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| Titel |
A 1-Ga history of melting in the western Mediterranean Subcontinetal Lithospheric Mantle recorded in chromitites from the Ojén Ultramafic Massif (SW Spain) |
| VerfasserIn |
J. M. González-Jiménez, C. Marchesi, W. L. Griffin, R. Gutiérrez-Narbona, J.-P. Lorand, S. Y. O'Reilly, C. J. Garrido, F. Gervilla, N. J. Pearson |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061843
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| Zusammenfassung |
| Small (< 10 m) Cr- (Cr# = 0.84-0.60) and Al-rich (Cr# 0.57-0.40)] chromitite bodies are
enclosed in tabular dunite hosted in lherzolite of the Ojén ultramafic massif (SW Spain).
Field observations and thermochronology constrain the formation of chromitites circa 22 Ma,
before the emplacement of the massif into the lower crust. Estimation on the composition of
the chromitite parental melt suggests that they contained 0.1-0.3 wt% Cr2O3.Mass balance
calculations reveal that a typical Ojén high-Cr chromitite (5 m long and 0.3 cm thick)
contains 1.8-2.4Ã103 kg of Cr, implying that each chromitite body integrates 300-1500 m3 of
partial melts formed by melting of 7-29Ã103 m3 of peridotite. An Ojén high-Al chromitite
body of similar size contains 1.4-1.8Ã103 kg of Cr, integrating 250-1050 m3 of
partial melts formed by melting of 5-21Ã103 m3 of peridotite. These calculation
suggest that the chromitite bodies formed by focusing small melt fractions into
dunite channels after kilometer-scale infiltration in the Subcontinental Lithospheric
Mantle.
The examination of the in situ Os isotopic composition of accessory platinum-group
minerals (PGM) and base-metal minerals (BMM, including sulfides and arsenides) from both
chromitite-types and host-rock peridotites reveals similar distributions of Re-depletion
(TRD) model ages. TRD model ages calculated for chromitite-hosted PGM-BMM cluster at
~0.3 Ga and minor peaks extend back to ~ 1.4 Ga, whereas base-metal sulfides (BMS)
hosted in peridotite show a relatively minor age peak at ~0.2 Ga and two prominent peaks at
~ 0.6 and 1.2 Ga. The preservation of Proterozoic to Paleozoic Os model ages in the
chromitite-hosted PGM-BMM indicates that the small-volume melts that produced
the chromitites scavenged Os from relict phases in mantle peridotite during melt
transport and melt-rock reactions ca 22 Ma ago. These variable Os model ages
reflect the origin of Os from volumes of the subcontinental lithospheric mantle that
underwent multiple melting and metasomatic events since the Mesoproterozoic. The
preservation of these old events in mineral phases hosted in the small chromitite bodies
reflects the precipitation of small volumes of mantle melts and emphasizes that
the chromitites are key materials for studying the evolution of the upper mantle. |
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