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 language | English |
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Pages (from-to) | A299-A299 |
Number of pages | 1 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 74 |
Issue number | 12 supplement |
DOIs | |
Publication status | Published - Jun 2010 |
Externally published | Yes |
Event | Goldschmidt Conference (20th : 2010) - Knoxville, United States Duration: 13 Jun 2010 → 18 Jun 2010 |