Petrological Studies of Peridotites from Diapiric Serpentinite Seamounts in the Izu-Ogasawara-Mariana Forearc, Leg 125

Abstract

Refractory spinel peridotites were drilled during Leg 125 from two diapiric serpentinite seamounts: Conical Seamount in the Mariana forearc (Sites 778-780) and Torishima Forearc Seamount (Sites 783-784) in the Izu-Ogasawara forearc.Harzburgite is the predominant rock type in the recovered samples, with subordinate dunite; no lherzolite was found.The harzburgite is diopside-free to sparsely diopside-bearing, with modal percentages of diopside that range from 0% to 2%.Spinels in the harzburgites are chrome-rich (Cr/[Cr + Al] = 0.38-0.83;Fe 3+ /[Fe 3+ + Cr + Al] = 0.01-0.07).Olivine and orthopyroxene are magnesian (Mg# = 0.92).Discrete diopsides reveal extreme depletion of light rare earth elements.Primary hornblende is rare.The bulk major-element chemistry shows low average values of TiO 2 (trace), A1 2 O 3 (0.55%) and CaO (0.60%), but high Mg# (0.90).These rocks are more depleted than the abyssal peridotites from the mid-oceanic ridge.They are interpreted as residues of extensive partial melting (= 30%), of which the last episode was in the mantle wedge, probably associated with the generation of incipient island-arc magma, including boninite and/or arc-tholeiite.These depleted peridotites probably represent the residues of melting within mantle diapirs that developed within the mantle wedge.