RARE-EARTH ELEMENT INVERSION FOR MELT DISTRIBUTION - SENSITIVITY AND APPLICATION

Inversion of rare earth element (REE) abundances in MgO-rich (MgO grea ter-than-or-equal-to 6 wt.%) igneous rocks, described initially by McK enzie & O'Nions (1991), provides a useful means of constraining the di stribution of melt fraction with depth, X(z), in the mantle. Here, we investigate the sensitivity of the REE inversion procedure to noise in measured REE abundances and to variation in parameters which must be specified a priori. Inverted melt distributions are insensitive to rea listic amounts of random noise but are sensitive to systematic noise. Melt distributions are also sensitive to input parameters such as dept h and range of melting, source composition, and mantle mineralogy. To determine unique melt distributions, additional independent constraint s on at least two of the following input parameters are therefore requ ired: depth and range of melting (i.e., lithospheric thickness); sourc e composition; source mineralogy (dependent on mantle temperature). Th e successful replication of melt compositions where these three parame ters are well constrained and the agreement between inverted melt dist ributions and melt distributions calculated independently assuming ise ntropic melting during adiabatic upwelling indicate that the scheme is reliable. REE inversion therefore provides a means for both routinely constraining X(z) and for determining unknown input parameters.