Experimental Hydrous Partial Melting of Natural Pristine and Altered MORB Beneath Subduction Zones

Experimental determination of the hydrous phase relations and trace element partitioning behaviour of Mid-Ocean Ridge Basalt (MORB) constrains the conditions for melting of subducted oceanic crust. This study utilises natural pristine MORB (from the Kolbeinsey ridge, north of Iceland) and altered MORB from the altered oceanic crust (AOC, from the DSDP leg 46, Mid Atlantic, ~20°N), hydrated with about 15wt% water, in experiments using piston-cylinder apparatus to simulate pressures and temperatures equivalent to about 100 km depth (3.0 GPa, 800-1000°C). Our motivation of (re-)studying the melting behaviour of undoped subducted basalt is two-fold. First, previous studies that focused on the trace element behaviour 'doped' their starting materials with trace elements to facilitate analysis and positive identification of accessory phases. Only by applying the 'doping' method has it been found that allanite may exert a key control on the light rare earth element (approx. La-Sm) budget in subducted basalt. However, it is still a matter of debate whether the ubiquitous presence of allanite in these experimental studies is solely due to doping of La-Sm. In addition, bulk fluid-solid partition coefficients so obtained may not be uncritically applied to model the trace element transfer in nature because of the enhanced proportions of accessory phases in the experimental solid residue. Second, there appears to be a clear difference in the melting behaviour of K-free and K-bearing MORB (and sediment), even if K2O only appears in minor concentrations. These differences are difficult to quantify, as the various studies were carried out at differing P-T-XH2O conditions. In agreement with earlier studies, we find that the vapour-saturated solidus is shifted toward higher temperatures at 3 GPa in pristine MORB (800°C < Tsolidus < 850°C), because it is virtually K-free (approx. 0.03 wt% K2O), in contrast to altered MORB where melting starts at T < 800°C (approx. 0.26 wt% K2O). Textural evidence further suggests that the K-composition of the starting material likely contributes to the location of the elusive second critical endpoint. Textural evidence implies the presence of 2 immiscible fluids at 850°C in the pristine MORB sample (in agreement with the location of the second critical endpoint of Kessel et al. 2005, using K-free MORB). This is in contrast to AOC which appears to have had only a single fluid phase (in agreement with the conclusion of Klimm et al. 2008, simulating a synthetic AOC composition). Accessory phases, apart from rutile, were not yet positively identified in our run products. Instead, we will use trace element data combined with a mass balance approach and fractionation of key trace element ratios (e.g. U/Th; La/Th, etc.) to evaluate whether accessory phases are present or not. Our study suggests that subducted MORB may behave heterogeneously during partial melting in subduction zone environments, owing to significant chemical variations in the K2O content of pristine versus altered oceanic crust. Geochemical analysis of the resulting mineralogy and melt composition of this study will further assist in the understanding of element transfer from the subducted slab to the overriding mantle wedge.