuities, specifically at transform faults, partially crystallize at higher pressures than magmas erupted along 'normal' ridge segments. In order to evaluate the factors that control the plumbing systems of MORB, we have determined the pressures (P) of partial crystallization of magmas beneath the EPR from 8°N to 14°N using 2,837 published chemical analyses of basalt glasses. Pressures were calculated using the two different methods described by Herzberg, 2004 (H) and Kelley and Barton, 2008 (KB). These two methods are expected to yield minimum and maximum values for calculated pressures. However, for the samples used in this work both methods yield virtually identical pressures for the entire data set (average pressures: H=182.3×143.6 MPa and KB=200.9×153.3 MPa) and we elected to use results obtained with the KB method for detailed analysis of the results. Based on sample frequency, the EPR was divided into 10 sections that include 'normal' segments lacking axial discontinuities and segments that include discontinuities such as a transform fault, an overlapping spreading center (OSC), or a deviation from axial linearity, or one or more of these features. The average pressure of partial crystallization for 'normal' segments is 145.6×59.9 MPa, and is essentially constant along the whole length of the ridge studied. These results are consistent with crystallization at depths of ~ 5km near the base of the crust and, as with results obtained for the Juan da Fuca and Reykjanes ridge, provide no evidence for crystallization in the upper mantle along 'normal' ridge segments. The pressures of partial crystallization of magmas at or near axial discontinuities show a wider range than those for magmas erupted along normal ridge segments. The maximum pressures for samples from the Clipperton and Siqueiros fracture zones, for example, exceed 300 Mpa and range up to ~600 MPa. This confirms the findings of previous workers and appears to indicate partial crystallization in the upper mantle at depths of 10-20 km. It has been suggested that the greater depths of partial crystallization of magmas at transform faults and other ridge discontinuities reflects the presence of cooler mantle beneath these features. However, many of the samples that yield high pressures have anomalous chemical compositions (shown primarily in the concentrations of CaO and Al₂O₃) compared with magmas erupted along 'normal' ridge segments. We interpret these chemical anomalies to result from magma-crust interaction, and when samples with anomalous compositions are filtered out of the data base, we obtain pressures of partial crystallization for magmas erupted near ridge discontinuities that are similar to those obtained for magmas erupted along normal ridge segments. Work is underway to refine the results reported here, and it is possible this refinement will lead to a decrease in calculated pressures of partial crystallization.