Intact Single Crystals of Phlogopite from the Slowly-Cooled Adirondack Lowlands (NY, USA) Show that Ar and Sr Closure does not Reflect Volume Diffusion
Abstract
Phlogopite crystals as isolated, sub- to euhedral grains in regionally metamorphosed marble are readily recovered intact, without comminution, using mechanical or mild-acid extraction. Single crystals from the Grenville Province of the Adirondack Lowlands in New York are used to examine behavior of the K/Ar and Rb/Sr systems and mechanisms of radiogenic daughter loss during slow cooling ( ~1.5 ° C/m.y., from biotite and amphibole throughout the Lowlands). Selected crystals ( ~1 mm or less in diameter and ~1 mg or less), which are ~1000 Ma, are analyzed whole or as sections by 40Ar/39Ar and Rb/Sr methods. Using 40Ar/39Ar, detailed serial sampling via incremental-heating or spatial sampling by UV-laser ablation microanalysis is conducted. The degree of internal concordance with all types of 40Ar/39Ar analysis is very high with variations essentially within uncertainties and with no apparent gradients. No center to edge age gradients on (001) are present, contrary to expectation for diffusion control of closure with a cylindrical geometry, nor are any observed with depth profiling perpendicular to (001) growth faces. However, the ages for individual crystals within small rock volumes vary from negligible to ~100 m.y. for different marbles. For crystals from the same marble, Rb/Sr and 40Ar/39Ar ages show essentially the same average and dispersion indicating that the two methods behave similarly. Variations in crystal ages are not correlated with any obvious physical (e.g., size) or chemical parameters. Collectively, the relationships argue against the commonly assumed, thermally-activated, volume-diffusion control of closure of the Ar and Sr systems in these phlogopite crystals. A potential domain-size explanation is remote since it would require a two order-of-magnitude size difference for almost identical, translucent crystals. The relationships point to an alternate mechanism for loss of daughter isotopes, such as that induced by fluids or stress below the closure temperature for diffusion kinetics.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2001
- Bibcode:
- 2001AGUSM...V41B05F
- Keywords:
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- 1035 Geochronology;
- 1040 Isotopic composition/chemistry;
- 1099 General or miscellaneous;
- 3699 General or miscellaneous