Thermal Footprints in Time: Exploring the Crust using Thermochronometry Brent McInnes Director, John de Laeter Centre

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1 Thermal Footprints in Time: Exploring the Crust using Thermochronometry Brent McInnes Director, John de Laeter Centre Noreen Evans, Brad McDonald and Martin Danisik Curtin University

2 Introduction Heat is energy transferred between substances or systems due to a temperature difference between them. Heat energy drives fluid flow & mass transfer in mineral and petroleum systems, and so understanding the thermal history of a region is important in assessing its resource potential. Thermochronometry is the science of quantifying the thermal history of the Earth s crust. In data poor environments such as greenfields areas and frontier basins, mineral thermochronometry data may be the only source of thermal history data available to explorers. Thermochronometry can be used as a heat prospecting tool.

3 Thermal Footprints in Time Introduction of heat energy into the crust disrupts the steady state geothermal gradient Thermal resetting of low T mineral chronometers can be applied to heat prospecting The strength of the observable footprint is a function of: amount of introduced heat energy, heat transfer mechanism (mass transfer, diffusion, advection) geology (crustal depth, architecture) sensitivity and resilience of the mineral thermochronometer

4 Thermal History Toolkit and Hydrothermal Mineral Systems Zircon (U-Th)/He Apatite fission tracks Apatite (U-Th)/He Garnet U-Pb Zircon U-Pb Allanite U-Pb Monazite U-Pb Sphene U-Pb Hornblende 40 Ar- 39 Ar Muscovite 40 Ar- 39 Ar Biotite 40 Ar- 39 Ar K-feldspar 40 Ar- 39 Ar Sphene (U-Th)/He Thermal Footprints of Carlin Au Deposits: Arehart et al., 2003 Tosdal et al., 2003 Cline et al., Temperature ( C) 1000

5 Heat Prospecting & Diamond Exploration Tom Reddicliffe standing on kimberlite pipe Nov 2008 M405: Application of zircon double dating to diamond exploration

6 Economic Issues: Australian Diamond Exports Argyle Carats (M) (M) $AUD $AUD (100M) (100M)

7 Industry Drivers Australian Diamond Exploration Exodus 2002 Geophysics few bulls eye targets Geochemistry kimberlite indicator minerals (garnet) destroyed by weathering Industry response brute force, diamond detection surveys

8 Not for lack of trying

9 Lots of smoke, but no fire Jaques (2003) from Smith et al (1989) based on Ashton Exploration Data

10 Thermal Footprint of Diamond Pipe 8 12 Helium Partial Retention Zone If crustal zircons are homogeneously sampled and transported to surface during kimberlite eruption, then >80% zircons will originate below HePRZ.

11 Depth Zircon a diamond indicator mineral? Resilient: Zircons, like diamonds, are forever! Kimberlite end member zircon populations: Phenocrysts: 1. Identical U Pb and U He ages Xenocrysts: 2. Below Zr He PRZ will have old U Pb and young U He ages (hot zircon) 3. Above Zr He PRZ will have old U Pb and old U He ages (cold zircon) 238 U 206 Pb He ~900 C ~180 C (Reiners et al., Tectonophysics 2002)

12 Zircon (U-Th)/(He-Pb) Double-Dating 1. Remove outer 20 microns of zircon through air abrasion No need to make -ejection correction 2. Mount multiple zircons in epoxy and lightly polished to expose flat surface for SHRIMP analysis (~1 micron deep analytical pits) 3. Pluck zircon from SHRIMP mount, load in Nb foil packets & laser heat to ~1300 C for 4 He measurement by 3 He isotope dilution 4. Dissolve zircon in Parr bombs for U-Th analysis by ID-ICP-MS Abraded zircon U-Th-He Facility SHRIMP U-Th-Pb Facility

13 Ellendale Diamond Mine Case Study Darwin Ellendale Broome Argyle Merlin Cairns 500 km Brisbane Perth Diamond occurrence Diamond mine

14 Ellendale Diamondiferous Lamproite Pipes 50 pipes spread over 150km 2 Tuff deposits have positive relief Champagne glass shape Intrude Permo-Carb sediments Evans, McInnes, McDonald, Danišík, Jourdan et al Mineralium Deposita 48 (3),

15 Ellendale E9 Pipe Half-full champagne-glass form of E9 pipe (East view) showing lamproite tuff units (grey-green) draped over country rock sediments (brown).

16 Ellendale E9 Pipe, West Pit Grant Fm Fairfield Gp View of the NW wall of the E9 West Pit showing lamproite dyke intruded into Carboniferous siltstones of the Fairfield Group. Floor of the pit is lamproite dyke and lamproite tuff.

17 Ellendale 9 Lamproite: Phlogopite Ar Ar and Zircon He Dating Phlogopite Ar-Ar Zircon (U-Th)/He Lamproite emplacement at 22 Ma

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