Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary Os-rich PGM

Ahmed, Ahmed H.; Hanghøj, K.; Kelemen, P. B.; Hart, Stanley R.; Arai, Shoji

doi:10.1016/j.epsl.2006.03.021

Cited by 77 publications

(27 citation statements)

References 58 publications

(102 reference statements)

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“…Most researchers acknowledge the role of aqueous fluids or hydrous silicate melts in the petrogenesis of the Oman chromitites (Johan et al, 1983;Auge¤ , 1987;Lorand & Ceuleneer, 1989;Leblanc & Ceuleneer, 1992;Schiano et al, 1997;Arai et al, 2004Arai et al, , 2006Ahmed et al, 2006;Rollinson, 2008). The source of water and its involvement in the Maqsad MTZ chromitite petrogenesis is, however, controversial.…”

Section: Introductionmentioning

confidence: 99%

“…Shimizu et al, 2001;Arai et al, 2006Arai et al, , 2011. In contrast to chromites disseminated in volcanic rocks (Kamenetsky, 1996;Borisova et al, 2001;Kamenetsky et al, 2001Kamenetsky et al, , 2002, chromian spinel forming chromitite ores contains abundant inclusions of fluid, hydrous and anhydrous silicates, sulphides, and platinum group minerals (Auge¤ , 1987;Auge¤ & Johan, 1988;Lorand & Ceuleneer, 1989;Melcher et al 1997;Kamenetsky et al, 2001Kamenetsky et al, , 2002Arai et al, 2004;Spandler et al, 2005;Ahmed et al, 2006;Borisova et al, 2008). Inclusions of hydrous silicate minerals have been found in chromian spinel from chromitites in different tectonic settings, ranging from subduction zones to spreading ridges, and in their fossil equivalents exposed in ophiolites (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…The source of water and its involvement in the Maqsad MTZ chromitite petrogenesis is, however, controversial. Water might originate from either (1) hydrothermal fluid sources (Auge¤ & Johan, 1988) or (2) dehydration of a subducting slab (Schiano et al, 1997;Ahmed et al, 2006;Rollinson, 2008) or (3) re-melting of hydrated lithosphere in a spreading-ridge setting in response to episodic emplacement of mantle diapirs at the MTZ (Benoit et al, 1999;Python et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

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A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

Borisova¹,

Ceuleneer²,

Kamenetsky³

et al. 2012

Journal of Petrology

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110

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To decipher the petrogenesis of chromitites from the MohoTransition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of $500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1^50 mm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr þAl) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f O2 , of $3 log units below the quartz^fayalite^magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 9508C at subsolidus conditions, were overgrown by more oxidized host chromite (f O2 % QFM) in association with microcrystal inclusions of silicates (plagioclase An 86 , clinopyroxene, and pargasite) that were formed between 950 and 10508C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to $0·18C a À1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

Borisova¹,

Ceuleneer²,

Kamenetsky³

et al. 2012

Journal of Petrology

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110

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“…The ReOs isotopic systematics of IPGE-rich sulphides and alloys occurring as inclusions in chromitites from oceanic mantle have been used to track the geochemical evolution of the upper convecting mantle (e.g. Malitch 2004;Ahmed et al 2006;Shi et al 2007;Marchesi et al 2011;González-Jiménez et al 2012a, 2013a. However, a large body of analytical and experimental evidence indicates that the chromitites may undergo significant alteration during metamorphism (Evans and Frost 1975;Burkhard 1993;Barnes 2000Barnes , 2001Zaccarini et al 2005;Mukherjee et al 2010; Saumur and Hattori 2013; Barra et al 2014).…”

Section: Introductionmentioning

confidence: 97%

Thermal metamorphism of mantle chromites and the stability of noble-metal nanoparticles

Gonzáléz-Jiménez

Reich

Camprubí

et al. 2015

Contrib Mineral Petrol

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event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromitehosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS 2 , releasing minute particles of Ru-Os-Ir alloys <50 nm in diameter. The following short-lived thermal event that promoted dehydration in the chromitite had the opposite effect on nanoparticle stability, producing a significant coarsening of metal nanoparticles to dimensions larger than a micron. Based on such observations, we argue that IPGE nanoparticles can be exsolved and grown (or coarsen) from sulphide matrices during prograde metamorphism or heating and not exclusively upon cooling under magmatic conditions as it has been previously suggested. These results provide new insights on the relevant role of temperature and nanoparticle-host interaction phenomena in natural systems, shedding new light on the kinetic controls of nano-to micronscale IPGE particle distributions during metamorphism. Abstract The Loma Baya complex in south-westernMexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite-olivine assemblage was replaced by a secondary, porous intergrowth of Fe 2+ -rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This thirdgeneration chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating Communicated by Othmar Müntener. Electronic supplementary materialThe online version of this article (

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“…The dismembered ophiolite suites and island arc metavolcanosedimentary assemblages were formed during the arc stage (Stern et al 2004). The ophiolitic rocks were interpreted as formed in a supra-subduction zone (SSZ) setting (e.g., Abu El Ela 1996;El-Sayed et al 1999;El Bahariya and Arai 2003;Stern et al 2004;El Gaby 2005;Ahmed et al 2006;Azer and Stern 2007). They were formed due to seafloor spreading (Pearce 2003;Stern et al 2004) that opened the Mozambique Ocean between the West and East Gondwana supercontinents.…”

Section: Introductionmentioning

confidence: 99%

Pan-African (intraplate and subduction-related?) metasomatism in the Fawakhir ophiolitic serpentinites, Central Eastern Desert of Egypt: mineralogical and geochemical evidences

2011

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Fawakhir serpentinites are the most western ophiolitic ultramafics relative to the Pan-African collision suture at the Qift-Quseir road in the Central Eastern Desert of Egypt. Their location is the basis for their selection in examining the possible contribution of the westerly dipping subducted oceanic slab-related melt/fluid with the intraplate granitic intrusion-related melt/fluid in the metasomatism of the Neoproterozoic ophiolitic serpentinites in the Eastern Desert. Non-residual mineralogy and geochemistry of serpentinites (SF1) far from the post-collision A2-type Fawakhir granitoids and those of serpentinites (SF2) in the vicinity of the granitoid pluton were investigated. The Fawakhir serpentinites are harzburgitic in composition and the Cr# (0.66-0.80) and Mg# (0.32-0.50) of their unaltered spinel cores are indicators for their forearc setting, where they were formed in the oceanic mantle wedge. Based on the spinel Cr# and the whole rock Yb-V bivariate, the melt extraction from the primitive mantle is in excess of 18% up to 24%. The HREE pattern of the SF1 serpentinites refers to the fractional type of melting. The formation of nonresidual mineral phases particularly in SF2 samples (amphibole, biotite, apatite thorite, and monazite) and the enrichment of all serpentinites in trace incompatible elements refer to these two serpentinite groups having underwent modal metasomatism. It is suggested that viscous fluid/melt related to the Fawakhir granitoid emplacement metasomatized the SF2 serpentinites, causing a strong enrichment in LREE (display concave LREE; La N /Sm N = 3.32-6.25 and U-type HREE; Gd N /Yb N =1.14-2.69) and a slight enrichment in Zr (12-16.62 ppm). All serpentinites are enriched in fluid-mobile elements by aqueous fluids, but the SF2 are more enriched in these elements. The spiked B compared to the other fluid-mobile elements respectively) suggests that these elements were added to the obducted ophiolitic Fawakhir serpentinites by the percolation of subduction-related fluids at shallow depths. The contribution of B from shallow continental crust-related fluids is debated. Hosting the Fawakhir serpentinites for the gold deposit at Fawakhir Mine implies a possible genetic relation between gold mineralizations hosted in the ultramafic rocks of the ANS and the processes of recycling of the subducted oceanic slab and the interaction with the mantle. Detailed stable and radiogenic isotopic analyses of the mineralization zones are required to address this question.

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Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary Os-rich PGM

Cited by 77 publications

References 58 publications

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

Thermal metamorphism of mantle chromites and the stability of noble-metal nanoparticles

Pan-African (intraplate and subduction-related?) metasomatism in the Fawakhir ophiolitic serpentinites, Central Eastern Desert of Egypt: mineralogical and geochemical evidences

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