The link between oxygen isotope resetting, partial melting, and fluid flow in metamorphic terrains

Abstract

A correlation between the style of partial melting and synmeta‐morphic fluid flow exists in metapelites from the Mount Lofty Ranges, Reynolds Range, and Omeo Zone (Australia). Mount Lofty Ranges migmatites comprise granitic leucosomes in rocks that are still biotite rich, with no indications of other mafic minerals being formed along with the melts. By contrast, in the Reynolds and Omeo migmatites, garnet, cordierite, and/or spinel formed along with the melts. Oxygen isotope data are most consistent with the Mount Lofty Ranges undergoing significant fluid–rock interaction during regional metamorphism, which may have fluxed fluid‐present partial melting. By contrast regional metamorphic fluid flow in the Reynolds Range and Omeo Zone was limited, leading to partial melting via fluid‐absent reactions. The style of melting reactions may help to constrain the timing of isotopic resetting and fluid flow in metamorphic terrains, which is currently a contentious issue.

Mineral equilibrium constraints on the feasibility of diffusive H2O‐fluxed melting in the continental crust

“…The modelling results reproduce key features of diffusive H 2 O‐fluxed melting as reported in low‐ P metapelitic terranes, including the presence of cordierite and nominally anhydrous peritectic phases like garnet, the persistence of hydrous phases like biotite as well as the preferential consumption of K‐feldspar, quartz and plagioclase during melting (Cartwright & Buick, 1998; Cartwright et al., 1995; Ellis & Obata, 1992; Genier et al., 2008; Kisters et al., 2009; Ward et al., 2008; Wickham, 1987; Wickham & Taylor, 1985; Yardley & Barber, 1991). The maximum amount of melt estimated by Ward et al.…”

“…These examples involve instances where (a) the volume of melt observed is larger than that expected from dehydration melting; (b) melting occurred at relatively low temperatures and did not consume hydrous minerals or (c) melting occurred in relatively anhydrous lithologies. Rock types where melting through the addition of H 2 O have been described include composite grey gneiss complexes (Collins et al., 1989, 2016; Escuder Viruete, 1999; Finch, Hasalová, Weinberg, & Fanning, 2014; Jung et al., 2009; Jung, Hoernes, & Mezger, 2000; Lee & Cho, 2013; Mogk, 1992; Nédélec, Minyem, & Barbey, 1993; Pognante, 1992; Sawyer, 1998, 2010; White, Pomroy, & Powell, 2005; Wu et al., 2018; Yakymchuk, Zhao, Wan, Lin, & Longstaffe, 2019), metagreywackes (Brown, 1979; Fornelli, Piccareta, Del Moro, & Acquafredda, 2002; Genier, Bussy, Epard, & Baumgartner, 2008; Holness & Clemens, 1999; Milord, Sawyer, & Brown, 2001; Schwindinger & Weinberg, 2017) and low‐ P aluminous metapelites (Butler, Harris, & Whittington, 1997; Cartwright & Buick, 1998; Cartwright et al., 1995; Ellis & Obata, 1992; Kisters, Ward, Anthonissen, & Vietze, 2009; Ward, Stevens, & Kisters, 2008; Wickham, 1987; Wickham & Taylor, 1985; Yardley & Barber, 1991).…”

Charge contrast imaging of geological materials in the environmental scanning electron microscope

“…Fluid flow plays a major role in many crustal processes including large-scale mass and heat transfer during metamorphism (Ague 1994;Gerdes and Valley 1990;Hoisch 1991), isotopic resetting (Cartwright and Buick 1998;Graham et al 1998), the location of ore deposits (Cathles 1981), and the creation or destruction of porosity by fluid flow in sandstone reservoirs (Marret and Laubach 1997). In our examples, microstructures such as fractures and kink bands are clearly discernible in CCI.…”