Trace elements in olivines as probes of parental melt compositions in the western rift of East Africa

S. F. Foley, D. E. Jacob, Hugh St C O'Neill

Research output: Contribution to journalMeeting abstract

Abstract

Olivine is commonly the earliest abundant phase crystallizing in mafic igneous rocks, and therefore has the potential to record information about unfractionated melts. Distinguishing liquidus olivines from crystals with other origins is particularly important for MgO-rich rocks such as picrites, kimberlites, and olivine lamproites, for which the proportion of primary igneous olivines is debated. Olivine phenocrysts in ugandite (16-22wt% MgO) and leucite basanite from the western branch of the East African Rift [1] have been analyzed for up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1ppb. Trace element concentrations are remarkably uniform between large and small phenocrysts; leucite basanites (Mg# 59) have higher DCa and DAl, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75-80). Zonation is seen in elements with cation charges from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and may correlate with Ti and Al, but not P. Some early phenocryst cores have high Li or Ni, low Mn, or enrichments in many trace elements, whereas xenocrysts have exceptionally low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31-35, less than in lamproites [2], with which they demonstrate correlations with K2O, K2O/Al2O3 and K2O/Na2O in melt, but none with SiO2 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. V/Sc indicates fO2 = 'FMQ – 0.54 to +1.38 at the time of olivine crystallization. A comparison of point analyses and line scans shows that 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. [1] Rosenthal et al. (2009) Earth Planet. Sci. Lett. 284, 236– 248. [2] Foley & Jenner (2004) Lithos 75, 19–38.
Original languageEnglish
Pages (from-to)A299-A299
Number of pages1
JournalGeochimica et Cosmochimica Acta
Volume74
Issue number12 supplement
DOIs
Publication statusPublished - Jun 2010
Externally publishedYes
EventGoldschmidt Conference (20th : 2010) - Knoxville, United States
Duration: 13 Jun 201018 Jun 2010

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