UMass Geosciences Link
Ultrachron

 

 

The Ultrachron project was initiated primarily to explore the limitations and applications of trace element EPMA as related to geochronology. The inspiration for this lies in the observation that polygenetic monazite is common in multiply tectonized rocks, and that the spatial resolution required for a detailed analysis of these materials is at the micrometer scale. Accessory phase populations often cover a range in size, down to well below five micrometers. This is important as these small grains in many cases are either the only population present in some rocks, or represent mineral growth episodes distint from larger grains.

Almost all monazite is compositionally zoned, down to the micrometer scale, and these compositional domains are ofter related to specific ages in polygenetic grains. The electron probe offers the only access to both the composition and age at this spatial resolution.

Monazite grain from Legs Lake Shear Zone, western Churchill Province, Canadian Sheild. This is a map of uranium concentration (U Mb). Note that age correlates with U content in this case.  

EPMA geochonology compliments other geochronologic techniques (ID-TIMS, Ion probe, LA-ICPMS) by offering a method to evaluate zoning at exceptionally high spatial resolution. This technique also offers a way to correlate compositions of constituent phases in-situ, allowing quantitative assessment of reactions (and related microstructures) that result in accessory phase growth and/or breakdown, essentially allowing reaction dating in many cases.

Sources of discordance or mixed ages in isotopic analyses can be better understood, and a better overall picture of P-T-t-D evolution is realized by combining information from all these techniques.

U Mb compositional map (image width = 50um) of a monazite grain from a migmatite from the Lower Granite Gorge of the Grand Canyon, AZ. Note that the grain has a typical round exterior but contains a euhedral internal crystal form. Similar internal forms have been seen in many rocks. Although bulk monazite analyses yield a range of dates (upper figure), dates from the internal crystals are tightly clustered at 1689 Ma. Anatexis is interpreted to have occurred at 1689 Ma; overgrowths developed at a variety of times from 1670 Ma to 1620 Ma leading to the range in concordant single-crystal dates.
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