Saturday, February 26, 2011

What does TEMORA stand for?

Ion probe (or SIMS) geochronology has no shortage of acronyms. SHRIMP is the most obvious one, and that in turn has spawned a number of associated abbreviations, such as PRAWN, ZOC, and the various standards (QGNG, FC1, SL13 BR266, etc.). In SHRIMP (and other SIMS- CAMECA stands for something in French) analysis, standards are used because the ionization efficiency of both Pb and U is variable, so you need something with a known Pb/U ratio to correct for these effects. One of the current popular standards is called TEMORA.

So I shouldn’t have been surprised when an American visitor to the RSES at the ANU asked me what TEMORA stood for. I wish I had a snappy answer. Perhaps he was expecting something like

Thermally Equilibrated Magnetite-Orthoclase Rhyolitic Assemblage*


Time Evidently Made One Resplendent Anchorpoint


The Excellent Mother Of Radiogenic Architypes

Because the boring truth is that it doesn’t stand for anything. Temora is the name of a town in Western NSW. The standard comes from a specific outcrop of the Middledale gabbroic diorite, and the particular outcrop from which this zircon was extracted happens to be on the road to Temora. Like most sources of zircon standards, this is an evolved mantle melt.

Crustal rocks are generally not chosen for zircon standards, as the zircons in crustally derived igneous rocks have a greater chance of being inherited from the source rock. This would make them older than the igneous zircons, giving your ‘standard’ two populations with different ages. This particular gabbroic norite from the fields near Temora was found to be fairly well behaved, isotopically speaking, so it has been widely used as a U/Pb, Hf isotope, and oxygen isotopic standard.

As for the town of Temora, it is a small country town in winter wheat and sheep country. It’s main claim to fame is the aviation museum, which keeps a number of vintage aircraft in flying condition and occasionally puts on shows.

The Temora zircon was called TEMORA 1 in the paper that originally described it. I have no idea why it was capitalized in this way. But it’s not an acronym. Perhaps the author just needed to yell to be heard from that far out in the bush.

* This would be wrong: It’s a gabbroic diorite.

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