Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with Ar/Ar dating

(U-Th)/He chronometry of zircon has a wide range of potential applications including thermochronometry, provided the temperature sensitivity (e.g., closure temperature) of the system be accurately constrained. We have examined the characteristics of He loss from zircon in a series of step-heating diffusion experiments, and compared zircon (U-Th)/He ages with other thermochronometric constraints from plutonic rocks. Diffusion experiments on zircons with varying ages and U-Th contents yield Arrhenius relationships which, after about 5% He release, indicate E_a = 163-173 kJ/mol (39-41 kcal/mol), and D₀ = 0.09-1.5 cm²/s, with an average E_a of 169 ± 3.8 kJ/mol (40.4 ± 0.9 kcal/mol) and average D₀ of 0.46^+0.87_−0.30 cm²/s. The experiments also suggest a correspondence between diffusion domain size and grain size. For effective grain radius of 60 μm and cooling rate of 10°C/myr, the diffusion data yield closure temperatures, T_c, of 171-196°C, with an average of 183°C. The early stages of step heating experiments show complications in the form of decreasing apparent diffusivity with successive heating steps, but these are essentially absent in later stages, after about 5-10% He release. These effects are independent of radiation dosage and are also unlikely to be due to intracrystalline He zonation. Regardless of the physical origin, this non-Arrhenius behavior is similar to predictions based on degassing of multiple diffusion domains, with only a small proportion (<2-4%) of gas residing in domains with a lower diffusivity than the bulk zircon crystal. Thus the features of zircon responsible for these non-Arrhenius trends in the early stages of diffusion experiments would have a negligible effect on the bulk thermal sensitivity and closure temperature of a zircon crystal.We have also measured single-grain zircon (U-Th)/He ages and obtained ⁴⁰Ar/³⁹Ar ages for several minerals, including K-feldspar, for a suite of slowly cooled samples with other thermochronologic constraints. Zircon He ages from most samples have 1 σ reproducibilities of about 1-5%, and agree well with K-feldspar ⁴⁰Ar/³⁹Ar multidomain cooling models for sample-specific closure temperatures (170-189°C). One sample has a relatively poor reproducibility of ∼24%, however, and a mean that falls to older ages than predicted by the K-feldspar model. Microimaging shows that trace element zonation of a variety of styles is most pronounced in this sample, which probably leads to poor reproducibility via inaccurate α-ejection corrections. We present preliminary results of a new method for characterizing U-Th zonation in dated grains by laser-ablation, which significantly improves zircon He age accuracy.In summary, the zircon (U-Th)/He thermochronometer has a closure temperature of 170-190°C for typical plutonic cooling rates and crystal sizes, it is not significantly affected by radiation damage except in relatively rare cases of high radiation dosage with long-term low-temperature histories, and most ages agree well with constraints provided by K-spar ⁴⁰Ar/³⁹Ar cooling models. In some cases, intracrystalline U-Th zonation can result in inaccurate ages, but depth-profiling characterization of zonation in dated grains can significantly improve accuracy and precision of single-grain ages.     

Evaporites and siliciclastics of the Permian Nippewalla Group of Kansas, USA: a case for non-marine deposition in saline lakes and saline pans

The mid‐Permian Nippewalla Group of Kansas consists of bedded evaporites, red‐bed siliciclastics and grey siliciclastics deposited in a non‐marine environment. Lithologies and sedimentary features indicate lacustrine and aeolian deposition, subaerial exposure and palaeosol formation. Grey siliciclastic mudstones characterized by planar and convolute laminations, ostracods, peloids and plant material represent a freshwater‐brackish perennial lake facies. Bedded anhydrites containing gypsum‐crystal pseudomorphs, clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes. Bedded halites consist of chevron and cumulate crystals, dissolution surfaces and pipes and mudcracked microcrystalline salt crusts, which were deposited in saline pans dominated by flooding, evaporative concentration and desiccation. Chaotic halite, composed of red‐bed mudstone and siltstone with displacive halite crystals, formed in saline mudflats. Red‐bed mudstone and siltstone with little or no displacive halite, but with abundant cracking, root and plant features, suggest deposition in a dry mudflat. Red‐bed sandstone, composed of well‐sorted, well‐rounded quartz grains cemented with halite, indicate aeolian and rare shallow‐water deposition. Most deposition took place in halite‐dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. Evaporation, desiccation, flooding and wind played significant roles in this environment. The Nippewalla Group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non‐marine, acidic saline pan system in the mid‐continent of North America. The problem of distinguishing between ancient marine and non‐marine evaporites, as well as recognizing those evaporites deposited in acid settings, with detailed field, core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed. In addition, interpretations of mid‐Permian palaeoclimate data in the form of short‐term air temperature proxies within longer‐term wet–dry trends have been made. These data provide a new palaeogeographic and palaeoclimatic model for the mid‐Permian of western Pangaea.     

Textures in Partially Solidified Crystalline Nodules: a Window into the Pore Structure of Slowly Cooled Mafic Intrusions

Abundant glass is present along grain boundaries in coarse-grained,glass-bearing, crystalline gabbroic and peridotitic nodulesentrained and erupted in lavas from Iceland, Santorini and MaunaLoa (Hawaii), even when the total porosity is less than a fewvolume per cent. The glass films vary from a few microns toa few tens of microns thick, and are associated with stringsof small lensoid grain boundary pockets formed by impingementduring crystal growth. Additional porosity occurs as extensiveliquid-filled pockets adjacent to included grains within oikocrystsand as large triangular pockets formed by impingement of planar-sidedgrains. Interstitial material within glass films, and the irregularityof film thickness along a single grain boundary, suggest thatthe present pore structure is representative of the pore structurebefore entrainment and eruption. Pore geometry is consistentwith a dominant control by crystal growth during solidification,with little or no evidence for control by minimization of internalenergies driven by textural equilibration. Similarities betweenliquid distribution in the crystalline nodules and that of late-stage,interstitial phases in fully solidified mafic cumulates fromthe Rum and Skaergaard intrusions demonstrate that the crystallinenodules provide information about the latest stages of solidificationin slowly cooled mafic plutons. The highly permeable networkof intersecting liquid films, lenses and pockets may promotein situ crystallization in the solidifying mush, explainingthe common presence of adcumulates in such intrusions. KEY WORDS: textures; liquid distribution; mafic cumulates; crystalline nodules     

Use of archaeology to date liquefaction features and seismic events in the New Madrid seismic zone,Central United States

Prehistoric earthquake-induced liquefaction features occur in association with Native American occupation horizons in the New Madrid seismic zone. Age control of these liquefaction features, including sand-blow deposits, sand-blow craters, and sand dikes, can be accomplished by extensive sampling and flotation processing of datable materials as well as archaeobotanical analysis of associated archaeological horizons and pits. This approach increases both the amount of carbon for radiocarbon dating and the precision dating of artifact assemblages. Using this approach, we dated liquefaction features at four sites northwest of Blytheville, Arkansas, and found that at least one significant earthquake occurred in the New Madrid seismic zone between A.D. 1180 and 1400, probably about A.D. 1300 ± 100 yr. In addition, we found three buried sand blows that formed between 3340 B.C. and A.D. 780. In this region where very large to great earthquakes appear to be closely timed, archaeology is helping to develop a paleoearthquake chronology for the New Madrid seismic zone. © 1996 John Wiley & Sons, Inc.