Petrology and geochemistry of prograde deserpentinized peridotites from Happo-O’ne, Japan: Evidence of element mobility during deserpentinization

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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“…For example, the forsterite microcrystals may represent the dehydrated serpentine assemblage (e.g. Khedr & Arai, 2012). The Maqsad diapir (Fig.…”

Section: Three-stage Model For the Maqsad Mtz Chromitite Petrogenesismentioning

confidence: 99%

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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106

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“…The mantle olivine array is from Takahashi et al [33]; (B) The olivine forsterite content and MnO wt % of the Ulamertoq samples are compared with other metamorphic olivine from various studies and olivine from typical residual abyssal peridotite from Warren [34]. Compositions of metamorphic olivine from the Happo-O'ne peridotite complex [35], Tari-Misaka [36], Ohsa-Yama [37] in Japan, and abyssal peridotites [34] are also included for comparison.…”

Section: Olivinementioning

confidence: 99%

“…The orthopyroxene Al 2 O 3 content is also low (1.2-1.9 wt %). Clinopyroxenes associated with the homogeneous chromites and its peridotite host also have low Al 2 O 3 contents (0.32-0.68 wt %) with a wide-ranging Cr 2 O 3 (0.00-0.17 wt %) and less variable Mg# (0.96-0.97) ( Figure 6C [35], Tari-Misaka [36], Ohsa-Yama [37] in Japan, abyssal peridotites [34], and cratonic peridotites [38] are also included for comparison. [35], Tari-Misaka [36], Ohsa-Yama [37] in Japan, abyssal peridotites [34], and cratonic peridotites [38] are also included for comparison.…”

Section: Pyroxenesmentioning

confidence: 99%

Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

et al. 2018

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Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.

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“…Metamorphic olivine may form through deserpentinization reactions of altered ultramafic precursors. Such an olivine is generally high in Fo (>>90 wt.%) and often also MnO (>1 wt.%) and commonly contains magnetite inclusions formed from liberated excess iron (Arai, 1975;Vance & Dungan, 1977;Nozaka & Shibata, 1995;Nozaka, 2010;Khedr & Arai, 2012). Although scattered oxide inclusions other than Cr-spinel may occur within the recrystallized olivine grains, they probably are related to a late-stage breakdown of both clinopyroxene and olivine to secondary magnetite, which is commonly observed in the Kevitsa olivine pyroxenites.…”

Section: Group 1 Inclusionsmentioning

confidence: 99%

Characterization and origin of dunitic rocks in the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland: whole-rock and mineral chemical constraints

Luolavirta¹,

Hanski²,

Maier³

et al. 2018

Bull Geol Soc Finland

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The ca. 2.06 Ga Kevitsa intrusion is one of the ore-bearing mafic-ultramafic igneous bodies in the Central Lapland greenstone belt. A large disseminated Ni-Cu-(PGE) sulfide ore deposit is hosted by olivine-pyroxene cumulates in the lower ultramafic part of the intrusion, indicating involvement of a multiply saturated magma in the ore formation. There are also various dunitic rocks, which occur as numerous inclusions within the Kevitsa intrusion, most commonly in the economic resource area. On textural basis, two distinct types of inclusions are recognized: i) cumulate-textured (Kevitsa Dunite) and ii) recrystallized ultramafic inclusions. In addition, there also exists a separate dunitic body (Central Dunite) with a suface area of 0.6 x 1.0 km, cropping out in the central part of the intrusion.The Central Dunite and Kevitsa Dunite are similar olivine-chromite cumulates and with comparable whole-rock and mineral compositions, suggesting that they are cogenetic. A magmatic, rather than replacement origin for the dunitic cumulates is evidenced by their systematic mineral compositional trends consistent with magmatic fractionation. Wholerock major and trace element and mineral compositional data of the dunitic cumulates and Kevitsa olivine pyroxenites fall on the same linear trends and record similar REE characteristics indicating a genetic link between these two. The parental magmas for the dunitic cumulates were probably picritic and related to the picritic and basaltic volcanic rocks in the area. The high Fo content of olivine (up to ~89 mol.%) is consistent with a high-Mg parental melt.The recrystallized ultramafic inclusions are fine-grained and show a granoblastic/interlobate textures indicative for thermal textural readjustment. Two subgroups are identified: Group 1 shows a chemical affinity towards the dunitic cumulates and are interpreted as their recrystallized clasts. The Group 2 inclusions are compositionally comparable to the ~2.06 Ga komatiitic volcanic rocks in the area and are considered as dehydrated metavolcanic xenoliths. A decrease in the flow rate of the Kevitsa magmas due to entrapment of a high number of inclusions is proposed as a mechanism to promote settling of sulfides, contributing to the formation of the Ni-Cu-(PGE) ore.

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Petrology and geochemistry of prograde deserpentinized peridotites from Happo-O’ne, Japan: Evidence of element mobility during deserpentinization

Cited by 39 publications

References 63 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

Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

Characterization and origin of dunitic rocks in the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland: whole-rock and mineral chemical constraints

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