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| Titel |
Trace element partitioning in rock forming minerals of co-genetic, subduction-related alkaline and tholeiitic mafic rocks in the Ural Mountains, Russia |
| VerfasserIn |
J. Krause, G. E. Brügmann, E. V. Pushkarev |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029334
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| Zusammenfassung |
| The partitioning of trace elements between rock forming minerals in igneous rocks is largely
controlled by physical and chemical parameters e.g. temperature, pressure and chemical
composition of the minerals and the coexisting melt. In the present study partition
coefficients for REE between hornblende, orthopyroxene, feldspars, apatite and
clinopyroxene in a suite of co-genetic alkaline and tholeiitic mafic rocks from the Ural
Mountains (Russia) were calculated. The results give insights to the influence of the
chemical composition of the parental melt on the partitioning behaviour of the
REE.
Nepheline-bearing, alkaline melanogabbros (tilaites) are assumed to represent the most
fractionated products of the melt that formed the ultramafic cumulates in zoned
mafic-ultramafic complexes in the Ural Mountains. Co-genetic with the latter is a suite of
olivine gabbros, gabbronorites and hornblende gabbros formed from a tholeiitic parental
melt. Negative anomalies for the HFSE along with low Nb and Ta contents and
a positive Sr anomaly indicate a subduction related origin of all parental melts.
The nepheline gabbros consist predominantly of coarse-grained clinopyroxene
phenocrysts in a matrix of fine grained clinopyroxene, olivine, plagioclase, K-feldspar
and nepheline with accessory apatite. The tholeiitic gabbros have equigranular
to porphyric textures with phenocrysts of olivine, pyroxene and hornblende in a
plagioclase rich matrix with olivine hornblende, pyroxene and accessory apatite. Element
concentrations of adjacent matrix grains and rims of phenochrysts were measured with
LA-ICPMS.
The distribution of REE between hornblende and clinopyroxene in the tholeiitic rocks is
similar for most of the elements (DHbl-Cpx(La-Tm) = 2.7-2.8, decreasing to 2.6 and 2.4 for
Yb and Lu, respectively). These values are about two times higher than published data
(e.g. Ionov et al. 1997). Partition coefficients for orthopyroxene/clinopyroxene
systematically decrease from the HREE (DOpx-Cpx(Lu) = 0.31) towards the LREE
(DOpx-Cpx(Nd) = 0.01). The partition coefficients for plagioclase/clinopyroxene and
K-feldspar/clinopyroxene in the alkaline melanogabbros decrease from the LREE
(DPlg-Cpx(La) = 0.91, DK-fs-Cpx(La)=0.26) to the MREE (DPlg-Cpx(Sm) = 0.02,
DK-fs-Cpx(Sm) = 0.006), but both mineral pairs have similar DEu (DPlg-Cpx(Eu) = 0.25,
DK-fs-Cpx(Eu) = 0.23). Plagioclase/clinopyroxene partition coefficients for all REE in
the tholeiitic gabbros are 3-5 times higher, if compared to those of the alkaline
gabbros (DPlg-Cpx(La) = 1.7, DPlg-Cpx(Sm) = 0.034). Apatite/clinopyroxene partition
coefficients for the REE decrease from the LREE (DAp-Cpx(La) = 65 in alkaline and
120 in tholeiitic gabbro) to the HREE (DAp-Cpx(Lu) = 4.5 in alkaline and 5.3 in
tholeiitic gabbro). The lower partition coefficients for apatite/clinopyroxene and
plagioclase/clinopyroxene in the alkaline melanogabbros can be explained by higher
clinopyroxene/melt partition coefficients in this system. The higher Al2O3-content
in clinopyroxene from the alkali gabbros (Al2O3 = 3.5-7 wt.%), if compared to
clinopyroxene in the tholeiitic gabbros (Al2O3 = 2.0-4.5 wt.%) can account for a
stronger partitioning of the REE into clinopyroxene in the alkaline rocks (e.g. Gaetani
and Grove 1995). Experimental data by Gaetani (2004) also indicate a systematic
increase of the Cpx/melt partition coefficients for the REE with increasing Al2O3 and
Na2O contents of the parental melt in mafic systems. This is in agreement with the
assumed compositional differences between the alkaline and the tholeiitic parental
melts.
Gaetani, G.A., 2004. Contributions to Mineralogy and Petrology, Vol. 147, 511-527. Gaetani,
G.A., Grove, T.L, 1995. Geochimica et Cosmochimica Acta, Vol. 59, 1951-1962. Ionov,
D.A., Griffin, W.L., O‘Reily, S.Y., 1997. Chemical Geology, Vol. 141, 153-184. |
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