MULTI-STEP CHEMICAL ABRASION AND DISSOLUTION APPLIED TO COMPLEX ZIRCON FROM THE PROTEROZOIC MT. MARCY MASSIF ANORTHOSITE, NEW YORK

In U-Pb geochronology of zircon, the choice of analytical techniques is typically a tradeoff between precision and spatial resolution: isotope-dilution thermal ionization mass spectrometry (ID-TIMS) can provide precise ages (better than 0.1%) but targets single grain or grain fragments, whereas, microbeam techniques can target sub-grain regions but at an order of magnitude lower precision. Here, we provide preliminary data on closing this spatial resolution gap by leveraging partial dissolution techniques on the Mesoproterozoic Mt. Marcy anorthosite in New York, to access smaller quantities of sub-grain material for ID-TIMS analyses.

Proterozoic massif anorthosites are commonly considered to represent flotation cumulates developed in large, lower crustal mafic magma chambers. Recent high-precision U-Pb zircon geochronologic data from the Kunene anorthosite in Angola suggests that it represents an amalgamation of several intrusive events, raising questions about why and how multiple flotation cumulates developed at the same location over a period of more than 10 million years. This issue can be particularly challenging for zircon that has had a complex crystallization history and can result in mixed ages if metamorphic and igneous growth domains are dissolved together. To circumvent this problem, we are attempting to implement a multi-step chemical abrasion and dissolution procedure to individual zircon from the Mt. Marcy anorthosite. As expected, the first leaching step, which corresponds to the partial dissolution component of chemical abrasion typical chemical abrasion -ID-TIMS U-Pb zircon studies, dissolves zircon that has experienced Pb-loss. Subsequent dissolution steps then dissolve mixtures of metamorphic and igneous growth domains. We find that in some cases, the apparent dates converge over the final steps, and we attribute these results to the true igneous age. and Preliminary data provide igneous crystallization ages of 1156 Ma and are in good agreement, but more precise than previous U-Pb zircon geochronologic results from the Mt. Marcy anorthosite produced by secondary ion mass spectrometry.