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| Dandahra Creek Leucogranite |
This post is a follow-on from an earlier post which can be read here.
The dating of the Dandahra Creek Leucogranite was only conducted in the last couple of years. It is an example of using multiple techniques together to get an answer. The mineral Zircon is formed in magma chambers of granite and granite-like composition. This is a very stable mineral. Zircon locks up uranium in small amounts and this uranium undergoes radioactive decay to lead. By measuring the proportions of uranium to lead it is possible to determine how long ago the zircon had formed. In the past in some cases the whole zircon crystal have been used to determine the ratio. However, this method has some complications.
Not all of the zircon crystals in rocks show the same age. In the case of the Dandahra Creek Leucogranite some seemingly having much older ages. These crystals are actually inherited from the parent rock. The stability of the zircons means that they have not fully melted in the magma chamber. Often a good way to determine if a zircon is older than the magma chamber is to look at the shape and determine whether there has been any melting of the edges of the crystal. However, sometimes it is very hard to tell because the zircon often builds itself up again with an old core and a new crystal face.
To overcome the problem of age zoning in zircon crystals an alternative method was developed measure the ratio of lead and uranium. A high accuracy ion beam is aimed at the different portions of crystal. The ion beam vaporises the elements in that tiny area. The vapour is then measured for the abundance of each element and then the ratio of elements can be calculated. This is called the Sensitive High Resolution Ion Micro Probe or SHRIMP.
SHRIMP was a method developed right here in Australia. It is regarded as one of the most reliable ways to analyse microscopic crystals to determine when and how they formed. The need for the special machine came from dating the Rocks that make up the oldest parts of Western Australia which are the oldest in the world. It has no become a recognised tool around the world (Ireland et al 2008). There are 20 SHRIMP analysers around the world with four built in the last couple of years in Japan, China and Poland. Like Wi-Fi, the Hills-Hoist and Pavlova it is another example of Australian scientific ingenuity.
Not all of the zircon crystals in rocks show the same age. In the case of the Dandahra Creek Leucogranite some seemingly having much older ages. These crystals are actually inherited from the parent rock. The stability of the zircons means that they have not fully melted in the magma chamber. Often a good way to determine if a zircon is older than the magma chamber is to look at the shape and determine whether there has been any melting of the edges of the crystal. However, sometimes it is very hard to tell because the zircon often builds itself up again with an old core and a new crystal face.
To overcome the problem of age zoning in zircon crystals an alternative method was developed measure the ratio of lead and uranium. A high accuracy ion beam is aimed at the different portions of crystal. The ion beam vaporises the elements in that tiny area. The vapour is then measured for the abundance of each element and then the ratio of elements can be calculated. This is called the Sensitive High Resolution Ion Micro Probe or SHRIMP.
SHRIMP was a method developed right here in Australia. It is regarded as one of the most reliable ways to analyse microscopic crystals to determine when and how they formed. The need for the special machine came from dating the Rocks that make up the oldest parts of Western Australia which are the oldest in the world. It has no become a recognised tool around the world (Ireland et al 2008). There are 20 SHRIMP analysers around the world with four built in the last couple of years in Japan, China and Poland. Like Wi-Fi, the Hills-Hoist and Pavlova it is another example of Australian scientific ingenuity.
The age of the intrusion given for the Dandahra Creek Leucogranite using the SHRIMP method is 237.6 Ma (plus or minus 1.8Ma). This makes it the youngest example of the Stanthorpe Suite of Granites (Chisholm et al 2014) and nearly the youngest in the whole Standthorpe Supersuite (Thanks for the correction rockdoc!).
References.Bibliography:
*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia
*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales
*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4
*Ireland, T.R., Clement, S., Compston, W., Foster, J. J., Holden, P., Jenkins, B., Lanc, P., Schram, N. & Williams, I. S. (2008), "Development of SHRIMP", Australian Journal of Earth Sciences V55 p937–954
*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia
*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales
*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4
*Ireland, T.R., Clement, S., Compston, W., Foster, J. J., Holden, P., Jenkins, B., Lanc, P., Schram, N. & Williams, I. S. (2008), "Development of SHRIMP", Australian Journal of Earth Sciences V55 p937–954
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ReplyDeleteThe explanation of anomalous zircon ages is a bit simplistic. Also the age of the Nonnington Leucoadamellite near Tenterfield came out at 236m.y.
ReplyDeletehi rockdoc,
DeleteThanks for the correction on the ages. I've made some changes in the last paragraph to fix this mistake.
You are right that I've probably oversimplified the zircon dating problems. Do you think I've gone to far and made it too simplistic? I will happily make changes if you think I should include more and be more detailed in the info already provided.
It is great to get people like yourself who clearly are experienced with these matters to make comments on my posts.