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| Titel |
Osmium isotope systematics of Os-rich alloys and Ru-Os sulfides from oceanic mantle: evidence from Proterozoic and Paleozoic ophiolite-type complexes |
| VerfasserIn |
Inna Yu. Badanina, Kreshimir N. Malitch, Elena A. Belousova, Richard A. Lord, Thomas C. Meisel, Valery V. Murzin, Norman J. Pearson |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093205
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| Publikation (Nr.) |
 EGU/EGU2014-13509.pdf |
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| Zusammenfassung |
| This study presents a substantial data set of Os-isotope compositions of Os-rich alloys
and Ru-Os sulfides from deep portions of ophiolite sections from oceanic mantle.
These are represented by samples from different in age ophiolite-type massifs (i.e.,
Neoproterozoic Kunar in Northern Taimyr, Russia, and Hochgrossen in Eastern Alps,
Austria, Paleozoic Verkh-Neivinsk in Middle Urals, Russia, and Shetland in northern
Scotland). The investigation employed a number of analytical techniques, including electron
microprobe analysis, ID ICP-MS after high pressure acid digestion, and laser ablation
attached to multiple collector-inductively coupled plasma mass-spectrometry (LA
MC-ICP-MS).
Two distinct platinum-group mineral (PGM) assemblages have been recognized at the
Shetland and Verkh-Neivinsk localities: a ‘primary’ euhedral PGM assemblage, which occur
as inclusions in chromite, and a modified ‘secondary’ subeuhedral to anhedral PGM
assemblage observed in cracks filled by chlorite or serpentine, interstitially to chromite grains
[1]. A ‘primary’ PGM assemblage at Shetland is represented by solitary grains of laurite or
iridian osmium and composite grains of laurite + osmian iridium ± iridian osmium
that display well defined phase boundaries between two or three distinct PGMs. A
‘primary’ PGM assemblage at Verkh-Neivinsk is represented by Ru-Os-Ir alloy grains
that frequently mantled by ‘secondary’ Ru-Os sulfide and/or Ru-Os sulfarsenide
overgrowths.
The osmium isotope results identify (1) a restricted range of ‘unradiogenic’ 187Os/188Os
values for coexisting laurite and Os-rich alloy pairs that form ‘primary’ PGM assemblages at
Hochgrossen and Shetland (0.11860–0.11866 and 0.12473–0.12488, respectively); (2)
similar ‘unradiogenic’ 187Os/188Os values for both ‘primary’ and ‘secondary’ PGM
assemblages at Shetland (with mean 187Os/188Os 0.12419 and 0.12464, respectively) and
Verkh-Neivinsk (with distinct mean 187Os/188Os values), and (3) a wide scatter of
subchondritic 187Os/188Os values for ‘primary’ PGM assemblages at Kunar (i.e.,
187Os/188Os 0.11848–0.11239), Verkh-Neivinsk (0.11619–0.12565), and Hochgrossen
(0.11860–0.12450).
The whole-rock Os-isotope budget of chromitite at Shetland (0.1240±0.0006) is largely
controlled by laurite-dominant assemblages. In this case, the ‘secondary’ PGM assemblage
inherited the ‘unradiogenic’ Os-isotope signature of the ‘primary’ PGMs. No evidence for
other source contributions during later thermal events has been observed here. However, the
wide range of subchondritic 187Os/188Os values has been found in the ‘primary’ PGM
assemblages (e.g., laurite and Os-rich alloys) from the ophiolite-type complexes worldwide
[2 and references cited therein]. This wide range would be consistent with a model, in which
a prolonged history of melting events of parent ultramafic source rocks took place in the
mantle. This variability is in agreement with the conclusion that the Os-isotope system of
PGMs records multiple events during the chemical differentiation history of the
mantle [3] and could have been controlled by deep-geodynamic processes [4]. On the
other hand, the observed Os-isotope heterogeneity may be also attributed to the
presence of subcontinental lithospheric mantle (SCLM), characterized by highly
unradiogenic 187Os/188Os values (i.e., |
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