Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes

Geochemical monitoring of CO2 storage requires understanding of both innate and introduced fluids in the crust as well as the subsurface processes that can change the geochemical fingerprint of CO2 during injection, storage and any subsequent migration. Here, we analyse a natural analogue of CO2 storage, migration and leakage to the atmosphere, using noble gas and stable isotopes to constrain the effect of these processes on the geochemical fingerprint of the CO2. We present the most comprehensive evidence to date for mantle-sourced CO2 in south-east Australia, including well gas and CO2-rich mineral spring samples from the Otway Basin and Central Victorian Highlands (CVH). 3He/4He ratios in well gases and CO2 springs range from 1.21 to 3.07 RA and 1.23 – 3.65 RC/RA, respectively. We present chemical fractionation models to explain the observed range of 3He/4He ratios, He, Ne, Ar, Kr, Xe concentrations and δ13C(CO2) values in the springs and the well gases. The variability of 3He/4He in the well gases is controlled by the gas residence time in the reservoir and associated radiogenic 4He accumulation. 3He/4He in CO2 springs decrease 29 away from the main mantle fluid supply conduit. We identify one main pathway for CO2 supply to the surface in the CVH, located near a major fault zone. Solubility fractionation during phase separation is proposed to explain the range in noble gas concentrations and δ13C(CO2) values measured in the mineral spring samples. This process is also responsible for low 3He concentrations and associated high CO2/3He, which are commonly interpreted as evidence for mixing with crustal CO2. The elevated CO2/3He can be explained solely by solubility fractionation without the need to invoke other CO2 sources. The noble gases in the springs and well gases can be traced back to a single end-member which has suffered varying degrees of radiogenic helium accumulation and late stage degassing. This work shows that combined stable and noble gas isotopes in natural gases provide a robust tool for identifying the migration of injected CO2 to the shallow subsurface

The Warnie volcanic province : Jurassic intraplate volcanism in Central Australia

We wish to thank Santos Ltd. for providing us with the Snowball 3D seismic survey. In particular we wish to thank Jenni Clifford and Lance Holmes who provided helpful feedback and 2D seismic lines covering the Lambda 1, Orientos 2 and Warnie East 1 wells. We also wish to thank Beach Energy, in particular Rob Menpes, for the helpful discussions and feedback on the manuscript in addition to helping us with the analysis of the magnetic data. The work contained in this paper contains work conducted during a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil & Gas [grant number NEM00578X/1] and is fully funded by NERC whose support is gratefully acknowledged. Lastly, the two anonymous reviews of the manuscript are thanked for their insightful and constructive comments that significantly improved the work presented.Peer reviewedPostprin

U-series isotope and geodynamic constraints on mantle melting processes beneath the Newer Volcanic Province in South Australia

Copyright © 2007 Elsevier B.V. All rights reserved.Young (< 5 kyr) olivine- and clinopyroxene-phyric ne-hawaiites from Mounts Gambier and Schank in the Newer Volcanic Province in South Australia have been analysed for major and trace elements as well as for Sr and Nd isotopes and 238U-230Th disequilibria in order to constrain the mantle melting processes responsible for their origin. The rocks are relatively primitive (6.9-9.1% MgO), incompatible trace element-enriched alkali basalts with 87Sr/86Sr = 0.70398-0.70415 and 143Nd/144Nd = 0.51280-0.51271. Trace element modelling suggests that they reflect 3-6% partial melting in the presence of 2-8% residual garnet. Trends towards low K/K* are accompanied by decreasing 87Sr/86Sr and provide evidence for the involvement of hydrous phases during melting. 230Th excesses of 12-57% cannot be simulated by batch melting of the lithosphere and instead require dynamic melting models. It is argued that the distinction between continental basalts bearing significant U-Th disequilibria and those in secular equilibrium reflects dynamic melting in upwelling asthenosphere, rather than static batch melting within the lithosphere or the presence or absence of residual garnet. Upwelling rates are estimated at ∼ 1.5 cm/yr. A subdued, localised topographic uplift associated with the magmatism suggests that any upwelling is more likely associated with a secondary mode localised to the upper mantle, rather than a broad zone of deeply-sourced (plume) upwelling. Upper mantle, 'edge-driven' convection is consistent with seismic tomographic and anisotropy studies that imply rapid differential motion of variable thickness Australian lithosphere and the underlying asthenosphere. In this scenario, melting is linked to a significant contribution from hydrous mantle that is envisaged as resulting either from convective entrainment of lithosphere along the trailing edge of a lithospheric keel, or inherited variability in the asthenosphere. © 2007 Elsevier B.V. All rights reserved.Zoe Demidjuk, Simon Turner, Mike Sandiford, Rhiannon George, John Foden and Mike Etheridgehttp://www.elsevier.com/wps/find/journaldescription.cws_home/503328/description#descriptio

Decompression melting driving intraplate volcanism in Australia: Evidence from magnetotelluric sounding

Free to read A long-period magnetotelluric (MT) survey, with 39 sites covering an area of 270 by 150 km, has identified melt within the thinned lithosphere of Pleistocene-Holocene Newer Volcanics Province (NVP) in southeast Australia, which has been variously attributed to mantle plume activity or edge-driven mantle convection. Two-dimensional inversions from the MT array imaged a low-resistivity anomaly (10-30Ωm) beneath the NVP at ∼40-80 km depth, which is consistent with the presence of ∼1.5-4% partial melt in the lithosphere, but inconsistent with elevated iron content, metasomatism products or a hot spot. The conductive zone is located within thin juvenile oceanic mantle lithosphere, which was accreted onto thicker Proterozoic continental mantle lithosphere. We propose that the NVP owes its origin to decompression melting within the asthenosphere, promoted by lithospheric thickness variations in conjunction with rapid shear, where asthenospheric material is drawn by shear flow at a "step" at the base of the lithosphere

Steps in lithospheric thickness within eastern Australia, evidence from surface wave tomography

[1] A series of steps in the lithospheric thickness of eastern Australia are revealed by the latest seismic surface wave tomographic model and calculations of the horizontal gradient of shear wave speed. The new images incorporate data from the recent Tasmal experiment, improving resolution in continental Australia. Through comparisons with surface geology and geochemical studies, it is possible to infer that the steps in lithospheric thickness are related to boundaries between blocks of different age. The westernmost boundary marks the edge of the Archaean to Early-Proterozoic core of the continent. A second lithospheric boundary is observed in the central part of east Australia. To the west of this line, geochemical evidence suggests that there is Proterozoic lithospheric mantle, and this boundary may therefore represent the change from Proterozoic to Phanerozoic basement. The structure on the eastern margin of the continent is dominated by slow velocities, suggesting that in this area the continental lithosphere is very thin. There is a strong correlation between the slow wave speeds and the location of both the highest topography and recent volcanic activity. Inland of the continental margin, a zone of strong gradients in the seismic wave speed is observed, indicating a distinct step in lithospheric structure. If the step in lithospheric thickness was in place prior to volcanism, it may have acted as a boundary, with volcanism mainly occurring beneath the thinner lithosphere to the east