Apatite laser ablation Lu-Hf geochronology: A new tool to date mafic rocks

Abstract

Mafic rocks are the most common type of igneous rocks on Earth, however, constraining the crystallization age of mafic rocks can be challenging. Apatite is a common accessory phase in mafic rocks and is amenable to dating using the U–Pb system. However, the U–Pb system in apatite has a relatively low closure temperature (~350◦- 550 ◦C) and is therefore prone to resetting by later thermal and metasomatic events. Here, a recently developed Lu–Hf dating method using laser ablation reaction-cell mass spectrometry is applied to apatite from mafic rocks. The Lu–Hf system in apatite has a higher closure temperature (~650◦-750 ◦C) compared to U–Pb, increasing the chances of obtaining primary crystallization ages. Furthermore, the laser-ablation method allows rapid data collection compared to traditional solution-based Lu–Hf dating techniques. Four study areas were selected to compare the Lu–Hf vs U–Pb systematics of apatite in mafic igneous rocks: the Paleoproterozoic Sudbury Igneous Complex (Canada), the Neoproterozoic Borborema Province (NE Brazil), the Paleoproterozoic Fennoscandian Shield (Finland), the Archean Yilgarn Craton and adjacent Mesoproterozoic Albany Fraser Orogen (Western Australia). For all analyzed samples that have apatite trace element compositions typical of an undisturbed primary mafic igneous lithology, the Lu–Hf system retains primary igneous apatite crystallization ages, whereas the U–Pb system in the same grains often records isotopic disturbance or a cooling age. In few cases, the Lu–Hf system has also been disturbed in response to recrystallization, however, such disturbance is readily detected e potential of laser ablation apatite Lu–Hf dating to primary crystallization ages for otherwise difficult to date mafic rocks.