Noble gases in spinel peridotite xenoliths from Mt Quincan, North Queensland, Australia: Undisturbed MORB-type noble gases in the subcontinental lithospheric mantle

Abstract

Noble gas elemental and isotopic compositions have been determined for a suit of fresh spinel peridotite xenoliths from Mt Quincan, North Queensland (Northeast Australia). The observed helium, argon and xenon isotopic compositions are very similar to those reported for mid-ocean ridge basalts (MORB) in that they have following features: (1) 3He/ 4He ratios covering a narrow range from 8 Ra to 10 Ra, (2) 40Ar/ 36Ar (300–4000) and the 3He/ 36Ar (0.0012–0.07; corrected for some recent elemental fractionation) values that plot on the well-established MORB line, and (3) correlated excesses in 129Xe and 136Xe with regard to atmospheric xenon. MORB-like noble gas compositions suggest fluids ultimately derived from the asthenopshere are present in the subcontinental lithospheric mantle beneath northeastern Australia. The absence of any subduction-related noble gas signature precludes the preservation of fluids in the xenoliths added during Paleozoic subduction at the eastern margin of Australia. We also note that there is no mantle plume noble gas signature found in the Mt Quincan xenoliths, despite its location close to the suspected hotspot track across eastern Australia. These features are consistent with Sr and Nd isotopic characteristics of Mt Quincan xenoliths [Handler, M.R., Bennett, V.C., Carlson, R.W. 2005. Nd, Sr and Os isotope systematics in young, fertile peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle? Geochim. Cosmochim. Acta 69, 5747–5763.]. Thus, the fluid-entrapment is likely associated with subsequent rifting tectonic settings when asthenospheric fluid (and magma) could rise into the subcontinental lithospheric mantle and was able to overwrite any pre-existing signatures or even replace it with “MORB”-like mantle by physical removal of previously metasomatised lithospheric mantle. Additionally, differing from noble gas compositions previously considered typical for the subcontinental lithospheric mantle, the north Queensland data highlight the existence of noble gas isotopic heterogeneity in the subcontinental lithospheric mantle on a global scale.