Constraints in the MORB Melting Regime Based on the Lu-Hf and Sm-Nd Systematics

Abstract

Lu-Hf and Sm-Nd isotope systematics place the onset for MORB melting in the garnet field and are indicators for degree and depth of melting. In addition, recent results on U-Th partitioning indicates Th-disequilibria are also explained by placing the onset of melting in the garnet stability field. And, thirdly, Klein and Langmuir (1989) have shown that the normalized major element content of MORB also reveal information about the degree and the depth of melting. Especially Nas and Fes are important indicators whereby Nas is an indicator for the degree of melting and Fes is an indicator for the average depth of melting. We have taken the constrains derived f rom the 176Hf/177Hf, 143Nd/144Nd and 23~ systematics one step further and combined the constrains derived from the isotope systematics with those derived from major elements. The first set of four figures (Fig. 1.) shows the isotope data for two Atlantic Ridge segments with a clear 'local' trend. These two segments show positive correlations with both Fes and Nas and the isotope melting parameters, 8(Sm/~d) and 8(L,m0, indicating that the smaller degree melts have a larger portion of melt generated in the garnet stability field. This is consistent with previous explanations of incomplete mixing of the different malting columns in a triangular shaped melting regime. The high Nas, high 8(L.mO and high Fes melts represent a small degree of melting generated at relatively deep levels and this correspond to melts generated in of-axis melting columns. The low Nas, low Fes and low 8(L,/H0 are large degree of melts that were generated at relatively shallow level in he mantle which represents melts from an on-axis melting column. We also analyzed mid-ocean ridge basalts erupted at different ridge depth. Klei~ and Langmuir (1989) interpreted the 'global' trend and its correlation with ridge depth as follows: the deep ridge segments have high Nas and low Fes indicating a low degree of rdatively low pressure melting, while the shallow ridges represent a large degree of melting and a high average pressure during melting. The Mid-Cayman Rise and the American Antarctic Discordance are two of the de~pest parts of the ocean which both have high Nas and high Fes. However, both MCR and AAD