| Popis: |
We propose a numerical method for modeling the geochemical evolution of nonsteady state (changing volume) magma chambers using analyses of erupted lavas. The method allows estimation of the ratio between concentrations of an incompatible element in the erupted and incoming liquids (E1). The relations between E1, the dimensionless magma flux through the reservoir and time, are shown graphically. We apply this model to the behavior of magma chambers underlying three segments of the East Pacific Rise, between 17°00'S and 18°40′S, with different axial morphology. The geochemical data vary according to the axial morphology of the segments: the lavas sampled along a dome-shaped axis display more restricted compositional variations than the lavas from the axial caldera of collapsed segments. These geochemical features, thought to reflect variable magma chamber states, are consistent with a cyclical model of a successively inflating and deflating magma chamber. The results of our modeling imply that (1) the inflation could be about twice as long as deflation, and (2) the magmatic volume expelled from the reservoir during deflation is at least three times larger than the volume expelled during inflation. |
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