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Front. Earth Sci. | doi: 10.3389/feart.2018.00015

Partial melting of lower oceanic crust gabbro: Constraints from poikilitic clinopyroxene primocrysts

  • 1Institute of Petrology and Geochemistry, ETH Zurich, Switzerland
  • 2School of Earth and Ocean Sciences, Cardiff University, United Kingdom
  • 3School of Earth and Environmental Sciences, University of Manchester, United Kingdom
  • 4School of Physical Sciences, University of Tasmania, Australia

Successive magma batches underplate, ascend, stall and erupt along spreading ridges, building the oceanic crust. It is therefore important to understand the processes and conditions under which magma differentiates at mid ocean ridges. Although fractional crystallization is considered to be the dominant mechanism for magma differentiation, open-system igneous complexes also experience Melting-Assimilation-Storage-Hybridization (MASH, Hildreth and Moorbath, 1988) processes. Here, we examine crystal-scale records of partial melting in lower crustal gabbroic cumulates from the slow-spreading Atlantic oceanic ridge (Kane Megamullion; collected with Jason ROV) and the fast-spreading East Pacific Rise (Hess Deep; IODP expedition 345).
Clinopyroxene oikocrysts in these gabbros preserve marked intra-crystal geochemical variations that point to crystallization-dissolution episodes of the gabbro eutectic assemblage. Kane Megamullion and Hess Deep clinopyroxene core1 primocrysts and their plagioclase inclusions indicate crystallization from high temperature basalt (>1160 and >1200°C, respectively), close to clinopyroxene saturation temperature (<50% and <25% crystallization). Step-like compatible Cr (and co-varying Al) and incompatible Ti, Zr, Y and rare earth elements (REE) decrease from anhedral core1 to overgrown core2, while Mg# and Sr/Sr* ratios increase. We show that partial resorption textures and geochemical zoning result from partial melting of REE-poor lower oceanic crust gabbroic cumulate (protolith) following intrusion by hot primitive mantle-derived melt, and subsequent overgrowth crystallization (refertilization) from a hybrid melt. In addition, towards the outer rims of crystals, Ti, Zr, Y and the REE strongly increase and Al, Cr, Mg#, Eu/Eu* and Sr/Sr* decrease, suggesting crystallization either from late-stage percolating relatively differentiated melt or from in situ trapped melt.
Intrusion of primitive hot reactive melt and percolation of interstitial differentiated melt are two distinct MASH processes in the lower oceanic crust. They are potentially fundamental mechanisms for generating the wide compositional variation observed in mid-ocean ridge basalts. We furthermore propose that such processes operate at both slow- and fast-spreading ocean ridges. Thermal numerical modelling shows that the degree of lower crustal partial melting at slow-spreading ridges can locally increase up to 50%, but the overall crustal melt volume is low (less than ca. 5% of total mantle-derived and crustal melts; ca. 20% in fast-spreading ridges).

Keywords: Spreading ridge, Lower oceanic crust, partial melting, poikilitic gabbro, clinopyroxene, primocryst, Zoning, MORB petrogenesis

Received: 01 Dec 2017; Accepted: 12 Feb 2018.

Edited by:

Scott A. Whattam, Indian Institute of Technology Bhubaneswar, India

Reviewed by:

Sobhi J. Nasir, Sultan Qaboos University, Oman
Ian E. Smith, University of Auckland, New Zealand  

Copyright: © 2018 Leuthold, Lissenberg, O'Driscoll, Karakas, Falloon, Klimentyeva and Ulmer. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Julien Leuthold, ETH Zurich, Institute of Petrology and Geochemistry, Clausiusstrasse 25, Zurich, 8092, Switzerland, julien.leuthold@erdw.ethz.ch