Research School of Earth Sciences
In situ oxygen isotopic analyses of zircon from granites of the Bega Batholith, south-eastern Australia
The oxygen isotopic composition of granites is a sensitive indicator of both their source materials and the low-temperature processes that occurred as or after the magma cooled. Oxygen isotopes have been used extensively, particularly in the USA, for studies of granite genesis. Granite O isotopic compositions are normally measured on whole rocks or major minerals, but these are susceptible to late alteration. A major recent advance in the study of granite O isotopes has been the recognition that the O isotopic composition of zircon accurately reflects the O composition of the melt from which it precipitated (e.g. Monani & Valley, 2001). Because zircon grains are commonly strongly zoned and in many cases contain older cores, the ideal method of measuring the isotopic composition of the melt-precipitated components is in situ analysis by secondary ion mass spectrometry.
Over the past few years, the SHRIMP II ion microprobe has been progressively upgraded for the high precision isotopic analysis of light elements, including O. In addition, sample mounting procedures have been modified and analytical protocols developed to minimise variations in instrumental mass fractionation (Ickert et al., 2008). The resulting ability to measure the O isotopic composition of single 25 µm spots on a crystal with a precision and accuracy of better than 0.4‰ has opened up a range of new opportunities for the study of granite petrogenesis in Australia.
The Bega Batholith, SE of Canberra, consists of ~130 plutons of I-type granite covering ~8900 km2. These have been grouped into a series of suites that show remarkably systematic regional changes in chemical and isotopic composition. East to west, across the elongation of the suites and the batholith as a whole, in granites of any given general bulk composition there is, for example, a systematic decrease in Na, Sr, (Al, P) and increase in Ca, Sc, (Rb, V). East to west, the granites become more isotopically evolved, initial 87Sr/86Sr rising from 0.704 to 0.709 and εNd falling from +4.3 to -8.7 (Chappell et al., 1990). It is generally agreed that these changes are due to a westerly increasing sediment component in the magmas, but whether that originates in the lower or upper crust or mantle, and whether the magmas also contain a juvenile mantle component, are matters of vigorous debate.
The early O isotopic work done on eastern Australian granites by O'Neil and Chappell (1977) and O'Neil et al. (1977) showed a clear distinction between the whole-rock O isotopic compositions of the I- and S-type granites, consistent with the proposed contrasts in their source materials. Later Chappell et al. (1990) reported mean whole rock δ18O values for seven supersuites from the Bega Batholith ranging from 8.2 to 10.0‰. The range in individual granite compositions exceeded 6‰, some values reflecting the effects of late magmatic and/or post emplacement interaction with meteoric water.
A major study of the age and O isotopic composition of selected granites from the Bega Batholith is now nearing completion. Analyses of over 600 zircons from 30 plutons representing a range of bulk chemical compositions, mainly from the eastern and western sides of the batholith, has shown that there is a broad trend for the granites to be younger in the east than in the west. Contrary to expectations, however, with a few notable exceptions, the mean isotopic composition of the O in the granite zircons shows no clear regional trend. The range of mean O isotopic compositions of the zircons is much smaller than the range of whole rock compositions, consistent with the expectation that the zircon isotopes are much more resistant to alteration than those in the whole rock, thereby more closely reflecting the original O isotopic compositions of the magmas. The radiogenic and stable isotopic systems in the granites appear to be for the most part decoupled.
The next stage of this project will be to measure the Hf isotopic compositions of the same spots on the same grains as have been analysed for O. In combination with the O isotopes, this is expected to give a clearer indication of the relative contribution of bulk juvenile and crustal components to the magmas.
Chappell, B.W., Williams, I.S., White, A.J.R. & McCulloch, M.T. (1990)
Excursion Guide A-2, Granites of the Lachlan Fold Belt. BMR Record 1990/48: