FluidNET Abstract MSG - Cordierite's role in trace element mobilisation during partial melting and retrogression
Creators: name: Elisa E. Oliveira da Costa
name: Clare J. Warren
name: Barbara E. Kunz
name: Leo M. Kriegsman
name: Tom W. Argles
Publisher: name: Zenodo
Year: 2023

description: Cordierite’s role in trace element mobilisation during partial melting and retrogression

da Costa, E.O.1*, Warren C.J.1, Kunz B.E.1, Kriegsman L.M.2,3 and Argles T.W.11 School of Environment, Earth and Ecosystem Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom2 Department of Research & Education, Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden, Netherlands3 Department of Earth Sciences, University of Utrecht, Heidelberglaan 8, 3584 CS, Utrecht, Netherlands* Contact author: elisa.oliveira-da-costa@open.ac.uk___________________________________________________________________________Several processes are invoked to explain the genesis of granitoids enriched in critical metals such as Li, Be and Cs: (i) protolith enrichment, (ii) fractional crystallisation, (iii) metamorphic reactions, (iv) partial melting, and (v) assimilation. The extent to which each one of these processes control the enrichment of critical elements is still unclear; and specifically, during metamorphism and partial melting, little is known about the capacity of the main metamorphic minerals and reactions to sequester or release critical metals to the melt. To constrain how metamorphic and partial melting reactions influence the transfer of elements from metamorphic hosts to the melt, we studied the trace element concentrations of cordierite, which is a common sub-solidus and peritectic phase known to host Be2+ and Li+ in its tetrahedral and octahedral sites, respectively [1], and Cs+ in its channel structure [2].

We collected in-situ point analysis trace element data and maps with LA-ICP-MS in cordierites from high-grade metapelitic samples from different localities within the Variscan Orogen, Europe. Our data confirm that cordierite crystals commonly contain significant Li, Be and Cs, as well as P, Zn, Ga, Co and Ni. The comparison of trace element concentrations with temperatures (650 to 850 °C – estimated with the Na in cordierite thermometer [3]) show that Li, Be, Cs, Zn, P and Ga concentrations vary with temperature. The observed decrease of Li (550 to 100 ppm) and increase of Cs (2 to 4 ppm) with increasing temperatures from 700 to 850 °C agree with trends observed in experiments (84 to 14 ppm and 2 to 4 ppm, respectively – [4]). However, experimental concentrations of Be (105 to 22 ppm from 700 to 850 °C) strongly contrast with our results (0 to 35 ppm from 650 to 850 °C). Constraining the high concentration and temperature dependency of Be and Li in cordierite is essential to understand how reactions producing melt and peritectic cordierite may generate enriched melts.

In all our samples, cordierite shows varying degrees of alteration. Major element data acquired with SEM and EPMA show that their alteration usually involves the incorporation of water and decrease of all other major elements. LA-ICP-MS data reveal that Li, Be and Cs concentrations decrease sharply from fresh cores to pinitized rims of cordierites. These results provide preliminary evidence that fluid flux through cordierite-rich lithologies may be an important process for (re-)mobilising some of the critical metals in the continental crust.

References:[1] Bertholdi C et al. (2004) Lithos 78(4):389-409 [2] Yakubovich OV et al. (2004) Cryst. Rep. 49(6):953-963 [3] Tropper, P et al. (2018) Min. and Petr., 112(2):199-217[4] Evensen JM and London D (2002) Geoch. et Cosmoch. Acta 66(12):2239-2265