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Polymineralic inclusions in mantle chromitites from the Oman ophiolite indicate a highly magnesian parental melt

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posted on 2018-05-15, 10:46 authored by Hugh Rollinson, Lucan Memeri, Tiffany Barry
Polymineralic inclusions interpreted as melt inclusions in chromite from the dunitic Moho Transition Zone in the Maqsad area of the Oman ophiolite have been analysed and compositions integrated using a rastering technique on the scanning electron microscope. The inclusions now comprise a range of inter-grown hydrous phases including pargasite, aspidolite, phlogopite and chlorite, indicating that the parental melts were hydrous. Average inclusion compositions for seven samples contain between 23.1-26.8 wt% MgO and 1.7-3.6 wt % FeO. Compositions were corrected to allow for the low FeO concentrations using coexisting olivine compositions. These suggest that the primary melt has between 20-22 wt% MgO and 7-9.7 wt % FeO and has an affinity with boninitic melts, although the melts have a higher Ti content than most boninites. Average rare earth element concentrations suggest that the melts were derived from a REE depleted mantle source although fluid-mobile trace elements indicate a more enriched source. Given the hydrous nature of the inclusions this enrichment could be fluid driven. An estimate of the melt temperature can be made from the results of homogenisation experiments on these inclusions and suggests ~ 1300 oC, which implies for a harzburgite solidus, relatively shallow melting at depths of less than 50 km and is consistent with a boninitic origin. The current ‘basaltic’ nature of the chromite host to highly magnesian melt inclusions suggests that the dunitic Moho Transition Zone operated as a reaction filter in which magnesian melts were transformed into basalts by the removal of high magnesian olivines, particularly in areas where the Moho Transition Zone is unusually thick. We propose therefore that podiform mantle chromitites, even those with an apparent MORB-like chemical signature, have crystallised from a highly magnesian parental melt. The data presented here strongly support the view that this took place in a subduction initiation setting

History

Citation

Lithos, 2018, in press

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/School of Geography, Geology and the Environment

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  • AM (Accepted Manuscript)

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Lithos

Publisher

Elsevier

issn

0024-4937

Acceptance date

2018-04-30

Copyright date

2018

Publisher version

https://www.sciencedirect.com/science/article/pii/S0024493718301518

Notes

The file associated with this record is under embargo until 24 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.

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en

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