Rare earth and high field strength element partitioning between iron-rich clinopyroxenes and felsic liquids

Abstract

Rare earth elements are commonly assumed to substitute only for Ca in clinopyroxene because of the similarity of ionic radii for REE 3+ and Ca 2+ in 8-fold coordination. The assumption is valid for Mg-rich clinopyroxenes for which observed mineral/melt partition coefficients are readily predicted by the lattice strain model for substitution onto a single site (e.g. Wood and Blundy, 1997). We show that natural Fe-rich pyroxenes in both silica-undersaturated and -oversaturated magmatic systems deviate from this behavior. Salites (Mg# 48 to 59) in phonolites from Tenerife, ferrohedenbergites (Mg# 14.2 to 16.2) from the rhyolitic Bandelier Tuff, and ferroaugites (Mg# 9.6 to 32) from the rhyolitic Rattlesnake Tuff have higher heavy REE contents than predicted by single-site substitution. The ionic radius of Fe 2+ in 6-fold coordination is substantially greater than that of Mg 2+ , hence we propose that, in Fe-rich clinopyroxenes, heavy REE are significantly partitioned between 8-fold Ca sites and 6-fold Fe-Mg sites such that Yb and Lu exist dominantly in 6-fold coordination. We also outline a REE-based method of identifying pyroxene-melt pairs in systems with multiple liquid and crystal populations, based upon the assumption that LREE and MREE reside exclusively in 8-fold coordination in pyroxene.Contrary to expectations, interpolation of mineral/melt partition coefficient data for heavy REE does not predict the behavior of Y. We speculate that mass fractionation effects play a role in mineral/melt lithophile trace element partitioning that is detectable among pairs of isovalent elements with near-identical radii, such as Y-Ho, Zr-Hf, and Nb-Ta.

Partitioning of rare earth and high field strength elements between titanite and phonolitic liquid

“…Tiepolo et al (2000), they and Blundy and Wood (2003) in fact propose that the ionic radius of Ta 5+ is 0.01 -0.02 Å smaller that that of Nb 5+ . Olin and Wolff (2010) however showed that Y 3+ cpx/melt partitioning deviates from that of the HREE to an extent that the LSM cannot explain unless the Shannon (1976) value of the ionic radius for Y 3+ is grossly in error. Fasnia titanite displays similar behavior to a greater degree (Figs.…”

“…As described above, magmatic processes have scrambled crystals and glasses in the compositionally variable Fasnia phonolite, precluding confident identification of mineralglass pairs based on visual criteria. The method used by Olin and Wolff (2010) and outlined below is independent of major element mineral compositions, assumes that the LSM is a good first-order predictor of trace element partition coefficients, and does not rely on textural relationships.…”

“…Eu is omitted from this exercise due its unconstrained oxidation state, Pr, Nd, Ho, Tm and Lu are omitted because there is no ionic radius data reported for them in 7-fold coordination (Shannon, 1976), and Y is omitted because it not does behave as REE of similar radius (Olin and Wolff, 2010). Equilibrium between the high-Zr titanites and the different glasses is evaluated by assuming that isovalent cations entering a single lattice site will always follow a parabolic distribution in log D i vs. r i , using equation (1) rewritten as:…”

“…A change in the size of one lattice site may in turn affect other sites in the same mineral. These effects are seen in minerals that exhibit major solid solutions; pyroxene is a good example (Wood and Blundy, 1997;Olin and Wolff, 2010). However most igneous titanite (and even titanites synthesized from compositions well outside the range of natural magmas, e.g.…”

“…In this paper, we present the results of a LA-ICPMS study of titanites and associated glasses from the phonolitic Fasnia Member of the Diego Hernández Formation, Tenerife, Canary Islands (Edgar et al, 2007). Following Olin and Wolff (2010), we employ a method of identifying equilibrium mineral-melt pairs from natural samples that is independent of textural context. We then present titanite/melt D values for Y, Zr, Nb, REE, Hf, Ta, Th and U and use the data to estimate 7-fold coordination radii for REE 3+ not reported by Shannon (1976), and to approximately predict titanite/melt partitioning behavior of Ra, Ac and Pa. We then assess the results, paying particular attention to behaviors (melt structure effects, fractionation of cations of identical charge and radius) that are not predicted by the lattice strain model of Brice (1975) and Blundy and Wood (1994).…”