Experimental investigation of fluid transfer in sub-arc mantle conditions

Cassian Pirard and Jörg Hermann

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Fluid transport from the subducted slab to the locus of partial melting in the mantle wedge in volcanic arcs is a process which is still strongly debated. Two end member mechanisms are considered: 1) porous fluid transfer through the mantle or 2) focused fluid flow in dykes/channels. These two processes are very different and the composition of reacted fluids arriving at the locus of partial melting in the mantle wedge must have different trace element signatures depending on which process is involved.

The main goal of this experimental study is to constrain the change in composition of the fluid as a result of these two ways of fluid transport. Experiments were performed on natural San Carlos olivine representing a simplified mantle and various pre-synthesized, trace element doped, hydrous felsic glass identified as slab-extracted melts (Fig.1)(Hermann & Spandler, 2008). Synthesis piston cylinder experiments were carried out in gold capsules for a week in the range 700°-1100° C and 35kbar which represent average values for the extraction of slab-fluids into the mantle (Fig.2).

Porous fluid transport was simulated by mixing a 1 to 4 ratio of fine grained hydrous felsic glass with fine grained olivine. One end of the capsule was filled with carbon spheres in order to collect the reacted quenched fluid at the end of the run. These mixed charges show an olivine-orthopyroxene-biotite±garnet± amphibole assemblage in equilibrium with a fluid (Fig. 1a). Fluid traps collected in K₂O-rich experiments (amphibole barren) were analyzed with laser ablation ICP-MS. Fluid composition was calculated using Ce as internal standard and normalized on the initial felsic glass. It appears that the crystallization of phlogopite has a strong impact on the composition of the fluid. The K₂O/H₂O ratio is considerably diminished (Fig. 2) and the LILE have a strong affinity to follow potassium in phlogopite whereas LREE, MREE and HFSE tend to be enriched into the fluids. In the case of the H₂O-rich experiments, the presence of amphibole and biotite modify the system. Fluids are less abundant and most of the initial starting material is retained in a hydrous peridotitic mix.

Focused fluid was simulated by a layered experiment of hydrous felsic glass overlying coarse olivine grains. A carbon spheres fluid trap was placed over the olivine layer. Significant differences are observed in this type of experiment compared to the mixed experiments. A reaction zone consisting of an orthopyroxenite layer ±garnet only occurs at the interface between olivine and the felsic glass and neither phlogopite nor amphibole has been observed (Fig.1b). In consequence, the glass composition is very similar to the starting composition and the shielding provided by the garnet-orthopyroxenite reduced strongly interactions with olivine, keeping the K₂O/H₂O high. LILE remain high in the quenched glass and REE and HFSE are less affected with respect to the initial starting glass.

These two types of experiments show that there are strong differences in transport behaviour of LILE in the mantle wedge dependent on the fluid flow mechanism. The high K₂O/H₂O and LILE contents observed in arc lavas suggest that fluid transfer in sub-arc conditions can occurs by channelled flow. In case of porous flow, fluids are strongly affected by the crystallization of biotite and LILE are retained in the residue (Fig. 2). However, the melting of such hydrous peridotite residues containing both micas and amphiboles could potentially lead to the formation of arc lavas as well.

 

 

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Hermann J, Spandler C (2008) Sediment melts at sub-arc depths: an experimental study. Journal of Petrology 49:717-740