Mike Smith, NRG Steering Committee, and Lance Black, Geochronologist
Extract from National Rock Garden Newsletter No. 23, May 2022
Introduction
During April 2022, a large boulder of Middledale Gabbroic Diorite was collected from farmland near Temora and delivered to Canberra. Numerous large boulders had been moved from the farm paddocks and pushed to the edges of the fields to facilitate ploughing, planting and harvesting. In May 2021, the authors met the property owners who agreed to donate a really big boulder from their land to the National Rock Garden (NRG). The rock contains five dominant minerals: plagioclase, hornblende, pyroxene, ilmenite and haematite. Many years before, experts from Geoscience Australia conducted studies of this rock and recognised the diorite as a reliable source of isotopically undisturbed zircon crystals—these small mineral grains are very important for geological research work.
The NSW Rock Selection Subcommittee was tasked with finding the funds for the transportation of the rock to the ACT, the preparation for display by polishing a face of the specimen, and installing a stainless-steel plaque. A very generous donation has been made by a couple who have watched the steady progress of the NRG Steering Committee over the years, have attended a previous rock launch, and brought their grandchildren to the Federation Rocks display. This donation enabled the selected NRG specimen, weighing approximately 6 tonnes and shown here, to be raised from the ground using a crane truck.
Prior to being uploaded, the rock was rigorously washed while suspended above the ground. The property owners provided a fire-fighting truck with a tank of local bore water which achieved this task very efficiently.
The collection of the large boulder was attended by a reporter from The Temora Independent and the resulting front page article was published by the newspaper two days after the event. The text on page 4 of the newspaper focusses on the historic collection of specimens from the farm paddocks in this area.
Scientific importance of this rock
The Middledale Gabbroic Diorite has been proven to be a source of zircon crystals that permit reliable U-Pb isotopic dating. Now known as geochronology standard TEMORA-2, the zircons have become an extremely important world-wide reference material for geological dating.
The crystals are typically between 200 and 400 microns on their longer axis. Some examples of TEMORA-2 zircon crystals are shown in the image below. Note the 100 micron scale bar on the lower side of the image (100 microns is equivalent to one tenth of a millimetre).
The upper panel of the image shows a photomicrograph of zircon crystals illuminated in transmitted light. The lower panel shows the same zircon crystals imaged via a process called cathodoluminescence, which reveals growth zoning of the zircons.
These zircons have been impervious to chemical interaction since the crystallisation of their host diorite 417 million years ago. Specifically, the proportion of radioactive isotopes and daughter products is identical for all of the zircons, allowing the age measured for each crystal to be compared directly with the measured age of crystals whose age is not known.
One particular type of instrument that can undertakes these age comparisons is known as the SHRIMP, short for Sensitive High Resolution Ion MicroProbe, designed, and built at the Australian National University for several decades. One of these instruments was installed at GA at the end of 2007.
Placement in the National Arboretum
The NRG Steering Committee has been given approval to store a number of rocks inside the grounds of the National Arboretum Canberra, until we receive formal approval of our landscape design, when they will be moved to their final destination. After uplift and washing on the farm property, the rock was driven to the ACT and placed in our storage site as shown below.
The large rock already on the ground, in front of the green cabin of the truck is the 12 tonne boulder of Marlborough chrysoprase that was delivered a few years ago with the help of a Federal Government heritage grant.
Conclusion
The donation of private funding for the uplift, transport and preparation of this large rock for display is a very positive step in the development of the NRG in Canberra. Many of the NRG’s rocks are significant because of their use for large scale constructions (such as Sydney Town Hall, Opera House and Harbour Bridge, and Canberra’s new Parliament House), for monuments (such as the ANZAC Memorial in Hyde Park and the Canberra Foundation Stone in the ACT) and in sculpture (such as our Adelong Norite specimen).
The newly arrived diorite is very significant from the perspective of the scientific use of rocks. It will be placed in the Geoscience Knowledge cluster of the NRG’s display.
