(U-Th)/(He-Pb) double dating of detrital zircons

Abstract

Geochronology and thermochronology on detrital material provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terrains. In this paper we describe a method and case-studies of measurement of both U/Pb and (U-Th)/He ages on single crystals of zircon that improves the robustness of constraints in each of these areas by establishing both formation and cooling ages of single detrital grains. Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions. U/Pb dating is accomplished by laser-ablation ICP-MS in a small pit on the exterior of the crystal, and He dates are then determined on the bulk grain by conventional laser-heating and dissolution techniques. We present examples from Mesozoic aeolian sandstones, both modern and Paleogene fluvial sediments, and active margin turbidite assemblages from the Cascadia and Kamchatka margins. Important results include the fact that detritus from ancient orogens may dominate sediments thousands of kilometers away, crustal melting and exhumation appear to be spatially-temporally decoupled in at least two orogens, and first-cycle volcanic zircons older than depositional age are surprisingly rare in most settings except in the continental interior. In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset. We present a method for estimating the extent of resetting of each grain and the thermal history of the sample, based on coupled (U-Th)/(He-Pb) age patterns among all the grains. introduction Detrital materials in sediments and sedimentary rocks are commonly used to reconstruct timing and rates of past orogenic episodes, constrain models of paleogeography and sediment transport, establish volcanic eruptive histories, and estimate depositional ages. Although detrital studies often focus on petrographic, compositional, or other characteristics of detrital material, it is the geochronology of specific detrital minerals that provides the fundamental interpretive basis for most geologic insights about the temporal evolution of source terrains, as well as age of host units themselves. Zircon is the most commonly dated phase in detrital geochronology because it: a) is resistant to chemical and physical weathering; b) is abundant in most crustal rocks; and c) has relatively high concentrations of U and Th and low common Pb. These features make it highly suitable for geochronology and thermochronology

Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

“…By contrast, the U-Pb isotope system in zircon, having an effective closure temperature in excess of 900°C (Lee et al, 1997;Cherniak and Watson, 2001), essentially records the time of its crystallization in the igneous rock. The combination of these low-and hightemperature zircon chronologies has revealed that zircons with old (>400 Ma) FT or U-Th-He ages are extremely rare in continental sediments such as those from the Mississippi, Gange and Indus Rivers (Carter and Moss, 1999;Carter and Bristow, 2000;Rahl et al, 2003;Campbell et al, 2005;Reiners et al, 2005). This finding is consistent with the view that first-cycle zircons derived from old igneous basements are minor.…”

Thermochronology in Orogenic Systems

“…For example, crystallization or cooling ages of detrital minerals in a basin can be used to determine provenance (e.g., Adams and Kelley, 1998;Baldwin et al, 1986;Barbieri et al, 2003;Carrapa et al, 2006;Clift et al, 2006;Glotzbach et al, 2011;Gray and Zeitler, 1997;Hurford and Carter, 1991;Hurford et al, 1984;Kalsbeek et al, 2000;Kelley and Bluck, 1992;Kosler et al, 2002;Reiners et al, 2005;Spiegel et al, 2007;Vermeesch et al, 2009;von Eynatten et al, 1996;Watt and Thrane, 2001). By comparing depositional ages with the cooling ages for detrital minerals, the timescales for erosion and transport in orogenic settings can be ascertained (Brandon and Vance, 1992;Bullen et al, 2001;Cerveny et al, 1988;Copeland and Harrison, 1990;Corrigan and Crowley, 1992;Garver and Brandon, 1994;van der Beek et al, 2006van der Beek et al, , 2010.…”

Step heating of 40Ar/39Ar standard mineral mixtures: Investigation of a fine-grained bulk sediment provenance tool

“…Although classic petrographic and geochemical approaches are still commonly used (Morton, 1985;Garzanti et al, 1996), isotopic techniques offer improved resolution of source-sink problems. Examples include: U-Pb ages in zircons (see review by Fedo et al, 2003), 40 Ar/ 39 Ar age dating of detrital micas (Cerveny et al, 1988;Renne et al, 1990;Hodges et al, 2005) and K-feldspars (Copeland et al, 1990), combined U-Th/HePb double dating (Reiners et al, 2005), and fission track analyses on detrital zircons (Brandon and Vance, 1992) and apatites (Donelick et al, 2005). However, once grainsize falls below that of sand (i.e., silt), the availability of robust provenance techniques becomes limited.…”