Magma evolution along the East Pacific Rise between 11⁰N and 15⁰N

Abstract:

The fast spreading East Pacific Rise (EPR) between 11°N and 14°N is characterized by several discontinuities. Previous work suggests that magmas erupted at ridge discontinuities partially crystallize at higher pressures than magmas erupted along “normal” ridge segments. We have determined the pressures of partial crystallization of erupted magmas using 922 glass analyses compiled from Gale et al., (2013). The results indicate crystallization at a wide range of pressures (1 bar to ~ 700 MPa) over the whole length of this ridge segment with no apparent correlation between pressure and ridge discontinuities. This might indicate that some magmas begin to crystallize in the mantle. Alternatively, the compositions of some magmas have been modified by interaction with preexisting crust so that the calculated pressures may not represent the true pressure of partial crystallization. We compiled geochemical data for 178 whole-rock samples from Gale et al., (2013) to examine the possibility that some magmas have interacted with oceanic crust. We found no systematic changes in composition with latitude or ridge discontinuities, and values of Na₈ suggest near-constant degrees of melting at near constant temperature in the mantle beneath the ridge. However, normalizing trace element concentrations to the composition of average NMORB reveals that the southern and northern ridge segments have compositions similar to NMORB, although some samples are enriched in highly incompatible elements. In contrast, samples from the middle segment are depleted in highly incompatible elements relative to NMORB. The simplest interpretation is that magmas were ultimately derived from mantle sources variably depleted in highly incompatible elements. Plots of P₂O₅ and TiO₂ versus K₂O suggest that crystallization was not the only process occurring in the crust, and some magmas assimilated preexisting oceanic crust during ascent. This suggests the possibility that all magmas were ultimately derived from a mantle source depleted in incompatible elements relative to NMORB, but that some magmas became enriched in incompatible elements through interaction with the crust. Further studies will test this hypothesis, and seek an explanation for the high pressures of partial crystallization calculated for magmas erupted along the EPR.