Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D; fossilization of a dynamic melt lens at fast spreading ridges

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doi: 10.1029/2009GC002652
Author(s): France, Lydéric; Ildefonse, Benoit; Koepke, Juergen
Author Affiliation(s): Primary:
Université Montpellier II, Géosciences Montpellier, Montpellier, France
Other:
Leibniz Universität Hannover, Federal Republic of Germany
Volume Title: Geochemistry, Geophysics, Geosystems - G<sup>3</sup>
Source: Geochemistry, Geophysics, Geosystems - G>3`, 10(10). Publisher: American Geophysical Union and The Geochemical Society, United States. ISSN: 1525-2027
Note: In English. 98 refs.; illus., incl. 2 tables, sketch map
Summary: The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed as continuous outcrops. Our observations, which include the base of the sheeted dike being crosscut by gabbros, are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens. Dike assimilation is a possible mechanism for incorporating hydrated phases, which result from hydrothermal alteration, to the melt lens during upward migrations of its upper boundary. Upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths. This granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity. Clinopyroxenes present in these granoblastic lithologies are typically low in Ti and Al content, thus strongly contrasting with corresponding magmatic clinopyroxene. This may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex. Downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof, which represent the partly fossilized melt lens. Melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler. Melt lens migration, recrystallization of hydrothermally altered sheeted dikes during reheating stages, and assimilation processes observed in the Oman ophiolite are consistent with the observations made in IODP Hole 1256D. We propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption/replenishment stages or variations in the hydrothermal/magmatic fluxes.
Year of Publication: 2009
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; Al Ahmadi Hills; Arabian Peninsula; Asia; Dikes; East Pacific; Equatorial Pacific; Eruptions; Foliation; Fossilization; Gabbros; Geologic thermometry; Hydrothermal conditions; Igneous rocks; Integrated Ocean Drilling Program; Intrusions; Leg 206; Magma chambers; Magmas; Melting; Melts; Mid-ocean ridges; Mineral composition; North Pacific; Northeast Pacific; ODP Site 1256; Ocean Drilling Program; Ocean floors; Oman; Ophiolite; Pacific Ocean; Plutonic rocks; Sheeted dikes; Wadi Gideah
Coordinates: N223000 N233500 E0590000 E0580000
N064400 N064400 W0915600 W0915600
Record ID: 2010047363
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