Geochemistry of peridotites from ODP Leg 125 diapiric serpentinites

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; Fe3+/[Fe3+ + 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 TiO2 (trace), Al2O3 (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.

难熔尖晶石橄榄岩（refractory spinel peridotites）于第125航次从两座底辟蛇纹岩海山钻获：分别为马里亚纳弧前区的锥形海山（站位778至780），以及伊豆-小笠原弧前区的鸟岛弧前海山（站位783至784）。所获样品以方辉橄榄岩（harzburgite）为主要岩石类型，伴生少量纯橄榄岩（dunite），未发现二辉橄榄岩（lherzolite）。
该类方辉橄榄岩不含透辉石（diopside）或仅含微量透辉石，透辉石的模式百分含量介于0%至2%之间。方辉橄榄岩中的尖晶石属富铬组分，其Cr/(Cr+Al)比值为0.38~0.83，Fe³⁺/(Fe³⁺+Cr+Al)比值为0.01~0.07。橄榄石与斜方辉石均为富镁矿物，镁指数（Mg#）为0.92。孤立产出的透辉石表现出轻稀土元素（light rare earth elements）的极端亏损特征。原生角闪石极为稀少。全岩主量元素化学组成显示，TiO₂（痕量）、Al₂O₃（0.55%）与CaO（0.60%）的平均含量较低，而镁指数则高达0.90。
这类岩石的亏损程度高于洋中脊深海橄榄岩。研究认为其为大规模部分熔融（熔融程度达30%）后的残余物质，其中最后一期熔融事件发生于地幔楔中，可能与初始岛弧岩浆（包括玻安岩及/或弧型拉斑玄武岩）的形成过程相关。上述亏损型橄榄岩大概率代表了地幔楔内形成的地幔底辟体中熔融作用的残余产物。