Melt Generation and Transport During Basalt Petrogenesis:

Constraints from Measurements of ²²⁷Ac^-231Pa Activity Ratios

Uranium-series isotopes have provided unique insight into geochemical rates and processes during genesis of basaltic magma from the earth's mantle. Specifically, recent measurements of the longer-lived isotopes of the U-decay series have placed important constraints on melting rates (related to solid mantle upwelling rates) and melt fractions. However, there have been relatively few measurements of the shorter-lived isotopes of the U decay series in basaltic samples. As a result, time-dependant parameters such as melt transport rates and magma storage times are not well known. To better constrain these parameters, under this SGER award, the PIs propose to measure short-lived ²²⁷Ac (t1/2 =21.8 y) in young basaltic samples from mid-ocean spreading centers and intraplate volcanoes like Hawaii. This geochronologic system has the potential to be an important chronometer for dating of young mid-ocean ridge basalts in the time range of 10-100 years. Development of a new counting technique for ²²⁷Ac has simplified its analysis making this research possible for the first time.

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Sims, K.W.W., S. Pichat, M. Reagan, P. Kyle, H. Dulaiova, N. Dunbar, J. Prytulak, G. Sawyer, G. Layne, J. Blichert-Toft, P. Gauthier, M. Charette, and T. Elliott. (2013) On the timescales of magma genesis, melt evolution, crystal growth rates, and magma degassing in the Erebus volcano magmatic system using the ²³⁸U, ²³⁵U, and ²³²Th decay series. Journal of Petrology, 54, 235-271.