U − Pb geochronology of epidote by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool for dating hydrothermal-vein formation

Abstract

Abstract. Epidote – here defined as minerals belonging to the epidote–clinozoisite solid solution – is a low-μ (μ=238U/204Pb) mineral occurring in a variety of geological environments and participating in many metamorphic reactions that is stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U−Pb geochronometer has been hindered by the commonly high contents of initial Pb, with isotopic compositions that cannot be assumed a priori. We present a U−Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.01–38 µg g−1), and U (2.6–530 µg g−1) contents and with μ values between 7 and 510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps), and from the Heyuan fault (Guangdong Province, China). The investigated epidote samples show appreciable fractions of initial Pb contents (f206=0.7–1.0) – i.e., relative to radiogenic Pb – that vary to different extents. A protocol has been developed for in situ U−Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with a magmatic allanite as the primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U/206Pb and 207Pb/206Pb ratios, respectively, with quadrupole and multicollector ICP-MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb/206Pb versus 238U/206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb/206Pb ratio. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U−Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7±3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.2±4.3 and 16.9±3.7 Ma) epidote veins from the Grimsel area formed during the Handegg deformation phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb/206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 100 m. Vein epidote from the Heyuan fault is Cretaceous in age ( 107.2±8.9 Ma) and formed during the early movements of the fault. In situ U−Pb analyses of epidote returned reliable ages of otherwise undatable epidote–quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U−Pb dating of epidote, and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study

“…6b) performed U-Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of epidote grains in layer 1, obtaining a Tera-Wasserburg age of 19.2 ± 4.3 Ma. The Tera-Wasserburg regression revealed a single epidote generation at the current analytical precision (i.e., mean square of weighted deviates, MSWD, of 0.79), and the age is interpreted as the timing of epidote crystallization upon vein opening (Peverelli et al, 2021). The focus of this paper is the deformation mechanisms in layers 2-3.…”

“…Hydrothermal activity upon exhumation in Alpine times is recorded in the area primarily by the Grimsel Breccia Fault hydrothermal system (e.g., Hofmann et al, 2004;Belgrano et al, 2016;Diamond et al, 2018;Egli et al, 2018). U-Pb geochronology of hydrothermal epidote in veins returned Miocene ages (19.2 ± 4.3 Ma and 16.9 ± 3.7 Ma; Peverelli et al, 2021) related to fluid circulation occurring during the Alpine orogenic phases, already described also on the basis of cleft mineralization (e.g., Mullis et al, 1994;Janots et al, 2012;Berger et al, 2013Berger et al, , 2022Rossi and Rolland, 2014;Bergemann et al, 2017b;Ricchi et al, 2019). In addition, Permian ages (279 ± 29, 291 ± 50, and 275 ± 18 Ma) returned by other epidote veins revealed pre-orogenic fluid circulation in the Grimsel Pass area (Peverelli et al, 2022).…”

“…The object of the present investigation is Grimsel-1 epidote-quartz vein (Fig. 3; already used in Peverelli et al, 2021Peverelli et al, , 2022. This vein is subdivided into three layers: (1) layer with coarse-grained epidote (Fig.…”

“…40/60 in layer 2, whereas layer 3 is characterized by the near absence of epidote. Peverelli et al (2021; see their Fig. 6b) performed U-Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of epidote grains in layer 1, obtaining a Tera-Wasserburg age of 19.2 ± 4.3 Ma.…”

Short communication: On the potential use of materials with heterogeneously distributed parent and daughter isotopes as primary standards for non-U–Pb geochronological applications of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)

“…The use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for in situ geochronology is growing rapidly, and recent years have seen this technique being applied to many new minerals and isotope systems. Examples include in situ U-Pb dating of apatite (Chew et al, 2014(Chew et al, , 2011, carbonates (Roberts et al, 2017;Li et al, 2014;Guillong et al, 2020) and epidote (Peverelli et al, 2021), Rb-Sr dating of micas (Hogmalm et al, 2017;Zack and Hogmalm, 2016), alkali feldspar (Bevan et al, 2021) and celadonite (Laureijs et al, 2021), Lu-Hf dating of garnet, apatite and xenotime (Simpson et al, 2021), and a new approach to Re-Os dating of molybdenite (Hogmalm et al, 2019). One important challenge associated with this rapid growth is the development of matrixmatched primary standards to correct for elemental fractionation during LA-ICP-MS analysis.…”