In situ Lu–Hf geochronology of calcite

Abstract

Abstract. The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and 0.6 % obtained from calcite with Lu concentrations as low as ca. 0.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary precipitation ages over a significant amount of geological time, although further research is required to constrain the closure temperature. The in situ approach allows calcite to be rapidly dated while maintaining its petrogenetic context with mineralisation and other associated mineral processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to resolve the timing of complex mineral paragenetic sequences that are a feature of many ancient rock systems.

A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway

“…Recently obtained Lu-Hf age data from the sample used in this study indicates the garnets in the Fjørtoft sample, WGC2019J-31A, formed during the Mesoproterozoic (Simpson et al, 2021;Tamblyn et al, 2021). This is consistent with the presence of Late Mesoproterozoic-aged monazite inclusions inside garnet (Figure 8a; Terry et al, 2000b;Holder et al, 2015).…”

“…Based on the frequent observation of Mesoproterozoic-aged monazite and zircon inclusions inside garnet from Fjortoft both in our study (Figure 10a) and in others (e.g. Cuthbert & van Roermund, 2011;Liu & Massone, 2019;Terry et al, 2000a;Walczak et al, 2019), and Mesoproterozoic Lu-Hf dates from garnet (Simpson et al, 2021;Tamblyn et al, 2021), we confidently state that garnet growth in our sample from Fjørtoft predates the Caledonian Orogeny. To account for this, garnet was excluded from the Fjørtoft bulk composition used in P-T modelling.…”

“…Recent advances in Lu-Hf dating allowed for garnets from the same sample as the Fjørtoft metapelite used in this study (WGC2019J-31A) to be dated in situ. Mesoproterozoic garnet ages (Simpson et al, 2021;Tamblyn et al, 2021) indicate the Fjørtoft metapelite sample used in this study had a Mesoproterozoic-aged metamorphic protolith. These findings are corroborated by U-Pb results from monazite and zircon, which record unambiguous evidence of a Mesoproterozoic event (e.g.…”

“…Relicts of this event within the WGC are rare but tend to be more prevalent within allochthons that were thrust over the Baltican basement during the early stages of the Caledonian Orogeny (Jolivet et al, 2005). Garnet Sm-Nd and Lu-Hf dating has shown that garnet in some high and ultrahigh-pressure locations formed during the late Mesoproterozoic and early Neoproterozoic, ascribing a polymetamorphic character to some high-P assemblages (Root et al, 2005;Simpson et al, 2021;Tamblyn et al, 2021). Low H 2 O activity, coarse grain size, and limited deformation have been attributed to the sporadic preservation of these granulite facies assemblages during the subsequent Caledonian Orogeny (Austrheim, 1987;Krabbendam et al, 2000).…”

“…The older and more abundant population occurs as inclusions within garnet rims and in the matrix, while the younger population occurs solely within the matrix. Despite c. 423 Ma monazites occurring within the Mesoproterozoic-aged garnet (Simpson et al, 2021;Tamblyn et al, 2021), these grains are connected to the matrix via fractures. This connection with the matrix may have facilitated monazite recrystallization in response to fracture-hosted fluid migration (e.g.…”