Identification of growth mechanisms in metamorphic garnet by high‑resolution trace element mapping with LA‑ICP‑TOFMS

Rubatto, Daniela; Burger, Marcel; Lanari, Pierre; Hattendorf, Bodo; Schwarz, Gunnar; Neff, Christoph; Keresztes Schmidt, Peter; Hermann, Jörg; Vho, Alice; Günther, Detlef (2020). Identification of growth mechanisms in metamorphic garnet by high‑resolution trace element mapping with LA‑ICP‑TOFMS. Contributions to mineralogy and petrology, 175(7) Springer-Verlag 10.1007/s00410-020-01700-5

Preview	Text (Text, figures and supplementary material) Rubatto_CMP_2020_pre-print.pdf - Accepted Version Available under License Publisher holds Copyright. Download (265MB) | Preview
Preview	Text Rubatto2020_Article_IdentificationOfGrowthMechanis.pdf - Published Version Available under License Publisher holds Copyright. Download (5MB) | Preview

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 μm and limits of detection for the heavy rare earth elements (REE) down to 0.2 μg/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800–850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core–mantle boundary occurred over a length scale of ~ 200 μm. Garnet in an eclogite from the Sesia Zone, Western Alps (P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P–T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications.

Interest & Impact

Downloads

165 since deposited on 04 Aug 2020
52 in the past 12 months

Citations

5 Citations in Web of Science ®
6 Citations in Scopus

Search

in Google Scholar™

Services

Actions (login required)

Edit item

Actions (login required)

Edit item