The 226RA and 231PA systematics of axial, MORB, crustal residence ages, and MAGMA chamber characteristics at 9 to 10 degree N East Pacific Rise
Abstract
Mass spectrometric measurements of (230)Th-(226)Ra and (235)U-(231)Pa disequilibria for axial basalts are used to determine crustal residence ages for MORB magma and investigate the temporal and spatial characteristics of axial magma chambers (AMC) at 9-10 degrees N East Pacific Rise (EPR). Relative crustal residence ages can be calculated from variations in (226)Ra/(230)Th and (231)Pa/(235)U activity ratios for axial lavas, if (1) mantle sources and melting are uniform, and mantle transfer times are constant or rapid for axial N-MORB, and (2) (231)Pa/(235)U and (226)Ra/(230)Th in the melt are unaffected by shallow level fractional crystallization. Uniform Th, Sr, and Nd isotopic systematics and incompatible element ratios for N-MORB along the 9-10 degrees N segment indicate that mantle sources and transfer times are similar. In addition, estimated bulk solid/melt partition coefficients for U, Th, and Pa are small, hence effects of fractional crystallization on (231)Pa/(235)U ratios for the melt are expected to be negligible. However, fractional crystallization of plagioclase in the AMC would lower (226)Ra/(230)Th ratios in the melt and produce a positive bias in (226)Ra crustal residence ages for fractionated lavas.
- Publication:
-
Presented at the VM Goldschmidt Conference
- Pub Date:
- 1994
- Bibcode:
- 1994vmd..conf.....G
- Keywords:
-
- Basalt;
- Crystallization;
- Earth Crust;
- Geochronology;
- Magma;
- Mass Spectroscopy;
- Protactinium Isotopes;
- Radium Isotopes;
- Earth Mantle;
- Feldspars;
- Geophysics;
- Lava;
- Thorium Isotopes;
- Uranium Isotopes;
- Geophysics