Matthew T. Heizler

Multiple trapped argon isotope components revealed by 40AR39AR isochron analysis

Abstract Isotope correlation diagrams can be used to obtain geologically meaningful age information from complex, seemingly meaningless, 40 Ar 39 Ar step heating results. A violation of the common assumption that the trapped argon component has a 40 Ar 36 Ar equal to 295.5, the atmospheric value, has been observed for K-feldspar, biotite, muscovite and hornblende, the four terrestrial minerals most commonly dated by the 40 Ar 39 Ar method. This approach has revealed up to three separate trapped argon components in a single sample which are thermally distinct during laboratory heating; that is, as one 40 Ar 36 Ar component is exhausted, another dominates as the trapped phase. The recognition of these trapped reservoirs of excess 40 Ar explain why many release spectra have complex patterns. When multiple trapped argon components do not remain thermally distinct during laboratory heating, only limits on age and trapped argon composition can be obtained.

Effects of excess argon within large diffusion domains on K-feldspar age spectra

Abstract Unusual shapes of 40Ar/39Ar age spectra have long been attributed to the presence of excess argon. We recognize a distinctive age spectrum type brought about by extraneous argon trapped in large diffusion domains. Spectra of this type are characterized by little or no excess argon in low temperature steps, sensible apparent ages over the first ~40 to 50% 39Ar released, an abrupt increase to ages exceeding the permissible age of the sample, followed by either a gradual decrease in age until fusion or an anomalously old plateau segment. Detailed study of one such orthoclase reveals that the excess argon within these samples is likely located in radiogenic argon sites within large diffusion domains. Argon contained within these features is trapped when the domains close to argon loss at ~400°C. Thus, there exists a genetic difference between large-domain-trapped-argon-effect spectra and those saddle-shaped age spectra which result from excess argon trapped in anion vacancies at temperatures below ~350°C.

Geochronologic studies in central New England II: Post-Acadian hinged and differential uplift

{sup 40}Ar/{sup 39}Ar analyses of muscovite, biotite, and K-feldspar from central New England reveal a remarkable pattern of mineral ages: the ages are progressively younger from central to western New Hampshire and rise sharply near the Vermont border to ages more typical of post-Acadian cooling. This distribution is attributed to differential uplift via isostatic rebound of an anomalously thickened crust at the Bronson Hill anticlinorium. This explanation requires that between 6 and 8 km of normal fault motion has occurred on structures in western New Hampshire, not previously recognized to have accommodated this kind or magnitude of displacement. This hinged, differential uplift occurred from {approximately} 360-170 Ma and is consistent with a time constant for rebound of {approximately} 80 m.y.

Age and evolution of Western Brooks Range ophiolites, Alaska: Results from 40Ar/39Ar thermochronometry

Basalt, gabbro, and peridotite in the western Brooks Range structurally overlie Mississippian through Late Jurassic platform sediments that were deformed during Late Jurassic - Cretaceous time. 40Ar/39Ar step-heating experiments on hornblende, biotite, plagioclase, and potassium feldspar from gabbro, diorite, plagiogranite, and granite yielded complex release spectra. On isotope correlation diagrams, the incremental-heating data indicate that gabbro, diorite, and plagiogranite cooled below the closure-temperature for argon in amphibole between 187 and 184 Ma. Because the closure temperature of argon retention in amphibole is relatively high, these ages are interpreted to be the time since crystallization. Biotite and hornblende from metamorphic rock exposed along faults separating peridotite and gabbro from underlying basalt yielded plateau and isochron dates of 169 – 163 Ma; these dates are interpreted to record postmetamorphic cooling following intraoceanic detachment and transport of “hot” peridotite and gabbro over “cold” basalt and sediment. Detachment and faulting of the gabbros and peridotites is interpreted to have occurred approximately 20 Ma after crystallization, and is longer than the 5–10 Ma interval commonly observed in studies of other ophiolites. Fossils in flysch underlying the ophiolites indicate that the ophiolites were emplaced onto the continental margin during the Tithonian, approximately 15–20 Ma after initial detachment. The ages of feldspars (149 – 137 Ma) in the ophiolites and associated volcanic rocks are similar to the fossil ages of the underlying flysch and might record uplift during emplacement of the ophiolites onto the continental margin. A fission-track age of apatite implies that the ophiolitic rocks were overlain by >4 km of rock until ∼59 Ma ago, the time of regional uplift in the Brooks Range. Supplement Appendix 1 is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. document T92-005;

Mesozoic thermal evolution of the Yukon‐Tanana Composite Terrane: New evidence from 40Ar/39Ar data

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