Petrological mapping of a Low Velocity Zone (LVZ) induced by CO2-H2O-bearing incipient melts
Abstract:The link between volatiles and mantle melting has so far been illuminated by experiments, revealing that ppm concentration levels of carbon and other volatiles in the Earth’s mantle induce partial melting. Pressure-temperature conditions of incipient melting for CO2-H2O-peridotite  match fairly well with the upper part of the LVZ, as the redox melting  with the lower part. Recent experimental studies about the Earth mantle conductivity have shown the importance of small amounts of hydrated CO2-rich melts in the geophysical signature of the LVZ . Although such melts are stable under the P-T-fO2 conditions of the LVZ [1-2, 4-6], the variability of these parameters complicates the definition of their chemical composition.
Using Margules’ formalisms, we established a multi-component model describing the Gibbs free energy of melt produced by mantle melting in presence of CO2-H2O, that are carbonatite-carbonated melt-nephilinite-basanite and basalt with increasing degree of partial melting. This parameterization is calibrated on crystal-liquid, redox, fluid-liquid and liquid-liquid equilibria obtained by experimental studies in the P-T range 1-10 GPa and 900-1800°C.
We propose a calculation of the composition of melts produced in the oceanic LVZ as a function of ages (temperature) and chemical heterogeneities (water, alkalis). At about 80 km depth, we show that the composition of the melts is > 30 wt% SiO2 for ages < 30 Ma, and comes closer to the carbonatitic terms for older lithosphere. Besides lateral chemical variations, our model calculates the melt composition along an oceanic ridge adiabat, predicting an abrupt compositional transition between a H2O-rich carbonatitic melt and a carbonated silicate melt, between 130 km and 100 km. We propose a chemical mapping of the melt composition (and of the degree of partial melting) as a function of the distance to the ridge and of the depth. Our model represents an innovating attempt to connect the chemical variations between carbonated and silicated melts with the geophysical observations.
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