Trace element mapping by LA-ICP-MS: assessing geochemical mobility in garnet

Abstract

in garnet, which are argued to be largely dependent on the interplay between element fractionation, mineral reactions and partitioning, and the length scales of intergranular transport. Samples from the Peaked Hill shear zone, Reynolds Range, central Australia, exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events. Trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing U-Pb monazite and Sm-Nd garnet geochronology. Ultimately, the remarkable preservation of original growth zoning and its subtle modification by subsequent re-equilibration is used to 'see through' multiple superimposed events, thereby revealing a previously obscure petrological and temporal record of metamorphism, metasomatism, and deformation.

Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist, New Zealand

“…(), thus they are not compatible with diffusive re‐uptake. Such deviations from the bell‐shaped REE distributions representative of Raleigh fractionation are common (Baxter, Caddick, & Dragovic, ; Hollister, ) and likely result from diffusion‐limited REE uptake during growth, variable garnet growth grates, or growth or breakdown of a REE‐bearing phase(s) during garnet growth (Moore, Carlson, & Hesse, ; Raimondo et al., ; Skora et al., ).…”

“…If occurring significantly later than garnet growth, such process may cause younging of the Lu-Hf age record (Kelly et al, 2011). Discontinuous, <10 lm Mn-rich garnet rims in samples WC14-38, WC14-42, WC14-46, and zone 1 garnet rims in WC14-14 indicate that minor garnet resorption and diffusive element reuptake may indeed have occurred (Figure 4) Raimondo et al, 2017;Skora et al, 2006). Garnet in WC14-14 is compositionally distinctly different from garnet in other samples.…”

Complete metamorphic cycle and long‐lived anatexis in the c. 2.1 Ga Mistinibi Complex, Canada

“…Note a relative enrichment in LREE in the inner rim compared with other parts of the grain that could be due to the breakdown of monazite in the system. Such a Y+HREE zoning is commonly observed in garnet from metapelitic or felsic rocks and is typically considered to reflect Rayleigh distillation during growth (Hollister, ; Raimondo et al, ). Garnet in Sample 2049A shows strong zoning, with a garnet core exhibiting high Lu concentrations and low [Gd/Yb] N values and a large rim that is depleted in Y+HREE.…”

“…Data reduction was processed with the Iolite freeware (Paton, Hellstrom, Paul, Woodhead, & Hergt, 2011) using mean EPMA 29 Si data as an internal standard. Quantitative maps were generated using the XMapTool software for data processing (Lanari et al, 2014;Raimondo et al, 2017). REE pixel values were extracted from maps with the Areas selective tool.…”

“…A narrow outer rim is marked by a slight increase in Lu. Chondrite-normalized spectra extracted from the map show heterogeneous REE behaviours from core to rim (Figure 14f), suggestive of matrix-controlled equilibration (Raimondo et al, 2017). The HREE transition from a positive to a negative slope rimwards is associated with a decrease of three orders of magnitude in LREE.…”