Fluid Channel under the Subduction Zone

Arc lava has high water content, LILE & LREE enrichment compared to oceanic crust formed at mid-ocean ridges (Pearce et al., 2005) and has similar chemical composition to the average continental crust. Earlier studies (Ellam and Hawkesworth, 1988) have demonstrated that the arc lava’s properties are originated from subducted oceanic crust and its sedimentary covers. These signatures (slab signatures) transfer to the magma chamber by fluid-mediated mass transfer related to the dehydration of slab, at depths of 80-150km (Pirard and Hermann, 2015). But how fluids transfer to the arc lava’s magma chamber with little of interaction with surrounding peridotite remain unrevealed.

In this paper, authors test two ways of fluid transfer - porous flow and focused flow. In porous flow, fluid goes through the grain boundary, has higher probability of assimilation with mantle peridotite. In Focused flow, fluid goes through the channel such as vein, fracture, etc. Because channel is wider than grain boundaries, so there is fewer probability of the assimilation between fluid and wall rock. Both cases are tested on typical slab compositions & peridotite composition.

By doing this, we can see the mineral assemblage at reaction zone in each cases. In mixed experiment, slab composition are mixed with peridotite composition while they are separately layered in layered experiments before heat the capsule.

Figure 1. setup of experiment, Olivine(green), Opx(dark blue), Garnet(red), Mica(brown), Hydrous melt(light blue), Cpx(yellow)

After heating the capsules, only mixed experiment generates mica or amphibolite (hydrous mineral), the consequence of interaction between olivine (green in figure) and slab component. In layered experiment, however, fluid doesn’t generate hydrous minerals, remains as a hydrous melt. And also there is a rim composed of orthopyroxene, which limits further interactions between fluid (light blue part in figure) and olivine. Because of hydrous minerals presence, porous flow’s trace element patterns show depleted LILE, enriched HFSE, which is opposite to arc lava’s trace element pattern – LILEs are more enriched than HFSE. In addition, Pirard and Hermann find that concentration of Ce, the proxy for LREE more enriched and K, the proxy for LILE depleted during experiments, which is opposite to arc lava’s signatures.

So, this paper concludes that because nonexistence of hydrous mineral is necessary to explain the arc lava’s trace element signature, in respect of trace element pattern and hydrous mineral effect, focused flow is more suitable process to explain the arc lava generation occurs at depth of 100km, typical depth for the position of the subducted slab below volcanic arcs (Tatsumi, 1986).

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