Exposed ocean crust on Masirah Island, SE Oman: Crustal accretion and melt evolution at an ancient slow-spreading mid-ocean ridge

Jansen, Maximiliaan 2023. Exposed ocean crust on Masirah Island, SE Oman: Crustal accretion and melt evolution at an ancient slow-spreading mid-ocean ridge. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The accretion of oceanic crust at mid-oceanic ridges (MOR) accounts for the most voluminous magmatism on Earth and occurs across a range of spreading-rates. Slow-spreading ridges represent 50% of the present-day global MOR-system and produce heterogeneous oceanic crust that deviates fundamentally from the conventional 6 – 7 km layer-cake Penrose-crust formed at fast-spreading ridges. Instead, the sparse and intermittent magmatism at slow-spreading ridges requires plate separation to be partly accommodated by faulting, producing ocean lithosphere with a discontinuous igneous crust of variable thickness, commonly disrupted by large 'detachment' faults that exhume the deep crust and shallow mantle directly onto the seafloor. Due to the difficulty in accessing oceanic crust in situ, our knowledge of the processes occurring at MORs is still relatively limited and geologists often turn to ophiolites as their analogues. There is however an increasing awareness of a preservation bias towards ocean lithosphere formed at atypical spreading ridges, such as marginal ocean basins near subduction zones, and that potentially key differences between many ophiolites and ‘true’ MOR ocean lithosphere (especially in their chemical composition) complicate direct comparisons. The Masirah Ophiolite, exposed over ~650 km2 on an isolated island off the southeast coast of the Sultanate of Oman, is near-unique in that it is believed to have formed at a ‘true’ MOR, unaffected by the influence of subduction. It can therefore provide valuable geological insights into crustal accretion process along modern MORs. Although its basic structure was mapped the 1990s, previous studies did not examine Masirah from the perspective of modern MOR processes in any detail. Previous work determined that Masirah formed at a slow-spreading ridge in the young Indian Ocean at ~150 Ma, followed by an episode of alkaline magmatism during intraplate rifting some 20 Ma later, before being emplaced onto the Arabian continental margin in the late Cretaceous. The work presented in this thesis updates the geochronological model for the evolution of Masirah, describes a previously unrecognised type of ocean lithospheric architecture (‘Penrose on a diet’) that accreted at the paleo-spreading ridge, and investigates what mantle processes might be responsible for this style of accretion. In a key finding, new radiometric dates show that the ophiolite formed at 135 – 130 Ma, and that the alkaline magmatic activity overlapped with crustal accretion as an episode of ‘near-axis’ magmatism. Trace element compositions of the two magmatic suites show a large degree of overlap and define a continuum, supporting a transitional event where magmas, derived by variable degrees of melting of a heterogeneous mantle source, were initially delivered to the ridge axis, whereupon subsequent melts, derived from continued lower-degree melting of a more enriched mantle component, were delivered near off-axis. The ‘Penrose on a diet architecture’ of the Masirah lithosphere is noteworthy for having a thin igneous crust (2 km) and a thin lower crust with respect to the upper crust (lower crust : upper crust = ~0.4 : 0.6). Despite these characteristics suggesting a low melt supply, field relations nevertheless indicate the crust formed by magmatic spreading. This contradicts current crustal accretion models, which predict the formation of 6 – 7 km thick crust for conditions of high melt supply and spreading dominated by detachment faulting when melt supply is low. The Masirah Ophiolite shows that the variability of permissible slow-spreading crustal geometries is greater than previously thought and that a basaltic seafloor should not automatically be interpreted to correlate with a thick magmatic crust. The mantle section exposed on Masirah is characterised by highly refractory peridotites with a mineral chemistry indicating high degrees of melting, despite the overlying igneous crust being thin. By considering the effect of mantle fertility on the overall melt production, a model is advanced where upwelling of an ancient domain of depleted mantle led to an overall reduced melt supply and a high proportion of melts from fertile mantle components relative to those from the depleted peridotites. Consequently, observations from Masirah suggest that besides rates of upwelling and seafloor spreading, the time-integrated melting history of the mantle underneath modern MORs exercises an important control on the degree of mantle melting, as well as the resulting lithospheric architecture and compositions of the erupted mid-ocean ridge basalts.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > Q Science (General)
Funders:	NERC GW4+ DTP
Date of First Compliant Deposit:	23 March 2023
Last Modified:	24 Mar 2023 15:21
URI:	https://orca.cardiff.ac.uk/id/eprint/157900

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