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.

Committee Member

Charles B. Connor, 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). δ7 Li values for Moorea cluster at +3 ─ +5‰ for the freshest lavas, and 0 ─ +2‰ 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. δ7 Li 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 δ7 Li values for the Society and Hawaiian plumes. A more likely explanation is that recycled crust or sediments have minimal influence on the Li isotope signatures of hotspot plumes.

Using the Moorea data and geochemical data from other sources, I created a set of Power Point instructional modules for use in petrology classes to aid in teaching students about the effects of fractional crystallization and partial melting. I tested the module on fractional crystallization in two upper-level geology classes to assess its value in increasing student understanding. Both classes received a lecture about fractional crystallization. One class worked through the module as a homework exercise, while the other did not use the module. Students who worked through the module in addition to the lecture showed an increased understanding of the concept of fractional crystallization.

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