LinT, a simplified approach to oxygen-isotope thermometry and speedometry of high-grade rocks: An example from ultrahigh-temperature gneisses of southern Madagascar

Abstract

Accurate and precise temperatures of metamorphism and magmatism can be obtained from O-isotope compositions of minerals using a system of linear equations that defines fractionation and mass balance of O isotopes among all minerals in a rock. This approach-linearequation thermometry (LinT)-does not require that cooling rates or absolute O-diffusion parameters be known. If O diffusivities are known, closure temperatures for each phase can be calculated and inverted to determine a temperature-dependent cooling rate that is accurate to a factor of three. The highest and most precise temperatures will be preserved in rocks containing quartz and garnet, because O diffuses slowly in garnet and garnet-quartz O-isotope fractionation is relatively large. The technique will be most useful in rocks with low-variance mineral assemblages in which precise pseudosection analysis is inhibited by wide spacing of reactions in pressure-temperature space. Application of LinT to ultrahigh-temperature gneisses from Madagascar yields temperatures of 927 ± 31 °C, consistent with independent thermometry.

An extensive record of orogenesis recorded in a Madagascar granulite

“…525 Ma (Holder, Hacker, et al, 2018; Horton et al, 2016; Martelat et al, 2000; Tucker et al, 2014). The southern Anosyen domain reached peak conditions of 900–1000°C at 6–10 kbar (Boger et al, 2012; Holder & Hacker, 2019; Holder, Hacker, et al, 2018; Holder, Sharp, & Hacker, 2018; Horton et al, 2016; Jöns & Schenk, 2011). Peak temperatures were slightly lower to the west (Markl et al, 2000; Martelat et al, 1997) and north (Boger et al, 2012; Horton et al, 2016; Jöns & Schenk, 2011; Raith et al, 2008; although, see Martelat et al, 2012) and were lower still (≤800°C) in the Ikalamavony fold‐thrust belt to the north–northeast (Ganne et al, 2014; Grégoire et al, 2009; Holder & Hacker, 2019).…”