Microanalytical trace element techniques (such as ion probe or laser ablation ICP-MS) are hampered by a lack of well characterized, homogeneous standards. Two silicate glass reference materials produced by National Institute of Standards and Technology (NIST), NIST SRM 610 and NIST SRM 612, have been shown to be homogeneous and are spiked with up to sixty one trace elements at nominal concentrations of 500 μg g-1 and 50 μg g-1 respectively. These samples (supplied as 3 mm wafers) are equivalent to NIST SRM 611 and NIST SRM 613 respectively (which are supplied as 1 mm wafers) and are becoming more widely used as potential microanalytical reference materials. NIST however, only certifies up to eight elements in these glasses. Here we have compiled concentration data from approximately sixty published works for both glasses, and have produced new analyses from our laboratories. Compilations are presented for the matrix composition of these glasses and for fifty eight trace elements. The trace element data includes all available new and published data, and summaries present the overall average and standard deviation, the range, median, geometric mean and a preferred average (which excludes all data outside ± one standard deviation of the overall average). For the elements which have been certified, there is a good agreement between the compiled averages and the NIST data. This compilation is designed to provide useful new working values for these reference materials.

A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials

No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as approximately 4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404+/-8 Myr--about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (delta18O values from 7.4 to 5.0%), indicating that it formed from an evolving magmatic source. The evolved chemistry, high delta18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a delta18O value from 8.5 to 9.5%. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.

The effects of long alpha-stopping distances on (U‐Th)/He ages

High-precision stepped-heating experiments were performed to better characterize helium diffusion from apatite using Durango fluorapatite as a model system. At temperatures below 265°C, helium diffusion from this apatite is a simple, thermally activated process that is independent of the cumulative fraction of helium released and also of the heating schedule used. Across a factor of ∼4 in grain size, helium diffusivity scales with the inverse square of grain radius, implying that the physical grain is the diffusion domain. Measurements on crystallographically oriented thick sections indicate that helium diffusivity in Durango apatite is nearly isotropic. The best estimate of the activation energy for He diffusion from this apatite is E_a = 33±0.5 kcal/mol, with log(D_0) = 1.5±0.6 cm^2/s. The implied He closure temperature for a grain of 100 μm radius is 68°C assuming a 10°C/Myr cooling rate; this figure varies by ±5°C for grains ranging from 50 to 150 μm radius. When this apatite is heated to temperatures from 265 to 400°C, a progressive and irreversible change in He diffusion behavior occurs: Both the activation energy and frequency factor are reduced. This transition in behavior coincides closely with progressive annealing of radiation damage in Durango apatite, suggesting that defects and defect annealing play a role in the diffusivity of helium through apatite.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900348

Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite

The possibility of dating minerals by the accumulation of ^4He from U and Th decay
has been recognized for many years (e.g., Strutt 1905), but in the century since the idea
was first conceived, the method has rarely been applied successfully. After several
investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957;
Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973;
Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding
unreliable and usually low ages, presumably as a result of diffusive He loss possibly
associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the
method by proposing that in the case of apatite, He ages might be meaningfully
interpreted as ages of cooling through very low temperatures. Laboratory diffusion data
presented by these authors indicated a closure temperature of about 100oC, a value
supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al.
1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages
increase systematically with sample elevation in a mountain range, as expected for
exhumation-induced cooling through a low closure temperature. Based on the strength of
these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997;
House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has
focused on applications of apatite He thermochronometry. There is also renewed interest
in He dating of other U- and Th-bearing minerals both for dating mineral formation and
for thermochronometry. For example, Lippolt and coworkers have undertaken detailed
studies of He diffusion and dating of various phases, most notably hematite formed in
hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et
al. 1993; Wernicke and Lippolt 1994a,b).
Here I present an overview of recent techniques, calibrations, and applications of the
(U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in
this field. Much of this paper focuses on apatite, because the He behavior and requisite
analytical techniques are better established for this phase than for other target minerals,
such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior
required for thermochronometric applications, yet recent work is also considering applications
to direct dating, for example, of young tephras (Farley et al. 2001).

(U-Th)/He Dating: Techniques, Calibrations, and Applications

Recently groundmass samples of two high-K volcanic rocks (MDO-31A, 36-I) and a low-K basalt (BB-6) have been proposed for Quaternary KAr standardization. The ages given by the suppliers are ∼ 250 ka (MDO-31A), 4 ka (36-I) and 450 ka (BB-6). Sample 36-I could be used as zero-age sample for many purposes. 40Ar39Ar incremental heating measurements reveal no severe distortions for the KAr systematics of the groundmass samples. Therefore they should be suitable for Quaternary KAr age standardization. The BB-6 basalt standard shows a slightly distorted 40Ar39Ar age spectrum and obviously contains mineral phases with excess 40Ar. These properties restrict the suitability of BB-6 for KAr standardization to a certain degree. The geochemical biotite standard Mica-Fe showing both varying K and 40Ar concentrations and disturbed 40Ar39Ar patterns should not be used as reference material in KAr chronometry. First K and Ar results on a recently prepared Tertiary biotite (HD-B1) are presented. Four kg of this pure biotite (t=25 Ma, K=8.0%, 40Ar★=7.7 nl g−1 (STP) = 86% of total 40Ar) are provided for distribution to interested laboratories.

Examination of some proposed K-Ar standards: 40Ar39Ar analyses and conventional KAr data

(Uranium + thorium)/helium dating of apatite: experience with samples from different geochemical environments

Sanidine ages of various coal tonstein or tuff beds from six Central European Upper Carboniferous deposits have been determined using the 40Ar/39Ar stepwise heating method. All the sanidine samples show 40Ar/39Ar release spectra with well-defined plateaus. The measured ages are interpreted as the sedimentation ages of the coal tonsteins and tuffs. The results of these age determinations relate to the Carboniferous time scale and the authors thus suggest new age values for the Upper Carboniferous stage boundaries, which are ∼ 5–10 Ma higher than those proposed previously: base of Namurian: 326 Ma, base of Westphalian: 315 Ma, base of Stephanian: 306 Ma, base of Autunian: 300 Ma.

40Ar/39Ar ages of tonstein and tuff sanidines: New calibration points for the improvement of the Upper Carboniferous time scale

4He diffusion in 40Ar-retentive minerals

The loss of 39Ar from minerals in the course of neutron activation for 40Ar/39Ar dating is studied by directly measuring the loss rates in vacuum-sealed ampoules. Biotite shows 39Ar losses between 0.1% and 16%. These losses are predominantly due to diffusion processes from K-poor alteration-phase intergrowths in the biotites at the elevated temperatures during the irradiation. Estimates for the irradiation temperatures range from 150° to 180°C. Direct 39Ar recoil loss from biotite seems to be minor compared to difussion loss of recoil-implanted 39Ar. Precise 40Ar/39Ar dating of biotites therefore requires the measurement of the 39Ar losses during irradiation. Glauconite loses not only neutron-induced Ar isotopes (39Ar: 20–22%, 37Ar: 17–19%) but also radiogenic 40Ar (∼9%). Slight 39Ar losses are also observed for light micas (0.2% and 0.35%), hornblendes (0.1%) and sanidines (200 and 700 ppm).

Hans J. Lippolt

Papers

Examination of some proposed K-Ar standards: 40Ar39Ar analyses and conventional KAr data

(Uranium + thorium)/helium dating of apatite: experience with samples from different geochemical environments

40Ar/39Ar ages of tonstein and tuff sanidines: New calibration points for the improvement of the Upper Carboniferous time scale

4He diffusion in 40Ar-retentive minerals

Kinetics of Ar isotopes during neutron irradiation: 39Ar loss from minerals as a source of error in 40Ar/39Ar dating