TY - JOUR

T1 - Anatomy of a frozen axial melt lens from a fast-spreading paleo-ridge (Wadi Gideah, Oman ophiolite)

AU - Müller, T.

AU - Koepke, J.

AU - Garbe-Schönberg, C.-D.

AU - Dietrich, M.

AU - Bauer, U.

AU - Wolff, P. E.

N1 - Funding information: This study relies on field campaigns conducted thanks to the hospitality of the Omani people, and the Directory of Minerals at the Ministry of Commerce and Industry of the Sultanate of Oman, with special thanks to Drs. A. Al Alrajhi, M. Al Batashi, and M. Alaraimi. Careful sample preparation by O. Dietrich and J. Feige as well as skilled assistance by U. Westernströer and K. Bremer is gratefully acknowledged. Funding for this research was provided by grants from the Deutsche Forschungsgemeinschaft (Project KO1723/15-1 and KO1723/16-1, 2). The manuscript has been substantially improved after thorough reviews by Andrew Kerr, Scott A. Whattam and an anonymous reviewer.

PY - 2017/2

Y1 - 2017/2

N2 - At fast-spreading mid-ocean ridges, axial melt lenses (AMLs) sandwiched between the sheeted dyke section and the uppermost gabbros are assumed to be the major magma source of crust formation. Here, we present our results from a field study based on a single outcrop of a frozen AML in the Samail ophiolite in the Sultanate of Oman which presents a whole suite of different lithologies and complex cutting relationships: varitextured gabbro with relics of primitive poikilitic clinopyroxene is intruded by massive quartz diorites and tonalites bearing relics of assimilated sheeted dykes, which in turn are cut by trondhjemite dykes. The whole is cut by basaltic dykes with chilled margins. The geochemical evolutionary trend of the varitextured gabbros, including some of the quartz diorites and tonalites, can be best modelled by fractional crystallisation of an experimental MORB parental melt composition containing 0.4 to 0.8 wt.% H2O. Patchy varitextured gabbros containing domains of primitive poikilitic clinopyroxene and evolved granular networks represent the record of in situ crystallisation. Some quartz diorites, often with xenoliths of sheeted dykes and exceptionally high Al2O3 contents, show a bulk trace element pattern more in accord with melts generated by experimental partial melting of dyke material. Highly evolved, crosscutting trondhjemite dykes show characteristic trace element patterns implying a formation by partial melting of sheeted dykes under lower water activity which is indicated by relatively low Al2O3 contents. The late basaltic dykes with chilled margins crosscutting all other lithologies show a relatively depleted geochemical character with pronounced negative Nb-Ta anomalies implying a genetic relationship to the second phase of magmatic Oman paleo-ridge activity (V2). The field relationships in combination with the petrological/geochemical trends reveal multiple sequences of MORB-type magma cooling (resulting in fractional crystallisation) and re-heating (producing partial melting) during the formation of this special horizon; these are best explained by alternating cycles of vertical AML migration. Since the investigated outcrop shows many characteristic lithological and petrographic features that are well-known from the uppermost gabbros drilled at Site 1256 by the Integrated Ocean Drilling Program (IODP) in the equatorial Eastern Pacific, our results based on 3-D observation in the field help to elucidate the geological observations obtained from the 1-D drill core.

AB - At fast-spreading mid-ocean ridges, axial melt lenses (AMLs) sandwiched between the sheeted dyke section and the uppermost gabbros are assumed to be the major magma source of crust formation. Here, we present our results from a field study based on a single outcrop of a frozen AML in the Samail ophiolite in the Sultanate of Oman which presents a whole suite of different lithologies and complex cutting relationships: varitextured gabbro with relics of primitive poikilitic clinopyroxene is intruded by massive quartz diorites and tonalites bearing relics of assimilated sheeted dykes, which in turn are cut by trondhjemite dykes. The whole is cut by basaltic dykes with chilled margins. The geochemical evolutionary trend of the varitextured gabbros, including some of the quartz diorites and tonalites, can be best modelled by fractional crystallisation of an experimental MORB parental melt composition containing 0.4 to 0.8 wt.% H2O. Patchy varitextured gabbros containing domains of primitive poikilitic clinopyroxene and evolved granular networks represent the record of in situ crystallisation. Some quartz diorites, often with xenoliths of sheeted dykes and exceptionally high Al2O3 contents, show a bulk trace element pattern more in accord with melts generated by experimental partial melting of dyke material. Highly evolved, crosscutting trondhjemite dykes show characteristic trace element patterns implying a formation by partial melting of sheeted dykes under lower water activity which is indicated by relatively low Al2O3 contents. The late basaltic dykes with chilled margins crosscutting all other lithologies show a relatively depleted geochemical character with pronounced negative Nb-Ta anomalies implying a genetic relationship to the second phase of magmatic Oman paleo-ridge activity (V2). The field relationships in combination with the petrological/geochemical trends reveal multiple sequences of MORB-type magma cooling (resulting in fractional crystallisation) and re-heating (producing partial melting) during the formation of this special horizon; these are best explained by alternating cycles of vertical AML migration. Since the investigated outcrop shows many characteristic lithological and petrographic features that are well-known from the uppermost gabbros drilled at Site 1256 by the Integrated Ocean Drilling Program (IODP) in the equatorial Eastern Pacific, our results based on 3-D observation in the field help to elucidate the geological observations obtained from the 1-D drill core.

KW - Axial melt lens

KW - Crustal accretion

KW - Fast-spreading mid-ocean ridges

KW - Oman ophiolite

KW - Wadi Gideah

UR - http://www.scopus.com/inward/record.url?scp=85006994286&partnerID=8YFLogxK

U2 - 10.1016/j.lithos.2016.11.022

DO - 10.1016/j.lithos.2016.11.022

M3 - Article

AN - SCOPUS:85006994286

VL - 272-273

SP - 31

EP - 45

JO - Lithos

JF - Lithos

SN - 0024-4937

ER -