H₂O-rich mantle melting near the slab–wedge interface

To investigate the first melts of the mantle wedge in subduction zones and their relationship to primitive magmas erupted at arcs, the compositions of low degree melts of hydrous garnet lherzolite have been experimentally determined at 3.2 GPa over the temperature range of 925–1150 ^°C. Two starting compositions with variable H₂O contents were studied; a subduction-enriched peridotite containing 0.61% Na₂O, 0.16 K₂O% (wt%) with 4.2 wt% H₂O added (Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015) and an undepleted mantle peridotite (Hart and Zindler in Chem Geol 57:247–267, 1986) with 14.5% H₂O added (Till et al. in Contrib Mineral Petrol 163:669–688, 2012). Saturating phases include olivine, orthopyroxene, clinopyroxene, garnet and rutile. Melting extent is tracked from near solidus (~ 5 wt%) to 25 wt%, which is close to or beyond the point where clinopyroxene and garnet are exhausted. The beginning of melting is a peritectic reaction where 0.54 orthopyroxene + 0.17 clinopyroxene + 0.13 garnet react to produce 1.0 liquid + 0.88 olivine. The melt production rate near the solidus is 0.1 wt% °C⁻¹ and increases to 0.3 wt% °C⁻¹ over the experimentally studied interval. These values are significantly lower than that observed for anhydrous lherzolite (~ 1 wt% °C⁻¹). When melting through this reaction is calculated for a metasomatized lherzolite source, the rare earth element characteristics of the melt are similar to melts of an eclogite, as well as those observed in many subduction zone magmas. Moreover, since rutile is stable up to ~ 8 wt% melting, the first melts of a hydrous lherzolite source could also show strong high field strength element depletions as is observed in many subduction zone lavas. The silicate melts measured at the lowest temperatures and melting extents (< 10 wt%) are high silica andesites (56–60 wt% SiO₂) and contain very low Ca/Al and high alkalis. These deep low degree andesitic melts, if added to experimentally produced hydrous liquids from melting (20–25 wt%) of harzburgite residues at shallow pressures (1.0–1.2 GPa, Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015), can match the compositional characteristics of primitive natural basaltic andesite and magnesian andesite lavas found globally in arcs. In addition to a silicate melt phase, there is a small amount of silicate dissolved in the H₂O supercritical fluid that coexists with the silicate liquid and solids in our experiments. The composition of this fluid is in equilibrium with the Mg-rich minerals and it is granitic. The results presented here are used to develop a model for producing hydrous arc magmas. We hypothesize that mantle wedge melting produced by the flux of hydrous fluid from the slab occurs over a range of depths that begins at the base of the mantle wedge and ends at shallow mantle depths. These melts ascend and remain isolated until they mix in the shallow, hottest part of the mantle wedge. In this melting scenario, the metasomatic “slab melt” contributions to arc magmas is small (~ 5 wt%), but its effect on the alkali, REE and incompatible trace element budget of the derivative magmas is large and able to reproduce the trace elemental characteristics of the primitive andesites. Higher proportions of slab or sediment melt do not resemble primitive high magnesian arc andesites and basaltic andesites.