Graduation Year

2005

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Jeffrey G. Ryan, Ph.D.

Co-Major Professor

H. Leonard Vacher, Ph.D.

Keywords

Boron, Geochemistry, Mantle, Ocean island basalts, Quantitative literacy

Abstract

The purpose of this thesis is to examine the abundance systematics of Li, Be and B, and Li isotopic systematics in lavas from the Society Islands, an enriched mantle (EMII) intraplate site, to further characterize the chemical signatures in the sources for ocean island basalts that may result from subduction-related processes and mantle entrainment. The goal is to see how light-element and Li-isotope systematics vary during ocean-island volcanic evolution and during tropical weathering.B/K, B/Be and Li/V ratios in basaltic Moorea lavas are 0.0001-.0002, 0.6-2.0 and 0.01-0.05 respectively, and the more evolved samples are somewhat higher. These ratios are similar to those for other Society Island lavas, and lower than those for lavas from St. Helena, Erebus, Hawaii, Gough and Reunion, as well as analyzed mid-ocean ridge basalts (MORBs).

Li values for Moorea cluster at +3 +5 percent for the freshest lavas, and 0+2 percent for more weathered rocks.These new data from Moorea are consistent with earlier survey results from the Society Islands and indicate a mantle source that includes B-poor (subducted) materials. 7Li values for the freshest Moorea samples are similar to those of other Society Island lavas, suggesting that the EMII isotopic end-member records a Li-isotopic signature similar to that of MORBs. Dilution by entrainment of upper mantle material is unlikely due to differing B/K ratios and similar Li values for the Society and Hawaiian plumes.

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