Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas

• Published in: Contributions to mineralogy and petrology 2011-07, Vol.162 (1), p.1-20
• Main Authors: Foley, Stephen F., Jacob, Dorrit E., O’Neill, Hugh St. C.
• Format: Article
• Language: English
• Subjects: Basalt; Detection limits; Earth and Environmental Science; Earth Sciences; Geology; Magma; Marine pollution; Mineral Resources; Mineralogy; Original Paper; Petrology; Rocks; Trace elements; Volcanoes; Zonation
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-010-0579-y

Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb. A combination of point analyses with varying ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization of the chemical effects of contamination along cracks. Trace element concentrations are remarkably uniform between large and small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D Ca and D Al , and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80). Minor zonation is seen in elements with cation charges from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P. Early phenocryst cores with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an approximately linear correlation with K 2 O content, K 2 O/Al 2 O 3 and K 2 O/Na 2 O of the melt, but none with SiO 2 content or Mg#. D -values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0.011–0.018), Zn (0.44–0.49) and Ga (0.006–0.007) are lower. D V of various potassic rocks (0.015 in the Ugandan rocks) confirms the dependence on fO 2 calibrated by the Fe 3+ /(Fe 3+ +Fe 2+ ) of spinels; the Ugandan potassic rocks crystallized at fO 2  = FMQ to FMQ + 1. The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal peridotites, but not ocean islands. Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that large spot analyses produce the best results.