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| Titel |
Origin of platinum-group mineral assemblages in a mantle tectonite at Unst deduced from mineral chemistry and osmium isotopes |
| VerfasserIn |
Inna Yu. Badanina, Richard A. Lord, Kreshimir N. Malitch, Thomas C. Meisel |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079555
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| Zusammenfassung |
| This study assesses textural and mineral chemistry data, whole-rock and mineral separate
Os-isotope compositions for distinct platinum-group mineral (PGM) inclusion assemblages
in an isolated chromitite pod at Harold’s Grave, which occurrs in a mantle tectonite at
Unst in the Shetland Ophiolite Complex, Scotland. The investigation employed a
multi-technique approach and utilized a number of analytical techniques, including
electron microprobe analysis, ID ICP-MS after high pressure acid digestion, and LA
MC-ICP-MS.
Two distinct PGM assemblages have been recognized. They comprise a ‘primary’
euhedrally shaped (up to 15 μm in size) PGM assemblage, which occur as inclusions in
chromite, and a modified ‘secondary’ subeuhedral to anhedral PGM assemblage (up to 100
μm) associated with Ru-rich pentlandite observed in cracks filled by chlorite or serpentine,
interstitially to chromite grains.
A ‘primary’ PGM assemblage is represented by solitary grains of laurite or iridian
osmium and composite grains that display well defined phase boundaries between two or
three distinct PGM. The latter are dominated by laurite and iridian osmium, with subordinate
laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide
(possibly prassoite). The compositional variability of associated laurite and Os-rich alloys
at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of
Andrews and Brenan (2002) providing unequivocal information on conditions of
their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich
alloys estimated at 1200 – 1250Ë C and f(S2) of 10-0.39–10-0.07. The inconsistent
grouping of different primary PGM grains argues against an origin by subsolidus
exsolution from the chromite host, providing useful information on conditions of their
genesis.
The ‘secondary’ PGM assemblage is polyphase, with dominant laurite, intimately
intergrown with native osmium, irarsite and Ru-rich pentlandite. This assemblage is likely to
reflect processes such as in-situ serpentinisation, alteration during emplacement or regional
greenschist metamorphism.
Whole-rock platinum-group element (PGE) concentrations give negatively sloped
chondrite-normalized PGE patterns, typical of podiform chromitite, where refractory PGE
(Os, Ir and Ru) prevail over less refractory PGE (Rh, Pt and Pd). The osmium isotope results
identify similarly ‘unradiogenic’ 187Os/188Os values for ‘primary’ and ‘secondary’ PGM
assemblages (with mean 187Os/188Os values of 0.12419 and 0.12464, respectively), being
within uncertainty of the chromitite composition (0.1240±0.0006). This implies
that the whole-rock Os isotope budget is largely controlled by laurite-dominant
assemblages, supporting the conclusion that the ‘secondary’ PGM assemblage inherited
the subchondritic osmium isotope signature of the ‘primary’ PGM. No evidence
for other source contributions during later thermal events has been observed. The
Os-isotope data provide further support for an Enstatite Chondrite Reservoir model for
the convective upper mantle as defined by Walker et al. (2002) and are consistent
with origin of the complex as a Caledonian ophiolite formed in a supra-subduction
zone.
This study was supported by Russian Foundation for Basic Research (grant
12-05-01166-a to IYuB) and the Uralian Division of Russian Academy of Sciences (project
No 12-P-5-1020).
References:
Andrews, D.R.A., Brenan, J.M. (2002) Phase-equilibrium constraints on the magmatic
origin of laurite and Os-Ir alloy. Can. Mineral. 40, 1705-1716.
Walker, R.J., Prichard, H.M., Ishiwatari, A., Pimentel, M. (2002) The osmium isotopic
composition of convecting upper mantle deduced from ophiolite chromites. Geochim.
Cosmochim. Acta 66, 329-345. |
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