From the Hadean to the Himalaya: 4.4 Ga of felsic terrestrial magmatism
Detrital zircons as old as nearly 4.4 Ga offer insights into the earliest moments of Earth history. Results of geochemical investigations of these grains have been interpreted to indicate their formation in near-H2O saturated meta- and peraluminous magmas under a relatively low (15–30 °C/km) geotherm. A key feature in pursuing a petrotectonic model that explains the full spectrum of these observations is their seeming contrast to most Phanerozoic magmatic zircons, specifically their low Ti-in-zircon crystallization temperatures and inclusion assemblages. The ~22 Ma Arunachal leucogranites of the eastern Himalaya appear, however, to be a rare exception to this generality. They show large-ion lithophile covariance trends indicative of wet basement melting together with a normal distribution of magmatic crystallization temperatures about an average of 660 °C. In the same fashion as Hadean zircons, Arunachal leucogranite and host gneiss zircons are dominated by muscovite + quartz inclusions that yield formation pressures of 5–15 kbars. We suggest that the Arunachal leucogranites originated in the hanging wall of a megathrust that carried H2O-rich foreland sediments to depths of >20 km whereupon de-watering reactions released fluids that fluxed hanging wall anatexis. Modeling suggests the thermal structure of this continental collision environment may have been broadly similar to a Hadean ocean-continent subduction zone. The similarity of these two environments, separated by over 4 Ga may explain seemingly common features of the Hadean and Arunachal leucogranite zircons. Their key difference is the absence of metaluminous magmas in the continental collision environment, which is shielded from juvenile additions.
dating terrestrial impacts with Zircon (uranium-thorium)/helium geochronology
We investigate the presence of epitaxial overgrowth rims and 'reset' zircon, complete loss of radiogenic lead (Pb*), from terrestrial impactites to constrain the occurrence of such phenomenon in impact environments and their possible use in dating impact events. We also explore (U-Th)/He dating of zircon to evaluate this geochronometer in accurately identifying impact ages, particularly when no datable melt sheet exists. Our results show that (U-Th)/He ages of zircon from the brecciated, and presumably shocked target can accurately date an impact event and provides another tool to determine impact ages when no melt sheet exists, an alternative to problematic interpretations of commonly used apparent 40Ar/39Ar plateau ages. No evidence of epitaxial overgrowth rims and/or 'reset' zircon was observed, suggesting that zircons within shocked impactites have remarkably slow Pb diffusion and possibly explaining the relatively few 'reset' grains reported in terrestrial impactites.
Zircon saturation in impact melts
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. Modeling the occurrence and crystallization temperature spectrum for zircon in simulated impact melts; we predict the presence of such grains within impactites and evaluate the use of these grains in dating impact events. We also report U-Pb geochronology of sieve-textured, possibly poikilitic, zircon identified in the pseudotachylite 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 (Gnos et al., 2004; Liu et al., 2012; Grange et al., 2013) and used to determine the age of the Imbrium impact. Modeling in simulated lunar impact melts predicts crystallization of zircon in merely ~2% of events, due to the high [Zr] needed to nucleate zircon in lunar melt compositions. 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 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 under-saturated magma, as previously recognized in plagioclase. Although either scenario could be due to an impact, endogenic processes cannot be ruled out and thus lunar zircons may not be recording impact melting events. SIMS 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 identify the age of the Vredefort impact structure. Further understanding of the growth and isotopic effects on zircon associated with large impacts could form the basis of a tool to probe planetary impact histories.
Geochemical signatures and magmatic stability of terrestrial impact produced zircon
Understanding the role of impacts on early Earth has major implications to near surface conditions, but the apparent lack of preserved terrestrial craters >2 Ga does not allow a direct sampling of such events. Ion microprobe U–Pb ages, REE abundances and Ti-in-zircon thermometry for impact produced zircon are reported here. These results from terrestrial impactites, ranging in age from ~35Ma to ~2 Ga, are compared with the detrital Hadean zircon population from Western Australia. Such comparisons may provide the only terrestrial constraints on the role of impacts during the Hadean and early Archean, a time predicted to have a high bolide flux. Ti-in-zircon thermometry indicates an average of 773 °C for impact-produced zircon, ~100 °C higher than the average for Hadean zircon crystals. The agreement between whole-rock based zircon saturation temperatures for impactites and Ti-in zircon thermometry (at aTiO2=1) implies that Ti-in-zircon thermometry record actual crystallization temperatures for impact melts. Zircon saturation modeling of Archean crustal rock compositions undergoing thermal excursions associated with the Late Heavy
Bombardment predicts equally high zircon crystallization temperatures. The lack of such thermal signatures in the Hadean zircon record implies that impacts were not a dominant mechanism of producing the preserved Hadean detrital zircon record.
Experimental Calibration and development of sims technique for a Microscale White Mica Barometer
The Si per formula unit (Sipfu) content of potassic white mica varies according to the pressure dependent Tschermak substitution (Mg, Fe2+) + Si = Al + Al, from ideal muscovite [K(Al2)Al(Si3010)(F,OH)2], through intermediate phengite compositions, to the theoretical Al-celadonite endmember [K(Mg, Fe2+)Al(Si4010)(F,OH)2]. The composition of white mica inclusions within Hadean
(>4 Ga) Jack Hills zircons, together with results from Ti-in-zircon thermometry, have been used to suggest their formation in a low heat flow environment, perhaps analogous to a modern-day convergent plate boundary. However, only a small fraction of such inclusions are large enough to analyze using EPMA and we are thus developing a SIMS analysis method capable of μm -scale measurements, relating Si/Al ion yields to Sipfu. We have experimentally produced neoformed, sub-μm micas and stabilized seeded mica under appropriate P-T conditions and are pursuing growth of ca. 1 μm crystals for SIMS analysis. Because Si and Al are the primary variants in the Tschermak substitution, the Si/Al ratio provides a potential proxy for Sipfu. Analysis of a large suite of potassic white micas (both experimental and natural samples) across a range of Sipfu from 2.7-3.7 using EPMA shows a high
degree of correlation with molar Si/Al. We then used these characterized standards to determine the relative ion yields (28Si+/27Al+) under SIMS O- bombardment to achieve quantitative Si/Al measurements of μm-sized unknowns. This approach has high sensitivity due to: the high abundance of Si and Al in muscovite, the high proportion of 28Si and 27Al relative to other isotopes (92% and 100%, respectively), and the relatively high useful yields of both Si+ and Al+ (0.7% and 2.7%, respectively).
Interferences on 27Al+ and 28Si+ by 26MgH+ and 27AlH+ respectively, can be separated at a mass resolving power of only ~3000 permitting high secondary ion transmission. The potential for a ~1 μm lateral resolution probe of Si/Al in white micas would permit most of the mica inclusion population found in Hadean zircons to be characterized and opens up the possibility of a routine method of thermobarometry in muscovite inclusion-bearing zircons of all ages.
Origin and mixing timescale of Earth’s late venee
Experimental studies on the partitioning behavior of highly siderophile elements (HSE) between silicate and metallic melts imply that the Earth’s mantle should have been highly depleted in these elements by core formation in an early magma ocean. However, present HSE contents of the Earth’s mantle are ~3 orders of magnitude higher than that expected by experiments. The apparent over-abundance of HSE has commonly been explained by the addition of meteoritic material in the “late veneer” which describes the exogenous mass addition following the moon forming impact and concluding with the late heavy bombardment at ~3.8-3.9 Ga. The strongest evidence for this theory is that the platinum group element (PGE) contents in today’s mantle are present in chondritic relative abundances, as opposed to a fractionated pattern expected with metal-silicate partitioning.
Archean komatiites indicate that the PGE content of the Earth’s mantle increased from about half their present abundances at 3.5 Ga to their present abundances at 2.9 Ga. This secular increase in PGE content suggests a progressive mixing of the late veneer material into the Earth’s mantle. However, this time scale also implies that the whole mantle was relatively well mixed by 2.9 Ga.
We use a compilation of existing isotopic and trace element data in order to constrain the origin and composition of the late veneer. We use PGE abundances, W abundances and W isotopic compositions in chondritic meteorites and the primitive upper mantle to compute the amount of mass delivered during the late veneer and find the late veneer mass to be ~0.6 % the mass of the bulk silicate Earth (consistent with earlier estimates). We also use the 187Re-187Os and 190Pt-186Os systems to constrain the composition and timing of delivery of the impacting population.
We model the efficiency of mantle mixing in this time frame by using 3-dimensional numerical geodynamical simulations and geochemical constraints. Initial parameters include the amount of mass delivered in the late veneer and the Archean internal heating which is at least 4 times higher than the present values, due to the higher abundance of radioactive elements. Another important parameter is the mechanism of mass addition to the Earth. We test three end-member scenarios: (1) a single very large impactor accounting for the entire mass addition, (2) sprinkling of a large number of small impactors over the whole Earth which then mix into the mantle, or (3) by using a size/frequency distribution estimated from the lunar cratering record and corrected for the difference in gravitational cross section of the Earth and the Moon.