Tracking the prograde P–T path of Precambrian eclogite using Ti-in-quartz and Zr-in-rutile geothermobarometry

Abstract

A Fe-Ti-rich garnet, clinopyroxene, and quartz eclogite sample from the 1.0 Ga Sveconorwegian orogen, SW Sweden, contains abundant quartz, rutile, and zircon in distinct micro-textural sites: garnet core, garnet rim, and matrix, constituting an ideal case for investigation of the behavior of Zr-in-rutile and Ti-in-quartz at high-pressure and temperature. A P-T path, peaking at 16.5-19 kbar and 850-900 °C, has been constrained independently for the same rock by pseudosection modelling; input pressures from this model were used for trace element geothermometry of each garnet micro-textural domain. Trace element thermo(baro)metry, based on in situ Secondary Ion Mass Spectrometry analyses of Ti contents in quartz and Zr contents in rutile, yields P-T estimates of progressive crystallization of quartz and rutile along the prograde metamorphic path. For inclusions in the garnet cores, Zr-in-rutile geothermometry yields 700-715 °C and Ti-in-quartz 620-640 °C at 7 kbar. For inclusions in the garnet rims, temperature estimates are 760-790 °C (Zr-in-rutile) and 740-920 °C (Ti-in-quartz) at 12-18 kbar. Finally, matrix rutile records 775-800 °C and locally ~ 900 °C, and quartz records temperatures up to 900 °C at 18 kbar. Ti-in-quartz estimates for the metamorphic peak (inclusions in the garnet rims and matrix) conform to the pseudosection, but appear too low for the early prograde stage (garnet cores), possibly due to lack of equilibrium at T < 700 °C. The pseudosection shows that rutile was produced by continuous ilmenite breakdown during the early stages of prograde metamorphism, a reaction that was completed at ~ 730 °C. Rutile grains in the garnet rims and the matrix grew subsequently larger by recrystallization of previously produced rutile. However, recrystallized rutile does not predominantly record peak temperatures, but instead yield 745-840 °C between 12 and 18 kbar. In the pseudosection, this temperature range broadly coincides with a stage during which (Ti-bearing) hornblende was consumed and clinopyroxene produced (i.e., dehydration); the Zr contents thus appear to reflect the last stage of efficient rutile recrystallization, catalysed by fluids released by the dehydration of hornblende preceding the metamorphic peak. Concurrently, combination of the isopleths for Ti content in quartz and Zr content in rutile (i.e. independent from pseudosection modelling) yields pressure and temperature conditions in almost perfect agreement with the P-T path as deduced from the pseudosection. The variation in Ti concentration in quartz is small regardless of crystal size, and the Ti-in-quartz geothermometer provides both precise and accurate peak temperatures of 875-920 °C, without a significant diffusional reequilibration. The lack of significant Ti diffusion in quartz is consistent with an inferred short residence time at high temperature. This study illustrates that Zr-in-rutile and Ti-in-quartz geothermobarometry can robustly constrain prograde P-T conditions and yield further insights into recrystallization proc...

Thermal Structure of the Paleo‐Continental Subduction Zone: Insights From Quantitatively Constrained Prograde P–T Paths of Exhumed LT/UHP Eclogites in the Dabie Orogen

“…For the first case, despite not abundant, small grained rutile has been found in the cores of garnet porphyroblasts from different eclogites, and a few of them are intergrowth with or near calcic amphibole grains (Figure S2 in Supporting Information ), suggesting they have high potential to achieve chemical equilibrium during growth of calcic amphibole and temperature calculated by Ti‐in‐calcic amphibole thermometer is reliable. For the second case, previous studies have proven that the Zr content of rutile inclusions in garnet could survive from later heating and that a Zr‐in‐rutile thermometer based on rutile inclusions in garnet grains could robustly reflect the nucleation temperature of garnet even if the peak condition reaches ultrahigh temperature (e.g., Schantl et al., 2019; Tual et al., 2018). This suggests that the temperature recorded by rutile inclusion is robust.…”

Multi‐episodic formation of baddeleyite and zircon in polymetamorphic anorthosite and rutile‐bearing ilmenitite from the Chiapas Massif Complex, Mexico

“…The Zr partitioning and diffusion in rutile have been experimentally constrained (Cherniak et al, 2007; Ferry & Watson, 2007; Tomkins et al, 2007; Zack, Moraes, et al, 2004); however, controversy still exists regarding Zr diffusion rates (Blackburn et al, 2012; Cherniak et al, 2007; Ewing et al, 2013; Kooijman et al, 2012; Luvizotto & Zack, 2009; Meyer et al, 2011; Pape et al, 2016; Taylor‐Jones & Powell, 2015; Zack, Moraes, et al, 2004). Cherniak et al (2007) have shown that rutile is only moderately retentive for Zr, whereas geological studies indicate preservation of Zr‐in rutile cores even at UHT (Ewing et al, 2013; Kooijman et al, 2012; Pape et al, 2016; Tual et al, 2018).…”

Estimation of P-T Entrapment Conditions of a Subduction Fluid Using Elastic Thermobarometry: A Case Study from Cabo Ortegal Complex, Spain

“…Inclusion entrapment P-T conditions are additionally constrained by the crossing point of quartz-in-garnet isomekes and Ti-in-quartz isoplets, which serve as an independent thermometer to validate zircon-in-garnet elastic thermometry. The application of Ti-in-quartz geothermometer is a common method used to obtain P-T estimates of crystallization as quartz occurs in most crustal rocks over a wide range of P-T conditions (e.g., Thomas et al, 2010;Tual et al, 2018). Recently, it has been successfully applied not only on matrix quartz, but on inclusions trapped in garnet (Gonzalez et al, 2019).…”