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Petrology of mantle-derived ultramafics from the Owen fracture zone, northwest Indian Ocean: implications for the nature of the oceanic upper mantle
Hamlyn, P.R.; Bonatti, E. (1980). Petrology of mantle-derived ultramafics from the Owen fracture zone, northwest Indian Ocean: implications for the nature of the oceanic upper mantle. Earth Planet. Sci. Lett. 48(1): 65-79
In: Earth and Planetary Science Letters. Elsevier: Amsterdam. ISSN 0012-821X, more
Peer reviewed article  

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Keywords
    Fracture zones; Geomorphology; Lithogenesis; Mineral composition; Petrology; Marine

Authors  Top 
  • Hamlyn, P.R.
  • Bonatti, E.

Abstract
    An anomalous topographic high located 40 km southwest of the intersection of the Owen fracture zone with the Mid-Indian (Carlsberg) Ridge exposes mantle-derived ultramafic rocks for a vertical thickness of over 2 km. The rocks consist of partially serpentinized spinel lherzolite (ol 64%, opx 25%, cpx 9%, wp 2%) together with subordinate hatzburgite and rare dunite. Their coarse (protogranular) to porphyroclastic textures and mineral chemistry reveal a complex equilibration history and a strong resemblance to spinel lherzolite inclusions of the Cr-diopside group. Porphyroclastic olivine, enstatite and diopside have uniform composition irrespective of modal proportions and coexist with Al-rich spinel. Recrystallized pyroxenes exhibit lower Al2O3 and lower Di-En miscibility than porphyroclasts, consistent with partial subsolidus re-equilibration to lower T and P. Plagioclase, associated with a Cr-rich spinel, is a rare product of this subsolidus recrystallization and is indicative of final but incomplete equilibration within the plagioclase periodotite stability field. Textural relations of Al-rich spinel suggest an origin by two independent reactions. In the first association spinel occurs as coarse pyroxene-spinel clusters which may have formed from the breakdown of garnet during mantle upwelling beneath the Mid-Indian Ridge. In the second association spinel comprises part of the fine-grained recrystallized assemblage and probably formed via recrystallization of Al-rich pyroxene to Al-poor pyroxene.

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