The role of apatite fractionation and REE distribution in alkaline rocks from Mt. Etna, Sicily |
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| Authors: | T Busà R Clochiatti R Cristofolini |
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| Institution: | (1) Dipartimento di Scienze Geologiche, University of Catania, Italy, IT;(2) Laboratoire P. Sue (LPS), CEA – CERN, Saclay, France, FR |
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| Abstract: | Summary Melt inclusions in olivine and apatite, and REE distribution of apatite were studied in one of the least differentiated members
of the oldest alkaline succession of Mt. Etna. Apatite occurs both as microphenocrysts and as inclusions in olivine crystals,
even in the most Mg-rich ones (Fo82). In addition phenocrysts and groundmass are composed of plagioclase, clinopyroxene, olivine and magnetite.
Apatite is fluor-apatite, with rather homogeneous major element (measured by electron microprobe, EMP) and REE (measured by
laser-ablation microprobe, LAM, and by secondary ion mass spectrometer, SIMS) contents. REE are enriched when compared to
the whole rock, with contents in olivine-hosted apatite lower than or the same as those of the microphenocryst cores; these
in turn show lower REE values than their edges. Distribution coefficients, calculated from LAM data of microphenocryst edges
and whole rock analyses, are higher for LREE (8–12) than for HREE (5–4).
In the SiO2 vs. P2O5 diagram melt inclusions and whole rock samples define a trend that is consistent with continuous apatite extraction from
a “high P” basalt magma. Finally, whole rock data show LREE/HREE (La/Lu)n enrichment ratios from hawaiites to mugearites (=1.14), consistent with apatite fractionation, lower than those documented
for lavas of the “low P” type (enrichment ratio = 1.34–1.37), where conditions for apatite saturation were not established.
Received January 2, 2000; revised version accepted April 2, 2001 |
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