ESTIMATING THE SOURCE MANTLE COMPOSITION OF A MANTLE-DERIVED BASALT

When modeling petrogenesis of a mantle-derived basalt, an often-encountered difficulty is that the source mantle composition is unknown. Either a bulk Earth composition or a fixed depleted mantle composition is assumed based on the rare earth element pattern of the basalt (McKenzie and O’Nions, 1991). However, the source mantle composition may not match either the bulk Earth or the depleted MORB mantle. I propose a model to estimate source mantle composition using isotopic ratios in a mantle-derived basalt and a two-stage mantle evolution model. That is, based on the isotopic composition of a basalt, my model would allow estimation of the source mantle composition, which can then be used to discuss petrogenesis of the basalt.

My model assumes that a given radiogenic isotope ratio in a zero-age basalt can be modeled in a two-stage mantle evolution model, with the first stage being the primitive mantle, and the second stage being either a depleted or an enriched mantle relative to the primitive mantle. In the context of this model, there are two unknowns to derive one isotopic ratio (e.g., ¹⁴³Nd/¹⁴⁴Nd): the parent/daughter ratio in the second-stage mantle and the age of depletion or enrichment. When two isotopic ratios are used (e.g., ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr), one unknown is the age of mantle depletion or enrichment, and the other two unknowns (¹⁴⁷Sm/¹⁴⁴Nd and ⁸⁷Rb/⁸⁶Sr) may be related to the compatibility index (CoI) defined by Zhang (2014), meaning there is also only one unknown in the parent to daughter ratios. Hence, from two isotopic ratios, both the mantle depletion/enrichment age and the trace element pattern in the mantle may be solved. If three or more isotopic ratios are used, then the system is mathematically overdetermined, and nonlinear least squares may be used to find the depletion/enrichment age and the trace element pattern in the mantle. The model has been applied successfully to a number of basalts. However, for some basalts, the isotopic ratios do not seem to allow a simple two-stage evolution model. These results and complexities will be discussed.