Role of dunite in genesis of primitive MORB

Abstract

Both Hess Deep and the dredging sites of MCT are not associated with transform faults, 10),11) and the transform effect on the rocks discussed here will be minimized. Abstract: Dunite and related rocks from the Moho transition zone of the East Pacific Rise (EPR) and the Mid-Cayman Trough (MCT) are examined to know the spreading-rate dependence of their compositions. The rocks from both ridges are basically a reaction product between peridotite and primitive MORB. Despite the large difference of prevalent peridotite chemistry, depleted harzburgite for EPR and lherzolite for MCT, the dunite has less different compositional ranges. The Al 2 O 3 content of spinel differs only by 5 wt%, which may be equivalent to the difference of about 1 wt% of Al 2 O 3 in the melt. This is almost equal to the possible spreading-rate dependent compositional difference of observed Mg-rich MORB. The relatively unfractionated MORBs erupted are in equilibrium with the oceanic dunite of which composition is independent of or only slightly dependent on spreading rate. Dunite is calculated to be about half of the total oceanic mafic crust by weight. The dunite is concentrated as the Moho transition zone, and is sparsely distributed as small lenses within the uppermost mantle peridotite. Petrology and origin of the dunite-troctolite-

Chemical characteristics of chromian spinel in plutonic rocks: Implications for deep magma processes and discrimination of tectonic setting

“…Kelemen et al . 1995; Arai 2005). Plagioclase in the harzburgites–lherzolites is calcic, and is a melt‐impregnation product (e.g.…”

“…These rocks are expected to be in equilibrium with MORB in terms of mineral chemistry (e.g. Kelemen et al 1995;Arai 2005). Plagioclase in the harzburgites-lherzolites is calcic, and is a meltimpregnation product (e.g.…”

Mantle wehrlite from Hess Deep as a crystal cumulate from an ultra‐depleted primary melt in East Pacific Rise

“…Ordinary MORB evolve from primary incremental melts by mixing [ Johnson et al , 1990; Koga et al , 2001] and/or through interaction with peridotite [ Arai and Matsukage , 1996; Dick and Natland , 1996], during ascent from the partial melting zone. The ordinary MORB can be in equilibrium with dunite‐troctolite [ Kelemen et al , 1995; Arai , 2005], interpreted as a melt/harzburgite reaction at the shallowest mantle [ Arai and Matsukage , 1996; Dick and Natland , 1996]. We interpret that the wehrlite was precipitated at higher pressures [ Herzberg , 2004] from an ultra‐depleted MORB almost in ‐ situ segregated from residual harzburgite around the depth of final release of partial melt beneath EPR.…”

Petrologic profile of peridotite layers under a possible Moho in the northern Oman ophiolite: an example from Wadi Fizh

“…This contrast is possibly due to the difference in the order of crystallization of minerals in the melt involved in or resulting from the reaction; olivine was followed by plagioclase in Hess Deep, but by clinopyroxene in Wadi Fizh. This indicates that the melt involved in the formation of Moho transition zone rocks in the Oman ophiolite (Wadi Fizh) is different in chemistry from the ordinary MORB involved in the formation of a plutonic rock suite in Hess Deep (e.g., Dick and Natland, 1996;Arai and Matsukage, 1996;Arai, 2005). MORB is suggested to be involved in the formation of wehrlites and related rocks in the Moho transition zone of the southern Oman ophiolite (Koga et al, 2001).…”