U-Pb geochronology of grossular-andradite garnet

Abstract

This study presents a new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) based U-Pb geochronometric method for dating grossular-andradite (grandite) garnet. Grandite is a primary skarn mineral, therefore dating its growth directly dates hydrothermal activity. As zircon U-Pb geochronometry provides a high-resolution record of magmatic processes, grandite U-Pb dating has the potential to provide a complementary record for hydrothermal systems. This study characterizes four garnets of variable grossular-andradite content and age as potential reference materials for LA-ICPMS U-Pb geochronology: Willsboro Andradite (~1020Ma, Adirondacks, USA), Red and Yellow Mali Grandite (~200Ma, Southern Mali) and Lake Jaco Grossular (~35Ma, Coahuila, Mexico). Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb analyses of Willsboro andradite yield a mean 206Pb/238U date of 1022±16Ma. We use Willsboro Andradite as a primary reference material for LA-ICPMS U-Pb characterization of the two other garnets. The non-radiogenic Pb (Pbc) concentration in grandite is variable between specimens. Willsboro Andradite and Mali Grandites contain almost purely radiogenic Pb (Pb*), however Lake Jaco Grossular has a higher Pbc concentration comparatively and yields discordant U/Pb ratios. For specimens with highly variable Pb*/Pbc, linear regression is employed to derive a lower-intercept age. LA-ICPMS U-Pb dates for Yellow Mali Grandite (202±2Ma; 206Pb/238U weighted mean) and Lake Jaco Grossular (35±2Ma; lower intercept) agree with independent ID-TIMS U-Pb (202.0±1.2Ma) and (U-Th)/He dates (35±5Ma) derived for each specimen. The precision of LA-ICPMS U-Pb dates varies between 1–10% (2σ) for the analyzed garnets. Considering the short estimated lifespans of pluton-related hydrothermal systems (tens of thousands to a few million years), Neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels. As a performance test for the method, we present new U-Pb andradite data from a Late Miocene skarn system on Serifos Island, Greece. This garnet yields a lower-intercept age of 9.15±0.36Ma, in agreement with biotite Rb-Sr and zircon U-Pb dates from the causative pluton.