Testing the advantages of simultaneous in-situ Sm[sbnd]Nd, U[sbnd]Pb and elemental analysis of igneous monazite for petrochronological studies. An example from the late Archean, Penzance granite, Western Australia

Abstract

Monazite has the ability to incorporate a broad range of trace elements (TE), including SmNd, in addition to being a reliable UPb geochronometer. Nonetheless, many current isotopic tracer studies mostly focus on laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Hf-in-zircon or whole rock isotope dilution SmNd. The conditions necessary to produce reliable Nd-isotopic data from whole rock analyses (i.e. absence of weathering and alteration) restricts the number of samples suitable for this type of study. Conversely, accessory phases such as monazite, apatite, allanite, epidote and titanite are more resistant to weathering and alteration and have a greater chance of preserving their original Nd-isotopic signature.The Archean Penzance granite, in the Yilgarn Craton, Western Australia, is a relatively unweathered high field strength elements (HFSE)-enriched granite that is associated with the Teutonic Bore volcanic-hosted massive sulfide (VHMS) Camp. Here we compare newly acquired LA split-stream (SS)-MC-ICPMS analyses in monazite from the Penzance granite to previously collected whole rock Nd-isotopes, SHRIMP ages in zircon and monazite and Hf-in-zircon of a specific outcrop. The ƐNd0 of monazite from the Penzance granite have a weighted mean value of +1.6 ± 0.2 both for the values calculated from the SHRIMP single population age and from individual LASS dates. This value is the same within uncertainty to the whole-rock ƐNd0 value of +1.99 ± 0.39, whereas ƐHf0 show similar juvenile values with mean value of +2.2 ± 0.5 (2 SE).These findings complement previous studies that argue that monazite has the potential to provide reliable time constrained isotopic and chemical data, which are comparable to, and at times superior than, Hf-in-zircon and whole-rock Nd-isotopes. Additionally, because monazite can record TE signatures that can be used to retrieve petrological information, this study proposes a LASS-MC-ICPMS routine that achieves simultaneous SmNd isotopes, U/Th-Pb dating and TE analyses in monazite, allowing for further petrological information to be obtained from a single analytical routine. This study also assesses the precision and accuracy in UPb ages in monazite reference materials, when additional masses are measured in the same routine. The application of the routine presented here facilitates the use of monazite as isotopic tracer in petrological and sedimentary studies.