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.
‘Blind time’ – current limitations on laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for ultra-transient signal isotope ratio analysis and application to individual sub-micron sized uranium particles