Middledale Gabbroic Diorite (age 417 my)
The medium to coarse grained Middledale Gabbroic Diorite crops out as a ~3 km2 high-level mafic igneous rock near Temora, NSW, where it intrudes the Lower Paleozoic Bronxhome and the Combaning Formations in the Lachlan Orogen. Dominant primary minerals are plagioclase feldspar, hornblende, pyroxene, ilmenite and haematite. Although the rock is extremely fresh, late stage (deuteric) alteration has led to varied replacement of augite by cummingtonite, hypersthene by actinolite, and hornblende by chlorite. Secondary epidote is also common.
Selected in 1998 as a potentially useful unit for defining part of the chronology of this Palaeozoic terrane, this rock has since become the source of one of the most important world-wide reference standards for geological dating. Now known as TEMORA 2, the potential of this standard was initially established by scientists from Geoscience Australia, the Australian National University, and the University of Toronto.
Its popularity is based on the quality (and to a lesser extent, quantity) of its zircon, a constituent trace-mineral. Zircon dating is based on the radioactive decay of uranium and thorium to lead. The strength of the TEMORA zircon is that it has essentially neither lost nor gained any of those elements since host rock crystallisation 417 million years ago (very close to the boundary between the Silurian and the Devonian Periods). This means that even different zircon grains within this rock consistently yield the same age, a vital prerequisite for a geochronological standard.
Over recent decades, micro-beam methodologies using such instruments as the versatile and widely used SHRIMP (an ion-microprobe developed and manufactured at the ANU) have become vitally important to geochronology. These instruments, however, do not immediately yield numerical ages. In order to compensate for instrumental bias, the derivation of a numerical age requires the unknown zircon to be concurrently analysed with a zircon of known age (the reference standard). It is crucial that the isotopic composition of the standard is as homogeneous as possible, because any heterogeneity will translate into extra uncertainty in the age derived for the zircon being dated. The value of the TEMORA zircon for this task is exemplified by its popularity, with it now being used in more than a hundred different U-Pb dating laboratories throughout the world. And, since becoming accepted as a preferred reference standard for this phase of geological research, the TEMORA zircon has also been proven to be a quality isotopic standard for hafnium and for oxygen isotopic research, which provide additional valuable information on a rock’s history.
Over recent years the Middledale Gabbroic Diorite outcrop has been well visited. Some overseas scientists have been particularly keen to pay homage to the site that has played a critical role in the reliability of the ages being produced in their home laboratories.
A decade and a half ago, the age of the Middledale Gabbroic Diorite was only poorly known. Now it is one of the most frequently and best dated rocks, not only in Australia, but throughout the world.
Black LP, Kamo SL, Allen CM, Aleinikoff JN, Davis DW, Korsch RJ and Foudoulis C, 2003a. TEMORA 1: a new zircon standard for Phanerozoic U-Pb geochronology. Chemical Geology, 200, 155-170.
Black LP, Kamo SL, Williams IS, Mundil R, Davis DW, Korsch RJ and Foudoulis C, 2003b. The application of SHRIMP to Phanerozoic geochronology; a critical appraisal of four zircon standards. Chemical Geology, 200, 171-188.
Black LP, Kamo SL, Allen CM, Davis DW, Aleinikoff JN, Valley JW, Mundil R, Campbell IH, Korsch RJ, Williams IS and Foudoulis C, 2004. Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205, 115-140.
Photo: Surface outcrop of Middledale Gabbroic Diorite, near Temora, NSW
Text and photo courtesy Lance Black, Canberra