FOREARC PERIDOTITES FROM NORTH MARIANA: UNDERSTANDING MANTLE MELTING

The Izu-Bonin-Mariana arc (IBM), off the coast of Japan, represents the largest intra-oceanic subduction zone where the Pacific plate is being subducted below the Philippine Sea plate. The inner trench wall of the forearc has preserved the entire igneous sequence formed as the Pacific plate started plunging into the mantle. To understand the reason behind subduction initiation and its influence on the extent of mantle melting in the arc, it is important to study the forearc region, especially the peridotites.

Forearc Peridotites were collected on cruise KH-14-05 of the Japanese Research Vessel, Hakuhomaru, by dredging from the landward slope of the trench wall in northern Mariana area off the coast of Japan. The samples were obtained from a depth ranging from 5229m to 7590m below sea level. Harzburgites and dunites were recovered by dredging, although the majority of the samples have been altered to serpentine due to high water influx. The altered samples retain their original texture, and have the relict olivine and orthopyroxene grains preserved as well.

Petrographic study of these samples shows that olivine grains are altered and have given way to serpentine. Large, broken grains of enstatite have been found within the serpentine mesh, with modal percentages approaching 50–55% in several of the samples. In addition to serpentinization, chlorite replacing enstatite is observed. Clinopyroxene is rare, and at places, associated with amphibole rim. The harzburgite spinel analysis yields chrome numbers that vary from 0.60–0.88, with the less altered samples having higher values. The TiO₂ content of the spinels varies from 0.02–0.07 and wt% of Al oxide ranges from 5–15% in the respective spinels analyzed.

The initial petrographic and major element geochemical study indicates that the peridotites are highly refractory and have experienced a high degree of partial melting in the subduction regime. The variation in chrome number, along with varied Al content of the samples, results from variable degrees of partial melting in northern Mariana area. Further data, such as major and trace element analyses, are required to understand the origin of these peridotites as well as melting in the mantle wedge of the IBM arc.