Abstract
The temperature sensitivity of the U-Pb apatite system (350–570 °C) makes it a powerful tool to study thermal histories in the deeper crust. Recent studies have exploited diffusive Pb loss from apatite crystals to generate t-T paths between ~350–570 °C, by comparing apatite U-Pb ID-TIMS (isotope dilution-thermal ionisation mass spectrometry) dates with grain size or by LA-MC-ICP-MS (laser ablation-multicollector-inductively coupled plasma-mass spectrometry) age depth profiling/traverses of apatite crystals, and assuming the effective diffusion domain is the entire crystal. The key assumptions of apatite U-Pb thermochronology are discussed including (i) that Pb has been lost by Fickian diffusion, (ii) can experimental apatite Pb diffusion parameters be extrapolated down temperature to geological settings and (iii) are apatite grain boundaries open (i.e., is Pb lost to an infinite reservoir). Particular emphasis is placed on detecting fluid-mediated remobilisation of Pb, which invalidates assumption (i). The highly diverse and rock-type specific nature of apatite trace-element chemistry is very useful in this regard—metasomatic and low-grade metamorphic apatite can be easily distinguished from sub-categories of igneous rocks and high-grade metamorphic apatite. This enables reprecipitated domains to be identified geochemically and linked with petrographic observations. Other challenges in apatite U-Pb thermochronology are also discussed. An appropriate choice of initial Pb composition is critical, while U zoning remains an issue for inverse modelling of single crystal ID-TIMS dates, and LA-ICP-MS age traverses need to be integrated with U zoning information. A recommended apatite U-Pb thermochronology protocol for LA-MC-ICP-MS age depth profiling/traverses of apatite crystals and linked to petrographic and trace element information is presented.
Highlights
Apatite [Ca5 (PO4 )3 (F,Cl,OH)] is a very common accessory mineral in igneous, metamorphic and clastic sedimentary rocks
An appropriate choice of initial Pb composition is critical, while U zoning remains an issue for inverse modelling of single crystal ID-thermal ionization mass spectrometry (TIMS) dates, and LA-ICP-MS age traverses need to be integrated with U zoning information
Paths between ~350–570 ◦ C, by comparing apatite U-Pb ID-TIMS dates with grain size or by LA-MC-ICP-MS age depth profiling/traverses of apatite crystals, and assuming the effective diffusion domain is the entire crystal
Summary
Apatite [Ca5 (PO4 ) (F,Cl,OH)] is a very common accessory mineral in igneous, metamorphic and clastic sedimentary rocks. Extracting thermal history information from a particular mineral—decay scheme is dependent on several key assumptions that are difficult to validate These include (i) the daughter isotope(s) have been lost by diffusion (e.g., there is an appropriate relationship between diffusion length scale, and age), (ii) the intrinsic diffusion properties of the mineral phase derived from laboratory experiments can be extrapolated down temperature to geological settings, and (iii) the nature of the grain boundaries are known, i.e., is the daughter isotope lost to an infinite reservoir, or not [9]?. Numerous subsequent apatite U-Pb dating studies have shown that apatite often yields strongly discordant U-Pb dates While such discordance is expected in thermochronology which exploits daughter Pb loss, it became increasingly apparent that apatite usually incorporates significant initial Pb ( known as common Pb or Pbc ). U-Pb age standards and data reduction schemes employing 207 Pb- or 204 Pb-based Pbc corrections to age standards and unknowns [12,13,14] means that U-Pb dating of apatite by LA-ICP-MS (including the generation of apatite intra-grain U-Pb date transects) is routinely possible
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