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The role of the axial melt lens in crustal accretion at fast-spreading mid-ocean ridges

Loocke, Matthew 2016. The role of the axial melt lens in crustal accretion at fast-spreading mid-ocean ridges. PhD Thesis, Cardiff University.
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Abstract

Fast-spreading mid-ocean ridges (MOR) are underlain by a thin, quasi-steady-state melt or crystal mush body at the base of the sheeted dykes, referred to as the axial melt lens (AML). Although the AML is thought to play a key role in the development of MOR basalts (MORB), debate persists regarding the composition of the AML and the role it plays in the accretion of the lower crust. I address this question by studying a suite of varitextured gabbronorites from the Hess Deep rift valley in the equatorial Pacific Ocean which are interpreted to have formed in the AML of the East Pacific Rise. This unique sample set provides an unparalleled opportunity to conduct the first comprehensive investigation of the AML at a fast-spreading MOR. To facilitate this study, I here develop a method for the quantitative assessment of compositional distribution (QACD) in whole-thin-section element maps. QACD facilitates rapid data collection and processing to generate mineral modes, element and molar-ratio maps, and quantifying full-sample compositional distributions. My application of QACD to the Hess Deep AML suite reveals that mineral phases within the AML here are too evolved to be in equilibrium with MORB. I test the broader applicability of this conclusion by conducting detailed mapping and sampling of an analogous AML horizon in the Oman Ophiolite (Wadi Saq, Ibra Valley). This section is characterised by an evolved sheeted dyke complex rooting into a quartz diorite-hosted AML, supporting the supposition that the AML accommodates the fractionation of highly-evolved melts. I propose a model wherein the AML is predominantly fed by small volumes of evolved interstitial melts expelled from the underlying crystal mush. In the months preceding decadal eruption events, short-lived, focused injections of primitive melts into the AML mix with the extant highly-fractionated melt and trigger eruptions. This model reconciles the apparent mismatch between the volcanic and plutonic records and inferences made on geophysical and petrological grounds. I suggest that the AML is an active player in the development of MORB, permitting the fractionation and storage of evolved melts expelled from the underlying crystal mush and recording the mixing of that material with primitive melt, hence fulfilling more of a passive role with respect to lower crustal accretion than previously proposed.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 24 January 2017
Last Modified: 04 Jun 2017 09:37
URI: http://orca.cf.ac.uk/id/eprint/97663

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