Towards the definition of a petrological Low Velocity Zone (LVZ)

Abstract

International audienceCarbon and others volatiles that are present in the Earth's mantle at ppm concentrationlevels, induce partial melting. CO2-H2O-rich melts are stable under the P-T-fO2conditions of the Low Velocity Zone (LVZ). Recent experimental studies about theEarth mantle conductivity have shown the primordial importance of small amounts ofhydrated CO2-rich melts in the geophysical signature of the LVZ. Nevertheless, thechemical composition of these melts is difficult to capture as it depends on T-P andredox state.Using Margules formalisms, we established a multi-component model describing theGibbs free energy of melt produced by mantle melting in presence of CO2-H2O that arecarbonatite-carbonated melt-nephilinite-basanite and basalt with increasing degree ofpartial melting. This parameterization is calibrated on crystal-liquid, redox, fluid-liquidand liquid-liquid equilibria obtained by experimental studies in the P-T range 1-10 GPaand 900-1800°C.We propose a calculation of the composition of melts produced in the oceanic LVZ as afunction of age. At about 80 km depth, we show that the composition of the melts is >30wt% SiO2 for ages <20 Ma, and comes closer to the carbonatitic terms for olderlithosphere. Besides lateral chemical variations, our model calculates the meltcomposition along an oceanic ridge adiabat, predicting an abrupt compositionaltransition between a H2O-rich carbonatitic melt and a carbonated silicate melt, between140 km and 160 km. With the distance to the ridge, this transition is shifted to lowerdepths between 70 and 90 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 thedepth. The chemical variations between carbonated and silicated melts may be relatedto the geophysical observations