We explore the formation conditions and inheritance probability of zircon in impact melts and the implications of using zircon geochronology to investigate planetary impact histories. By modeling the occurrence and crystallization temperature spectrum for zircon in simulated impact melts, we predict the presence of such grains within impactites. We also report U-Pb geochronology of sieve-textured, possibly poikilitic, zircon identified in the pseudotachylyte and granophyre units present within the largest known terrestrial impact crater (Vredefort, South Africa) to explore the accuracy of these grains in dating impact events at an impact structure of known age. Zircons with similar textures have been recently interpreted as growing in an impact melt in lunar meteorite SaU 169 and used to determine the age of the Imbrium impact. Modeling in simulated lunar melt compositions predicts crystallization of zircon in merely ~2% of melting events, in this case via impact. The modeled crystallization temperature spectrum is significantly below Ti-in-zircon crystallization temperatures reported from lunar samples. Zircon formation within an impact melt is dictated by saturation of [Zr] and requires a high abundance for lunar melt compositions. This essentially rules out the possibility of zircon growing in equilibrium with lunar meteorites. Poikilitic textures may be inherited from the lunar crust, presumably due to rapid decompression and/or resorption into an undersaturated magma, as previously recognized in plagioclase. Although either scenario could be due to an impact, endogenic processes cannot be ruled out, and thus lunar poikilitic zircons may not be recording impact melting events. Secondary ion mass spectrometry U-Pb analysis of zircon with similar textures from Vredefort clearly shows that these grains are inherited from the Archean target rocks, with varying degrees of Pb loss, and consequently cannot be used to accurately identify the age of the Vredefort impact structure. Further understanding of the growth and isotopic effects on zircon of shock and heating associated with large impacts could provide another tool that can be used to probe planetary impact histories.

Abstract It is axiomatic that application of closure theory – the foundation of isotope-based thermochronology – requires an empirical diffusion model. It is therefore surprising that the majority of thermochronological studies have not met this requirement. The advent of the multi-diffusion domain (MDD) model transcended this limitation yielding both diffusion and age information via routine 40 Ar/ 39 Ar step-heating of K-feldspar. Observed correlations between age and Arrhenius spectra show that Ar diffusion occurs by the same mechanisms in nature as in the laboratory. Under certain conditions, these data permit the recovery of a unique, cooling history. The community reaction included some unproductive lines of argument but some stimulated refinements of the MDD model that benefited the development of thermochronology. The MDD model was recently applied to muscovite upon recognition that the same diffusion mechanism operates in vacuum step-heating as in nature. The advent of 40 K– 40 Ca closure profile dating opens up a new thermochronological approach. Initial results confirm that muscovite intragrain defects can restrict effective diffusion length scales in white micas from 10–100 s of microns. Our hope for the future of the MDD model is that it be subject to aggressive and sceptical testing by the community in which quantification is valued over assertion.

The Main Central Thrust shear zone is the dominant crustal thickening feature in the Himalayas, largely responsible for the extreme relief and mass wasting of the range. Along the Bhagirathi River in NW India, the Main Central Thrust is several kilometers thick and separates high-grade gneisses of the Greater Himalayan Crystallines from Lesser Himalayan metasedimentary rocks. Th-Pb ion microprobe ages of monazite dated in rock thin section from the Greater Himalayan Crystallines are Eocene (38.0 ± 0.8 Ma) to Miocene (19.5 ± 0.3 Ma), consistent with the burial of the unit during imbrication of the northern Indian margin and subsequent exhumation due to Main Central Thrust activity, respectively. However, two samples directly beneath the Main Central Thrust yield 4.5 ± 1.1 Ma ( T = 540 ± 25 °C and P = 700 ± 180 MPa from coexisting assemblage) and 4.3 ± 0.1 Ma (five grains) matrix monazite ages, suggesting Pliocene reactivation of the structure. Hydrothermal monazites at the base of the Main Central Thrust shear zone record Th-Pb ages of 1.0 ± 0.5 Ma and 0.8 ± 0.2 Ma, the youngest ever reported for the Himalayas. These ages postdate or overlap activity along structures closer to the Indian foreland and show that the zone of Indo-Asia plate convergence did not shift systematically southwestward from the Main Central Thrust toward the foreland during the mountain-building process. Instead, age data support out-of-sequence thrusting and reactivation consistent with critical-taper wedge models of the Himalayas.

Abstract The hypothesis that the Himalayan crystalline core originated by ductile channel flow of partially molten mid-crust from beneath the Tibetan Plateau is critically reviewed. The proposal that widespread shallow anatexis exists beneath southern Tibet today is inconsistent with numerous observations (e.g. ‘bright spots’ restricted to a single rift and evidence that they represent aqueous fluids rather than molten silicate; the seismogenic southern Tibetan Moho; 3 He/ 4 He data indicating the presence of mantle heat and mass in the rift valley; the likelihood that any melt present is due to late Neogene calc-alkaline magmatism; the lack of Tertiary migmatites in the crustal section exposed in the uplifted rift flank of the Yangbajain graben; the lack of Gangdese zircon xenocrysts in the Greater Himalayan Crystallines (GHC); and the broadly coherent stratigraphy in the GHC). Evidence advanced in support of this model is equally or better explained as resulting from localized Neogene calc-alkaline magmatism. A recently developed rapid denudation/channel flow model does explain key petrogenetic and thermochronological features of the Himalaya, but is inconsistent with several geological constraints, most notably the small portion of the collision front over which focused erosion has localized exposure of the GHC. It is concluded that no evidence has yet been documented that requires the existence of partially molten crust flowing in a channel from beneath the Tibetan Plateau to form the Himalaya.