207Pb-excess in carbonatitic baddeleyite as the result of Pa scavenging from the melt

Abstract

For the last two decades, the end of the voluminous phase of eruptions of the Siberian Traps large igneous province has been constrained by a U-Pb date of discordant baddeleyite collected from the Guli carbonatite intrusion with the assumption that the discordance resulted from unsupported 207Pb. In this study we have re-analysed baddeleyite from the same intrusion and found two types of discordance: (1) due to 207Pb-excess, and (2) radiogenic lead loss from high U mineral inclusions. The former implies that baddeleyite is an efficient scavenger of protactinium during crystallisation, leaving the magma depleted in this element. Together with a published high precision U-Pb date of 252.24 ± 0.08 Ma for the Arydzhansky Formation, our new date of 250.33 ± 0.38 Ma for the Guli carbonatite constrains the total duration of the voluminous eruptions of the Siberian Traps LIP at 1.91 ± 0.38 million years. The lower intercept of the (231Pa)/(235U) corrected discordance line yields a date of 129.2 ± 65.0 Ma, which points to the widespread Early Cretaceous rifting in East and Central Asia.