Stochastic melting of the marble cake mantle: evidence from local study of the East Pacific Rise at 12°50′N

Abstract

Isotopes (Nd, Sr and Pb) and trace elements (REE, Ba, Sr, Rb) have been measured on a set of basaltic glasses from a restricted area (40 × 10 km) at 12°50′N on the East Pacific Rise. The huge variation of incompatible element concentrations (factor 70 for Ba concentrations), and the variable degrees of correlation between element concentrations cannot be explained by usual models of melting and fractional crystallization. A rough correlation between the Ce/Yb ratio and the isotopic ratios favors a “source effect” for the genesis of the glasses. We have developed a model including both partial melting process acting on a heterogeneous mantle source with two components (peridotites and pyroxenites; “marble cake mantle” of Allègre and Turcotte) and fractional crystallization. The purpose of this model is not to obtain values of the four parameters involved (degree of melting in the peridotites, in the pyroxenites, proportion of pyroxenites involved in the melting, degree of fractional crystallization) for each analyzed glass, but to model the whole set of glasses by stochastic genesis and sampling of liquids. We have used the stochastic procedure for the four controlled parameters, currently generating 10, 000 “samples”. Our preferred model for this portion of the East Pacific Ridge is obtained with a degree of melting in the peridotites and in the pyroxenites varying uniformly from 6 to 20%, and from 6 to 50% respectively. The degree of mixing between liquids issued from the two sources varies from 0 to 100%, and the degree of fractional crystallization remains small, without noticeable effect on the concentrations, varying from 0 to 6%.