Stability of sapphirine + quartz in the oxidized rocks of the Wilson Lake terrane, Labrador: calculated equilibria in NCKFMASHTO

Abstract

The equilibrium coexistence of sapphirine + quartz is inferred to record temperatures in excess of 980°C, based on the stability of this assemblage in the simplified chemical system FeO-MgO-Al 2 O 3 -SiO 2 (FMAS) system. However, the potential for sapphirine to contain significant Fe 3+ suggests that the stability of sapphirine + quartz could extend to lower temperatures than those constrained in this ideal system. The Wilson Lake terrane in the Grenville Province of central Labrador preserves sapphirine + quartz-bearing assemblages in highly oxidized bulk compositions, and provides an opportunity to explore the stability of sapphirine + quartz in such rock compositions within the Naallows the effect of the stability of the additional phases, biotite, K-feldspar and melt, on the stability of sapphirine + quartz to be assessed. These phase relations are evaluated generally using P-T projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using P-T pseudosections, and the appropriate H 2 O and O contents to use in the modelled compositions are investigated using T-M H2O and T-M O pseudosections. The peak P-T estimates from a sapphirine + quartz-bearing sample are 960 to 935°C at 10 to 8.6 kbar, similar to estimates from orthopyroxene + sillimanite + quartz ± garnet-bearing samples. Whereas the sapphirine + quartzbearing sample is more Fe-rich than the orthopyroxene + sillimanite-bearing sample on an all-Feas-FeO basis, once the oxidation state is taken into account, the former is effectively more magnesian than the latter, accounting for the sapphirine occurrence.

Taking the temperature of Earth's hottest crust

“…The effect of H 2 O on the equilibria was evaluated using T /P -M H 2 O diagrams, as outlined in Korhonen et al (2011Korhonen et al ( , 2012, and the appropriate O (for Fe 3+ ) content was evaluated using T -M O diagrams (Diener and Powell, 2010;Korhonen et al, 2012). The H 2 O content was adjusted so that the final phase assemblage is stable just above the solidus, assumed to reflect the conditions at which this assemblage was in equilibrium with the last remaining melt Diener et al, 2008).…”

“…Examples where these newer methods have been applied to UHT granulites include the Napier complex (Harley and Motoyoshi, 2000;Harley, 2008;Hokada et al, 2008) and the Prydz Bay region (Kelsey et al, 2003b(Kelsey et al, , 2007 in East Antarctica, the Anapolis-Itauçu complex in the Brasiliá Belt of Brazil (Baldwin et al, 2005(Baldwin et al, , 2007Baldwin and Brown, 2008), the Southern Granulite Terrane in India (Clark et al, 2009), the Ivrea zone (Luvizotto and Zack, 2009;Redler et al, 2012;Ewing et al, 2013) and the Gruf complex (Galli et al, 2011) in the Alps, the Lewisian complex of northwest Scotland Zirkler et al, 2012), the Eastern Ghats Province in India (Korhonen et al, 2011(Korhonen et al, , 2013a, the Gawler Craton of southern Australia (Cutts et al, 2011(Cutts et al, , 2013, the Archean Pikwitonei Granulite Domain in Canada (Kooijman et al, 2012), the Wilson Lake terrane in Labrador, Canada (Korhonen et al, 2012), and the Musgrave Province in central Australia (Walsh et al, 2014). Most of these studies used either accessory mineral thermometry without any support from phase equilibria modeling or a pseudosection approach commonly in conjunction with Al-in-orthopyroxene thermometry but without confirmation from accessory mineral thermometry.…”

Challenges in constraining theP–Tconditions of mafic granulites: An example from the northern Trans‐North China Orogen

“…Other reasons for mismatches may include, but may not be limited to, the following: (1) uncertainties associated with the determination of modes (Van der Plas & Tobi, ); (2) uncertainties associated with the determination of the effective composition of the whole rock or equilibration volume (Palin, Weller, Waters, & Dyck, ; Stüwe, ), including appropriate O and H 2 O contents (Diener & Powell, ; Korhonen, Powell, & Stout, ; Korhonen, Saw, Clark, Brown, & Bhattacharya, ); (3) uncertainties associated with mineral analyses (Hodges & McKenna, ; Kohn & Spear, ) and the calculation of compositional parameters (including Fe 3+ when not determined separately; Pattison et al., ; Schumacher, ) from stoichiometric mineral formulae (Powell & Holland, ; Table S1); (4) assumptions about mineral growth and recrystallization, in particular that mineral compositions are able to maintain equilibrium with evolving P – T conditions (this is likely to be a problem where elements have different diffusion rates both within and between minerals (Fitzsimons & Harley, ; Frost & Chacko,; Kroll, Evangelakakis, & Voll, ; Pattison & Bégin, ; Pattison et al., ; Spear & Florence, ), and may be a particular problem in minerals with coupled substitutions, such as Na–Si v . Ca–Al in plagioclase); and (5) for phase equilibrium modelling, uncertainties associated with the thermodynamic data and the a–x models that lead to uncertainties on phase‐in/phase‐out boundaries, mole proportions and compositions of phases (Green, White, & Powell, ; Green et al., ; Palin et al., ; Powell & Holland, ).…”

Blueschist‐facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high‐pressure assemblages

“…However, similar to metabasic rocks, the assemblages are sensitive to the amount of available H 2 O and Fe 2 O 3 (cf. Korhonen et al. , 2012).…”