Douglas Bruce McKay MSc thesis abstract

Thesis Title: 
Aspects of Gallium Geochemistry in Upper Mantle-Derived Lherzolite Xenoliths and Continental Alkaline Volcanic Rocks
Douglas Bruce

Various aspects of the geochemistry of Ga in upper mantle-derived garnet lherzolite xenoliths (from the Bultfontein Floors mine dump in Kimberly, South Africa), spinel lherzolite xenoliths (from Mount Pordnon, Australia), and continental alkaline volcanic rocks (from the Freemans Cove, Balcones, Urach and Hegau suites of Canada, the U.S.A., and Germany, respectively) have been investigated.  Ga abundances were determined by RNAA.  The feasibility of using epithermal INAA to generate accurate Ga data was investigated.  Analysis of U.S.G.S. standard granite G-2 suggests that epithermal INAA utilizing 630.1 KeV72Ga data has the potential to produce geologically useful Ga data.

The abundance of Ga in the lherzolite xenoliths varies by a factor of approximately 10 from 0.52 ± 0.14 ppm to 5.23 ± 0.44 ppm.  The spinel lherzolite xenoliths contain appreciably more Ga (2.78 ± 0.26 ppm to 5.23 ± 0.44 ppm) than the garnet lherzolite xenoliths (0.52 ± 0.14 ppm to 1.44 ± 0.18 ppm).  The sequence of enrichment of Ga in the minerals comprising the lherzolites is, from lowest to highest Ga content, olivine (0.20 ± 0.04 ppm to 0.72 ± 0.11 ppm), clinopyroxene (1.31 ± 0.13 ppm to 4.92 ± 0.32 ppm), garnet, if present (3.17 ± 0.16 ppm to 6.03 ± 0.48 ppm) phlogopite, if present (7.65 ± 0.21 ppm), and spinel, if present (43.49 ± 0.91 ppm to 65.91 ± 2.10 ppm).  Ga mineral/mineral distribution coefficients have been calculated.  Preliminary results suggest the distribution of Ga between certain mineral pairs (e.g., DGaopx/pcx) is temperature dependent and might be profitably utilized as a geothermometer.

The abundance of Ga in the continental alkaline volcanic rocks varies by a factor of approximately 3.5 from 12:14 ± 0.32 ppm to 41.97 ± 1.20 ppm.  Ga behaved as an incompatible trace element during the genesis of these rocks.  Model-derived constraints for Ga bulk-solid/melt distribution coefficients vary from 0.16 (Balcones suite) to 0.28 (Hegau suite).  Intrasuite variations in the Ga/Al ratios of the mafic continental alkaline volcanic rocks range from approximately 14% (Hegau and Urach suites) to approximately 20% (Freemans Cove suite).  Ga/Al ratios of primary melts from the Freemans Cove and Balcones suites decrease slightly (from 2.86 ± 0.16 top 2.28 ± 0.13, and from 4.12 ± 0.24 to 3.35 ± 0.19 respectively) with increasing degrees of partial melting.

A copy of the thesis can be downloaded here