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

摘要

Noble gas elemental and isotopic compositions have been determined for a suit of fresh spinel peridotitexenoliths from Mt Quincan, North Queensland (Northeast Australia). The observed helium, argon and xenonisotopic compositions are very similar to those reported for mid-ocean ridge basalts (MORB) in that theyhave following features: (1) ~3He/ ~4He ratios covering a narrow range from 8 Ra to 10 Ra, (2) ~(40)Ar/~(36)Ar (300-4000) and the ~3He/~(36)Ar (0.0012-0.07; corrected for some recent elemental fractionation) values that plot onthe well-established MORB line, and (3) correlated excesses in ~(129)Xeand ~(136)Xewith regard to atmosphericxenon. MORB-like noble gas compositions suggest fluids ultimately derived from the asthenopshere are present inthe 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 Paleozoicsubduction at the eastern margin of Australia. We also note that there is no mantle plume noble gas signaturefound in the Mt Quincan xenoliths, despite its location close to the suspected hotspot track across easternAustralia. 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 peridotitexenoliths 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 riftingtectonic settings when asthenospheric fluid (and magma) could rise into the subcontinental lithosphericmantle and was able to overwrite any pre-existing signatures or even replace it with "MORB"-like mantle byphysical removal of previously metasomatised lithospheric mantle. Additionally, differing from noble gascompositions previously considered typical for the subcontinental lithospheric mantle, the north Queenslanddata highlight the existence of noble gas isotopic heterogeneity in the subcontinental lithospheric mantle ona global scale.