Element mobility during regional metamorphism in crustal and subduction zone environments with a focus on the rare earth elements (REE)

This paper explores bulk-rock geochemical data for a wide array of metamorphosed mafic, quartzo-feldspathic, pelitic, and metacarbonate rocks using a quantitative mass-balance approach to assess fluid-driven element mobility—particularly of the rare earth elements (REE)—in regional metamorphic and some high-pressure subduction zone environments (40 examples; over 240 individual analyses). Most examples are from focused fluid flow settings, such as veins and lithologic contacts, where fluxes are large and metasomatic signals are thus strong. A variety of REE behaviors are observed, including little or no REE mobility (roughly a third of the data set); light REE (LREE), mid-REE (MREE), and/or heavy (HREE) mobility; europium "anomalies"; overall REE losses; and local REE redistribution. The REE are typically fractionated by mass transfer, with the exception of several examples that underwent fairly uniform overall losses of REE. The fractionation reflects strong mineralogical controls on REE uptake/loss by a comparatively small number of phases. Examples include: HREE mass changes associated with garnet, xenotime, and sphene; LREE and MREE changes associated with apatite, monazite, and allanite; and Eu changes associated with plagioclase and lawsonite. As mineralogy is a dominant control, the nature of the metasomatism is not strongly correlated with metamorphic grade, other than obvious mineralogical differences between settings (e.g., plagioclase in Barrovian metamorphism, lawsonite in subduction zones). Extensive mobilization of non-REE major and trace elements can happen without significant open-system transport of the REE. If REE mobility occurs, it is always accompanied by mobilization of other non-REE. When mobile, neighboring REE (e.g., Sm and Nd) typically have strongly correlated mass changes indicating that both were mobilized to about the same degree. Although individual examples of metasomatism can show correlations between patterns of mass transfer for the REE and the non-REE, little such correlation is evident across the entire data set, with the exception of P. Once again, this highlights the importance of individual minerals in controlling REE systematics. Broad correlations of REE and P mobility suggest REE transport by P complexes, or REE and P transport together by some other complexing agent. Mass changes for REE and Y are more strongly coupled, reflecting the geochemical similarity of these elements and perhaps indicating a role for Y complexing as well.