Zircon U–Th–Pb–He double dating of the Merlin kimberlite field, Northern Territory, Australia
Introduction
Kimberlites and lamproites act like conveyor belts, transporting fragments from depths in excess of 150 km to the Earth's surface in time spans of hours to days (Kelley and Wartho, 2000), juxtaposing high pressure and high temperature mantle minerals with shallow crustal materials at the surface. Current diamond exploration strategies include the search for kimberlite indicator minerals (KIMS) such as chromite, picroilmenite, Cr-pyrope and Cr-diopside in stream sediment and soil samples (Fipke et al., 1995, Muggeridge, 1995, Nowicki et al., 2007, Hoal et al., this issue, Read and Janse, 2009). KIMS are, however, prone to chemical dissolution in tropical weathering conditions and therefore, alternative approaches involving the geochemical analysis of resistant minerals such as zircon would enhance exploration success in tropical regions such as Australia, India and Brazil.
Section snippets
Zircon in kimberlite and U–Th–Pb–He systematics
Mantle zircon is a relatively rare accessory mineral in kimberlite and when it is found, it is usually in the form of rounded, megacrystic (mm to cm size), gem-like crystals containing one or more perfect cleavages (Kresten et al., 1975). These morphological properties, combined with the very low concentration of U, Th, REE and Ti (Ahrens et al., 1967, Belousova et al., 1998, Belousova et al., 2002, Hoskin and Ireland, 2000, Page et al., 2007) differentiate mantle zircon from crustal zircon,
Samples and methods
The geology and setting of the Merlin Kimberlite Field is discussed by Lee et al. (1998). A total of 14 kimberlite pipes in a fault-controlled corridor 10 km long by 5 km wide were discovered during the mid-1990s. The kimberlite pipes do not currently outcrop and have no visible surface expression. All pipes contain diamonds with grade estimates of 0.2–0.3 ct/t, with an exceptional 104.7 carat, white, gem quality stone recovered in 2002. The deposits are hosted within the Early Cambrian
Results and discussion
The double dating results for the FCT zircon sample (Fig. 4, Table 1) demonstrate that the (U–Th)/He ages are equivalent within 2σ error to the U/Pb ages on all but one zircon grain. The SHRIMP double dating technique yields results comparable to those obtained using the ELA-ICPMS double dating technique (Rahl et al., 2003).
The SHRIMP zircon U/Pb ages for the Sacramore kimberlite (n = 14) range from 1500–2400 Ma (Fig. 5a, Table 2, Table 3), indicating that the zircon grains in the mineral
Conclusions
This study indicates that (U–Th)/He dating of xenocrystic zircon contained with kimberlites can be used to determine kimberlite emplacement age (e.g., Blackburn et al., 2008), which for the Sacramore pipe in the Merlin Kimberlite Field is 368 ± 4 Ma. There are two possible processes that could account for the thermal resetting of the zircon (U–Th)/He ages: (i) an origin from below the helium partial retention zone at > 6 km crustal depth or, (ii) prolonged exposure to ambient temperatures in
Acknowledgements
We are grateful to Tom Reddicliffe of North Australian Diamonds for his ongoing support of this research, as well as Michael Kammermann and Paul Shaw for supplying and preparing the samples. Angelo Vartesi and Travis McNaughton drafted the figures. Steve Reddy provided the CL images and Cam Scadding and Allen Thomas (University of Western Australia and TSW Analytical™) assisted with ICPMS U and Th analysis. Allen Kennedy and Hao Gao supported the SHRIMP analysis at Curtin University. We thank
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