A compilation of new and published 3He/4He data in Australia reveals a surprisingly pervasive yet partitioned ongoing process of transfer of mantle-derived volatiles to the near surface groundwater system and into oil and gas reservoirs.
Design and Methodology
Comparison of helium isotope data with mantle tomographic images show that high 3He/4He values in groundwater correlate with domains of low velocity mantle and with sharp mantle velocity contrasts.
Original data and results
Deep lithospheric boundaries (150-200 km) between high velocity mantle in western Australia and lower velocity mantle in eastern Australia, provide the first order control of mantle degassing and are interpreted to represent Precambrian structures that are currently being neotectonically reactivated (e.g. Tasman line and Torrens hinge zone). Mid-lithospheric (75- 100 km) low velocity domains underlie zones of mantle degassing in central and western Australia. Rather than uniform upward flux of mantle volatiles into the lithosphere, our data delineate variably fertile mantle source regions and lithospheric conduit zones. Understanding the conduit systems for the deeply-derived fluids require holistic geologic models, but we envision asthenospheric (MORB) sources in eastern Australia related to transfer of basaltic magma and accompanying volatiles from asthenosphere to lithosphere over the last several Ma. Helium leakage in non-volcanic areas is likely sourced from metasomatized lithospheric mantle. Helium and CO2 volatile transport through the crust takes place by (and facilitates) microseismicity, and water-rock interactions of deep geothermal fluids with hydrocarbons and deep basin brines introduce Cl, metals, and radiogenic Sr into aquifers. This continental scale fluid convection system is driven by small scale sublithospheric convection induced by plate reorganization events related to initiation of transpression in New Zealand ~ 5 Ma and accompanied reactivation of lithospheric zones of weakness.
Implications involve a new paradigm for the Great Artesian Basin (GAB) which underlies 22% of the Australian continent and is one of the largest groundwater basins in the world. This new paradigm involves endogenic fluid inputs into the J-K aquifer system that leak up faults from below the aquifer, partition the GAB into hydrogeologic subbasins, and cause variable degradation of water quality.