One has to consider plate tectonics, the age of the of the volcanic ash or indurated phase (e.g. tonsteins) and that of distant plutons,using a refined technique such as single-crystal zircon U-Pb dating, the microchemistry of glass inclusions in volcanic quartz, paleowinds, , and erosion of the the ultrasilicic volcanic ash, just to name several.
Hello Paul,
I remember reviewing some papers many years ago where the authors asked themselves the same question for some Ordovician super-eruption ash layer that had been first been "finger-printed" correlated across Europe and N America (then once continent).
The authors, I recall, used two techniques on these ancient silicic ash beds.
They measured ash bed thickness everywhere (isopach map) and the increase in thickness normally points towards the source, but this does not always work for the largest of éruptions as George Walker demonstrated for the AD180 Taupo ultraplinian eruption for example (Walker 1980; and other papers by CJN Wilson and GPL Walker from early 1980s).
In this particular study, it generally allowed to roughly locate the source region though.
More precise in this particular case had been to systematically measure the average maximum size of the phenocryst fraction present in the ash bed (for example measure up maximum size of 10% coarsest tail of the phenocryst fraction..then construct a maximum crystal isopleth map).
This proved much more accurate, leading them to locate the source area for this huge silicic eruption much more narrowly. At least two papers were published about this I think I recall (I think one was in "Geology").
I am sorry I cannot recall the name of the authors of the couple of papers that reported this. I reviewed this almost 20 years ago.
Hoping this helps.
Happy holidays Paul and everyone,
Best wishes,
Gerald
I will give you the example of ash from the Toba super volcano that was identified by John Westgate of the Univ. of Toronto on the basis of fission track dating and chemical analysis.
The samples of the ash were obtained by the ocean drilling program thousands of miles away across the Indian Ocean. No one knew the source of the ash until the ash at Toba Lake was sampled when other scientists realized Toba was a volcano. The ash samples gave identical ages and chemical compositions with those taken across the Indian Ocean.
The article referenced below may give you some ideas for criteria that could be used to match high-silica volcanic ash with a source intrusive complex.
Mercer, C.N., Hofstra, A.H., Todorov, T.I., Roberge, Julie, Burgisser, Alain, Adams, D.T., and Cosca, Michael, 2015, Pre-eruptive conditions of the Hideaway Park topaz rhyolite: insights into metal source and evolution of magma parental to the Henderson porphyry molybdenum deposit, Colorado: Journal of Petrology, v. 56, No. 4, p. 645-679.
Dear Paul, I think as already suggested by Martin Menzies, that the best should be to try to correlate as much as possible proximal and distal tephra. This can allow you at least to make a more confident attribution and dating of the possible source avoiding just one single data set and /or number date. Of coarse there are many possible sources but recently many papers on tephrostratigraphy have been published, especially in quaternary journals (eg.doi:10.1016/j.quascirev.2011.07.012; http://dx.doi.org/10.1016/j.quascirev.2012.09.009; http://dx.doi.org/10.1016/j.quascirev.2015.03.006; http://dx.doi.org/10.1016/j.quascirev.2014.04.002 and others) that could help to direct your effort. Data and literature related to very old volcanism are much less because often the proximal part is poorly known or unknown. Have a good job.
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