Possible Effects of Spreading Rate on Morb Isotopic and Rare Earth Composition Arising from Melting of a Heterogeneous Source

Plots of Sr and Nd isotopic ratios, and La/Sm ratios of MORB from the Pacific and Atlantic oceans show a decrease in variability with increasing spreading rate, a decrease in the average value of the Sr isotopic ratio and La/Sm ratio, and an increase in the average Nd isotopic ratio. Pb isotopic ratios do not exhibit a similar correlation. In modeling this data we assume a mantle source for MORB that is heterogeneous in concentration, isotope ratios, and melting on a scale much smaller than the region from which melts are drawn. Viewed on a larger scale the mantle source would appear homogeneous by averaging over a fixed distribution of heterogeneous material. The Sr isotopic composition of melt derived from such a source is calculated using a simple sub-ridge melting and segregation model. If increasingly incomplete extraction of sub-ridge melt correlates with an increase in spreading rate, this model generates Sr isotopic variations similar to those seen in the MORB data set. This illustrates that melt extraction processes may well significantly modify mantle isotopic signatures, which has important implications for deciding whether isotope and REE ratios seen to vary in MORB should be attributed to process (i.e., melt extraction) or to the mantle source.