Multiple episodes of melt-rock reaction at the slab-mantle interface: Formation of high silica primary magmas in intermediate to hot subduction zones.

Abstract:

The release of hydrous siliceous slab partial melts triggers complex physicochemical interactions in the sub-arc mantle that result in the formation of a wide variety of primary arc magmas. Here we investigate the phase equilibria of multiple episodes of rock-melt reactions occurring at the slab-mantle interface in intermediate to hot subduction zones. We perform a series of piston cylinder experiments at 3 GPa, and 800 - 950°C. Natural rhyolite (JR-1) and a synthetic KLB-1 composition (10 wt. % H₂O added) were used to represent the initial interaction between the hydrous slab-derived partial melt and the mantle wedge, respectively, explored at two rock-melt ratios (75-25 and 50-50). The reacted melt escapes and the solid residue interacts with a fresh slab partial melt (JR-1), producing a new reacted melt and a new residual phase, and so on.

The melts are dacitic to rhyolitic in composition (72-80 wt.%) showing a gradual increase of alkalis (1.5 to 7.5 wt.%) and high Mg# (40-87). The melts formed at 950°C and ratio 75-25 show highest MgO (1.4 wt.%), CaO (2.21 wt.%), FeO (1.6 wt.%), and Al₂O₃ (18.2 wt.%) values and low in SiO₂ (73 wt.%), K₂O (4 wt.%) and Na₂O (2 wt.%) compared with the melts at 800°C. At 950°C and ratio 50-50, the melts are enriched in MgO (3.8 wt.%) content and low in K₂O (1 wt.%) and Na₂O (0.5 wt.%) compared to the melts at 800°C. Pyroxenes + mica ± amphibole ± garnet/spinel are formed at 950°C and at both rock-melt ratios. At 800°C, mica + clinopyroxene + coesite ± amphiboles are favored by the new partial melt influx while orthopyroxene proportions decrease significantly.

The experimental melts are depleted in MgO, CaO, and FeO compared to the 12656 natural arc lavas, implying a required minimum interaction with hotter mantle sources. The major elements (SiO₂, TiO₂, Al₂O₃, MgO, CaO and FeO) and the Mg# show similarities with the Mexican Miocene trondhjemites [1], suggesting the contemporary contribution of high silica primary magmas to the new-crust genesis. The steady formation of mica, amphiboles, Ca-Na-rich pyroxenes, and coesite decrease the bulk residue density from ~3360 to 2910Kg/m³ and favor the formation of buoyant diapirs, a plausible mechanism to transfer major and trace elements from the slab to the hotter mantle wedge.

[1] Gómez-Tuena et al., 2008.