Petrologic characteristics of the upper mantle beneath island arcs

Abstract

The mantle wedge is an important locus for material recycling, magma generation, and fluid transportation from the slab, and should therefore be thoroughly examined to better understand these processes. Peridotite xenoliths transported to the surface by arc magmas, or by other magmas, may be representative of the upper part (lithosphere) of the mantle wedge. Fore-arc peridotites exposed on the seafloor also represent the uppermost part of the mantle wedge. We summarize their modal composition, mineral chemistry, equilibrium temperature, and redox state, and discuss the implications for mantle-wedge processes. The arc peridotites are thought to derive mainly from the spinel to plagioclase-peridotite stability fields. They are varied in character, depending on their history as well as the tectonic setting (e.g., fore-arc, volcanic front, and backarc) of their source regions. Some arc peridotites, especially those from the fore-arc to the volcanic front, are harzburgites and contain high-Mg olivine and high-Cr spinel, with high degrees of partial melting. They also show metasomatism, silica enrichment (i.e., formation of secondary orthopyroxene at the expense of olivine), and hydration (i.e., precipitation of Ca-amphiboles and/or phlogopites). The presence of tremolite, which is indicative of low temperatures and/or depleted (Al-poor) chemistry, is characteristic of sub-arc mantle peridotites. The equilibrium temperature is relatively low ( ) with the exception of the Noyamadake peridotites, SW Japan arc, which are characterized by high temperatures (). Some peridotites from the Western Pacific show high oxygen fugacities relative to abyssal peridotite (although a few peridotites show relatively low oxygen fugacities or contain secondary veins composed of highly reduced minerals such as metals and alloys). This indicates the importance of local reducing agents in the mantle wedge.

Abyssal Peridotite as a Component of Forearc Mantle: Inference from a New Mantle Xenolith Suite of Bankawa in the Southwest Japan Arc

“…The two-pyroxene thermometer [48,49] gave a very limited range of equilibrium temperature (993 to 1053 • C), which is included in the range of low-temperature peridotite xenoliths from the SW Japan arc [2,4,6] (Figure 12a).…”

“…is within that of abyssal peridotite or that of the peridotite xenoliths from the SW Japan arc [4] ( Figure 12b). [4,6]. Temperatures were calculated after Wells [48].…”

“…Ranges of equilibrium temperature and oxygen fugacity of the Bankawa lherzolite in comparison with peridotite xenoliths from the SW Japan arc. Histograms are from Arai and Ishimaru [4,6]. Temperatures were calculated after Wells [48].…”

“…One of the characteristics of the subarc mantle is heterogeneity both in chemistry and in lithology [1,2]. The number of localities of mantle xenoliths is very small from arc areas as compared with other settings [3,4], and this leads to a difficulty in obtaining a detailed picture of the lower crust and the upper mantle of arcs, which essentially have complicated tectonic histories [4][5][6]. Some supra-subduction zone ophiolites indicate a complicated evolution history of arcs.…”

“…Those peridotites recovered near the trench may be representative of a near-edge part of the mantle wedge. Both the edge and the volcanic front sides of the lithospheric mantle wedge have been considered to be mainly composed of depleted peridotites, i.e., harzburgites [6]. The other part of the lithospheric mantle wedge, however, has not been sampled as xenoliths, and its composition has been left unclear.…”