Closed-system behaviour of the Re-Os isotope system recorded in primary and secondary platinum-group mineral assemblages: evidence from a mantle chromitite at Harold's Grave (Shetland ophiolite complex, Scotland)

Inna Yu. Badanina, Kreshimir N. Malitch, Richard A. Lord, Elena A. Belousova, Thomas C. Meisel

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Osrich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.
LanguageEnglish
Pages174-185
JournalOre Geology Reviews
Volume75
Early online date17 Dec 2015
DOIs
Publication statusE-pub ahead of print - 17 Dec 2015

Fingerprint

laurite
chromitite
Platinum
platinum
ophiolite
Isotopes
Minerals
isotope
mantle
secondary mineral
osmium isotope
mineral
Osmium
enstatite chondrite
osmium
pentlandite
iridium
obduction
serpentinization
greenschist

Keywords

  • platinum-group minerals
  • chromitite
  • osmium
  • mantle
  • Harold's grave
  • Shetland ophiolite complex

Cite this

@article{9f40043e134a4bc2bdd7f85699a3a833,
title = "Closed-system behaviour of the Re-Os isotope system recorded in primary and secondary platinum-group mineral assemblages: evidence from a mantle chromitite at Harold's Grave (Shetland ophiolite complex, Scotland)",
abstract = "This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Osrich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.",
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Closed-system behaviour of the Re-Os isotope system recorded in primary and secondary platinum-group mineral assemblages : evidence from a mantle chromitite at Harold's Grave (Shetland ophiolite complex, Scotland). / Badanina, Inna Yu.; Malitch, Kreshimir N.; Lord, Richard A.; Belousova, Elena A.; Meisel, Thomas C.

In: Ore Geology Reviews, Vol. 75, 17.12.2015, p. 174-185.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Closed-system behaviour of the Re-Os isotope system recorded in primary and secondary platinum-group mineral assemblages

T2 - Ore Geology Reviews

AU - Badanina, Inna Yu.

AU - Malitch, Kreshimir N.

AU - Lord, Richard A.

AU - Belousova, Elena A.

AU - Meisel, Thomas C.

PY - 2015/12/17

Y1 - 2015/12/17

N2 - This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Osrich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.

AB - This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Osrich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.

KW - platinum-group minerals

KW - chromitite

KW - osmium

KW - mantle

KW - Harold's grave

KW - Shetland ophiolite complex

UR - http://www.sciencedirect.com/science/journal/01691368

U2 - 10.1016/j.oregeorev.2015.12.014

DO - 10.1016/j.oregeorev.2015.12.014

M3 - Article

VL - 75

SP - 174

EP - 185

JO - Ore Geology Reviews

JF - Ore Geology Reviews

SN - 0169-1368

ER -