The fall and rise of metamorphic zircon

Abstract

Zircon geochronology and geochemistry are increasingly important for understanding metamorphic processes, particularly at extreme conditions, but drivers of zircon dissolution and regrowth are poorly understood. Here, we model Zr mass balance to identify P-T regions where zircon should dissolve or grow. Zirconium contents of major metamorphic minerals were assessed from published data and new measurements, and models were constructed of mineralogical development and zircon abundance for hydrous MORB and metapelitic compositions along representative P-T paths. Excluding zircon, the minerals rutile, garnet, and hornblende strongly influence Zr mass balance in metabasites, accounting for as much as 40% of the whole-rock Zr budget. Clinopyroxene and garnet contain more Zr than plagioclase, so breakdown of plagioclase at the amphibolite to eclogite facies transition, should cause zircon to dissolve slightly, rather than grow. Growth of UHP zircon is predicted over a restricted region, and most zircon grows subsequently at much lower pressure. In metapelites, zircon is predicted to undergo only minor changes to modal abundance in solid state assemblages. Partial melting, however, drives massive zircon dissolution, whereas melt crystallization regrows zircon. From a mass-balance perspective, zircon growth cannot be attributed a priori to the prograde amphibolite-eclogite transition, to UHP metamorphism, or to partial melting. Instead, zircon should grow mainly during late-stage exhumation and cooling, particularly during oxide transitions from rutile to ilmenite and melt crystallization. As predicted, most zircons from HP/UHP eclogites of the Western Gneiss Region and Papua New Guinea substantially postdate eclogite formation and maximum pressures.

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

“…Rubatto et al 2006;Kelsey et al 2008;Kelsey and Powell 2011;Yakymchuk and Brown 2014;Kohn et al 2015). Therefore, to understand the evolution of zircon growth along the continuous metamorphic field gradient in Val Strona di Omegna and avoid misinterpretations due to compositional differences, we collected nine aluminous metapelitic samples (Figs.…”

“…While it is stable over a wide range of metamorphic conditions, modelling predicts growth mostly in restricted parts of a P-T path (e.g. Yakymchuk and Brown 2014;Kohn et al 2015). For new metamorphic zircon to form in hightemperature (HT) metamorphic rocks, either the presence of a hydrous fluid or melt phase is required (e.g.…”

“…Recent studies have improved our understanding of how to interpret metamorphic zircon in the context of the P-T evolution of rocks by modelling zircon growth during metamorphism (e.g. Kelsey et al 2008;Kelsey and Powell 2011;Yakymchuk and Brown 2014;Kohn et al 2015; for a recent review of zircon petrochronology, see Rubatto 2017). A number of studies have found that zircon chemical and isotopic signatures can be altered by various mechanisms: Pb can be locally lost (e.g.…”

Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides

“…Metamorphic precipitation of zircon could be a result of the breakdown of and/or exsolution from various Zr-bearing phases (Davidson and van Breemen, 1988) such as garnet, amphibole, (clino)pyroxene and ilmenite (e.g., Fraser et al, 1997;Degeling et al, 2001;Möller et al, 2003;Söderlund et al, 2004;Harley et al, 2007;Kelsey et al, 2008;Morisset and Scoates, 2008), hemo-ilmenite (Morisset et al, 2005), baddeleyite (Bingen et al, 2001;Söderlund et al, 2004), rutile (Harley et al, 2007;Tomkins et al, 2007;Morisset and Scoates, 2008;Kelsey and Powell, 2011;Ewing et al, 2013Ewing et al, , 2014Pape et al, 2016), epidote, titanite (Kohn et al, 2015), chlorite (Fraser et al, 2004), and biotite (Vavra et al, 1996). Zircon coronae have been reported around Martian baddeleyite as a result of shock metamorphism (Moser et al, 2013).…”

“…Zirconium is a common component of rutile, in which its content can reach 10 000 ppm (e.g., Ewing et al, 2013); thus, Zr distributions generally reflect the formation and decomposition of rutile. The temperature dependence of Zr solubility in rutile can have a fundamental impact on the zircon growth rate (Kohn et al, 2015) and controls zircon stability (Kelsey and Powell, 2011). The zirconium-in-rutile thermometer for the rutile-quartz-zircon system was calibrated by a number of authors (e.g., Watson et al, 2006;Ferry and Watson, 2007;Tomkins et al, 2007;Lucassen et al, 2010;Ewing et al, 2013), who have shown a large temperature-and pressuredependent solubility of Zr in rutile.…”