Petrology of mantle-derived ultramafics from the Owen fracture zone, northwest Indian ocean: Implications for the nature of the oceanic upper mantle

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%, sp 2%) together with subordinate harzburgite 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 (Fo 89.0–90.4), enstatite (En 89.5–91.0; Al 2O 3; 3.0–6.5%) and diopside (Wo 43–46En 51–50Fs 6–4; Al 2O 3: 4.4–7.0%; Cr 2O 3: 0.74–1.68%) have uniform composition irrespective of modal proportions and coexist with Al-rich spinel. Recrystallized pyroxenes exhibit lower Al 2O 3 and lower Di-En miscibility than porphyroclasts, consistent with partial subsolidus re-equilibration to lower TandP. Plagioclase (An 91–96), 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. On the assumption that these ultramafics have been subjected to processes operative at accretionary plate boundaries (convective mantle upwelling, partial melting, removal of basaltic melt, incorporation into the sub-Moho lithosphere), and that they are mineralogically and chemically representative of the upper mantle in the northwest Indian Ocean, the following conclusions may be drawn: (1) The upper mantle in this region consists principally of depleted lherzolite with pockets of harzburgite and dunite. (2) Unlike in ophiolites, lherzolite, rather than harzburgite, is the residue of partial melting beneath the Mid-Indian Ridge and forms the substratum to oceanic crust. This is in agreement with the eutectic-like behavior of primitive abyssal tholeiites at elevated pressures (7.5–10.5 kbar). (3) Based on the preservation and interpretation of relic pyroxine-spinel clusters, extensive melting of ascending mantle material must take place following garnet breakdown at about 100 km beneath the ridge. (4) The restriction of plagioclase to subsolidus assemblages is indicative of magma segregation at a depth greater than 20 km, the high- P stability limit of plagioclase.