{sup 226}Ra and {sup 231}Pa systematics of axial MORB, crustal residence ages, and magma chamber characteristics at 9--10{degree}N East Pacific Rise

Conference · Wed Jun 01 04:00:00 EDT 1994

OSTI ID:10156836

Goldstein, S J; Murrell, M T ^[1]; Perfit, M R ^[2]; Batiza, R ^[3]; Fornari, D J ^[4]

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1. Los Alamos National Lab., NM (United States)
2. Univ., of Florida, Gainesville, FL (United States). Dept. of Geology
3. Univ., of Hawaii, Honolulu, HI (United States)
4. Woods Hole Oceanographic Institution, MA (United States). Dept. of Geology and Geophysics

Mass spectrometric measurements of {sup 30}Th-22{sup 226}Ra and {sup 235}-U{sup 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 {sup 226}Ra/{sup 230}Th and {sup 231}Pa/{sup 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) {sup 231}Pa/{sup 235}U and {sup 226}Ra/{sup 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 {sup 231}Pa/{sup 235}U ratios for the melt are expected to be negligible. However, fractional crystallization of plagioclase in the AMC would lower {sup 226}Ra/{sup 230}Th ratios in the melt and produce a positive bias in {sup 226}Ra crustal residence ages for fractionated lavas.

• Research Organization: Los Alamos National Lab., NM (United States)
• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: W-7405-ENG-36
• OSTI ID: 10156836
• Report Number(s): LA-UR--94-1668; CONF-9408102--1; ON: DE94013084
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000
AGE ESTIMATION
BASALT
GEOPHYSICS
GEOSCIENCES
ISOTOPE RATIO
MAGMA
MASS SPECTROMETERS