Compositional variation in the chevkinite group: new data from igneous and metamorphic rocks

Abstract

Electron microprobe analyses are presented of chevkinite-group minerals from Canada, USA, Guatemala, Norway, Scotland, Italy and India. The host rocks are metacarbonates, alkaline and subalkaline granitoids, quartz-bearing pegmatites, carbonatite and an inferred K-rich tuff. The analyses extend slightly the range of compositions in the chevkinite group, e.g. the most MgO-rich phases yet recorded, and we report two further examples where La is the dominant cation in the A site. Patchily- zoned crystals from Virginia and Guatemala contain both perrierite and chevkinite compositions. The new and published analyses are used to review compositional variation in minerals of the perrierite subgroup, which can form in a wide range of host rock compositions and over a substantial pressure- temperature range. The dominant substitutions in the various cation sites and a generalized substitution scheme are described.

238 U– 230 Th dating of chevkinite in high-silica rhyolites from La Primavera and Yellowstone calderas

“…Differentiation of metaluminous silicic magma and/or increasing peralkalinity typically results in crystallization of chevkinite with higher La, Ce, Mn, Fe, and Nb concentrations, and lower Al, Ti, Mg, Ca, and MREE concentrations (Macdonald et al, , 2009 Primavera and Yellowstone chevkinite follow this general trend and are consistent with increasing magnitude of differentiation and/or peralkalinity of their host rocks. West Yellowstone chevkinite has lower average Th/U, Mg/Fe, Fe and Th, and higher average Mn, Zr, and Nb with decreasing age (Fig.…”

“…Concentrations of light rare earth elements (LREEs) typically range between 9-25 wt.% (Ce, La) 2 O 3 (Macdonald et al, , 2009, giving chevkinite some of the highest observed crystal/melt partition coefficients for LREE Troll et al, 2003). Heavy rare earth element concentrations in chevkinite are typically 1-3 orders of magnitude lower than LREE .…”

“…% ThO 2 with Th/U ratios up to~60 (Macdonald et al, , 2009 where A = rare earth element (REE), Ca, Th, U; B = Fe 2+ ; C = Ti, Al, Zr, Nb, Mg, Mn, Fe 3+ , Fe 2+ and D = Ti (Macdonald and Belkin, 2002), with the most abundant REE being La, Ce, Pr, Nd, and Sm. In chevkinite, charge balancing for REE incorporation follows: McDowell, 1979;Green and Pearson, 1988) and:…”

Hydrothermal alteration of a chevkinite-group mineral to a bastnäsite-(Ce)-ilmenite- columbite-(Fe) assemblage: interaction with a F-, CO2-rich fluid

“…2b, are presented in Table 1, and supplemented in Supplementary Table 1a by five analyses from Macdonald et al (2012). The phase has been classified as chevkinite-(Ce) using the empirical discriminant of Macdonald et al (2009) (Fig. 7) Compositions of the altered chevkinite (Zone B) are given in Table 1 and Supplementary Table 1a.…”

“…Fig. 7 Empirical discrimination plot of chevkinite and perrierite (Macdonald et al 2009). The unaltered chevkinite-(Ce) plots in the chevkinite field, whereas the altered chevkinite spreads into the perrierite field.…”