Hans Joachim Lippolt

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

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.

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

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.

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

Abstract U-He dating, which during its history has been very problematic, might find special application in the field of hydrothermal mineralizations where other methods generally fail. The mineral apatite is a potential candidate for this method. In order to shed light on various aspects of apatite dating by the (U + Th)/He method, two samples each from different geochemical environments (plutonic, pneumatolytic and hydrothermal) were probed. Isotope dilution techniques were used for concentration analyses from which 4He model dates, also called “helium indices” after N.B. Keevil, were calculated. 4He diffusion measurements were carried out for comparison of the 4He retentivities between the samples. Comparisons of the apatite He indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the (U + Th)/He dating method applied to apatite yields an age of cooling rather than an emplacement age. The calculated closure temperatures are similar to those obtained by the fission-track method (∼ 100°C). Our results suggest that the high-temperature apatites (plutonic and pneumatolytic) may be more suitable for He dating than hydrothermal apatite. Important criteria for the dating of apatite by (U + Th)/He are the presence of moderate radionuclide concentrations (tens of ppm) and a minimum abundance of inclusions.

4He diffusion in 40Ar-retentive minerals☆

Abstract 4He vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in K-Ar chronometry such as hornblende, nepheline and others. The study aims at 4He diffusion parameters for uppermost crustal temperatures and thus at He retentivities of the minerals. The results indicate long-time near-quantitative 4He retention in hornblende, pyroxene, langbeinite and perhaps nepheline, for 0.5 mm grains at least at temperatures below about 70°C. Sanidine and muscovite lose 4He over geologic periods at lower temperatures. Only the minerals of the first group are possible (U + Th)/He chronometers or geothermometers provided that other dating problems, such as alpha loss and U migration, can be settled. Closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170°C. Tentative U-He indices indicate that diffusion parameters of the same order of magnitude as determined for the He-retentive minerals may be used to select minerals to be further examined for their (U + Th)/He datability, e.g. ore minerals.

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

Abstract 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).

Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia

Pb isotope analyses have been carried out on 42 zircon grains from a Western Australian metaconglomerate using stepwise Pb-evaporation directly in the ion source of a thermal ionization mass spectrometer. The metaconglomerate is from the Archean Jack Hills Metasedimentary Belt, and is known from ion microprobe (“SHRIMP”) analyses to contain a complex zircon population with ages between 4.2 Ga and 3.1 Ga. The same complex pattern of ages is found by the Pb evaporation studies. Five grains yielded minimum crystallization ages from 4.17 Ga to 4.07 Ga. The main population appears significantly younger, having been generated at about 3.55-3.3 Ga. The agreement between the two analytical approaches confirms the SHRIMP results and demonstrates the value of the stepwise-evaporation technique in determining the age patterns of mixed zircon populations. In many of the evaporative Pb isotope records the 207/206 ratios remained constant for all evaporation steps, which we interpret as evaporation from concordant zircon phases. However, for the majority of zircons 207/206 ratios increased with increasing evaporation temperature, and usually approached constant values during evaporation at the highest temperatures. This can be attributed to mixing of different radiogenic Pb components from either crystalline zircon phases of different ages or from domains of isotopically disturbed metamict zircon. Present results confirm > 4 Ga zircon ages in the metaconglomerate from the Jack Hills and substantiate formation of crust at a very early stage in the evolution of the earth. Results also confirm a major crust-forming event 3.55-3.3 Ga ago.

40Ar/39Ar dating of central European K–Mn oxides — a chronological framework of supergene alteration processes during the Neogene

K-bearing Mn oxides may potentially constitute useful objects for isotopic dating of ore-forming events. A comprehensive 40Ar/39Ar study performed on supergene K–Mn oxides sampled from different sub-alpine mountain terrains in Germany and France has been undertaken. The objective of these investigations was to provide new insight into how and when these secondary Mn accumulations may have formed. Developed in supergene environments at the expense of Mn2+/Mn3+-bearing precursor minerals, the Mn4+ oxides occur either as pseudomorphic ores or as cavity-fillings and linings. The isotopic ages range from 25 to 1 Ma, indicating intense chemical weathering, especially during the Miocene and Pliocene. It is yet too early to decide whether the age range represents a more or less continuous process or distinct weathering episodes. Formation of supergene Mn oxides may result from combined climatic and tectonic factors: local uplift, exhumation, and associated fracturing of rocks provided fresh mineral surfaces for percolating meteoric fluids that induced subsequent weathering under warm–temperate to subtropical conditions.

The cooling history of the late Pliocene Eldzhurtinskiy granite (Caucasus, Russia) and the thermochronological potential of grain-size/age relationships

Abstract Isotopic age investigations have been carried out on samples from the late Pliocene Eldzhurtinskiy granite (Great Caucasus, Russia) in order to elucidate the cooling history of this kilometere sized plutonic body. The samples were taken from a horizontal profile (from the edge to the centre of the granite) and from a vertical profile (from the top to a depth of 4570 m). The 40Ar/39Ar results on biotites ranging from 2.5 to 1.2 Ma indicate a vertical age zoning of the granite caused by regional uplift. An uplift rate of 4 mm/a is calculated from the age differences. Together with the results of fission track dating on apatite and zircon and the present-day rock temperatures the biotite ages can be used to evaluate cooling rates below 400°C. The cooling rates of about 180°C/Ma are relatively high and spatially and temporarily fairly uniform throughout the granite. A geothermal gradient of 45°C/km is derived from uplift and cooling rates; this is identical to the present-day borehole gradient. The 40Ar/39Ar ages of distinct biotite grain fractions from one and the same sample specimen are discordant. Generally, they show clear age/grain-size correlation, as expected from Ar diffusion theory for cooling systems. Thus, the biotite results support the additional detailed thermochronological information. The cooling rates in the temperature range of biotite closure, based on the grain-size/age correlations of the biotites, are close to 180°C/Ma. They provide evidence that within a certain temperature interval the time-temperature path of a rock may be determined exclusively by isotopic dating of different grain-size fractions of only one type of mineral.

Botryoidal hematite from the Schwarzwald (Germany): heterogeneous uranium distributions and their bearing on the helium dating method

Abstract Neutron-induced fission tracks of small grain mounts of two co-genetic botryoidal hematites (Eisenbach hematite-manganese mineralization district, Mittelschwarzwald, Germany) reveal substantial heterogeneous 235 U distributions. 235 U fission tracks in the three shells of one hematite yield total uranium concentrations of 26, 48 and 160 ppm ( 2σerrors of approximately20% ), respectively. The other hematite consists of a coherent mineralogical unit devoid of shell-type features, in which four localities have been probed with a range in total uranium between 26 and 600 ppm. Induced fission tracks from a larger section of the single unit hematite show a subparallel arrangement of five well-defined 235 U zones with densities ranging from about 10 6 tracks/cm 2 in zone 5 to approximately 25 × 10 6 tracks/cm 2 in zone 2. Additionally, in zones 3–5 gradual internal track density variations of over 100% can be recognized. A traverse shows that there is an overall decrease in radioactivity from zone 1 to zone 5. An autoradiograph confirms the existence of five zones with different activities and allows correlation of the high-activity zone 2 with a 0.5 mm thin internal hematite micro-layer. Analyses by mass spectrometer of the triple-shell hematite, resulted in the following concentrations: 26, 45 and 166 ppm uranium ( 2σerrors of1% ) and 0.55–3.25 μl/g 4 He ( 2σerrors of3% ). The four probed localities in the single unit hematite show smaller variations at 40–73 ppm uranium ( 2σerrors of0.5% ) and 0.75–1.37 μl/g 4 He ( 2σerrors of3% ). α-spectrometry of thorium yields values in the 1–5 ppm range for both hematites. The uranium and 4 He concentration differences measured allow construction of two internal (U, Th)-He isochrons yielding identical ages ( 2σerror ) of 156 ± 1 Ma and 155 ± 6 Ma. Concordance of both isochrons in our opinion is strong evidence for a geologically significant age. We interpret the data as emplacement ages. Further, we suggest that botryoidal hematite has the potential to be a prolific chronometer for the helium method on Mesozoic hematite-bearing mineralizations.

(U+Th)–He evidence of Jurassic continuous hydrothermal activity in the Schwarzwald basement, Germany

Abstract (U+Th)–He model and isochron ages of 21 botryoidal hematites collected from the Hohberg base-metal vein system in the Triberg granite complex, Mittelschwarzwald (Germany), cover most of the Jurassic period. The possibility of quantitative 4 He loss triggered by successive hydrothermal events, although conceivable for individual hematites — based on a 4 He retentivity study and calculated opening temperatures of hematite for 4 He of >(90–180)°C — can largely be ruled out. Thus the range of model and isochron ages indicates that the majority of the hematites are not cogenetic. This is supported by a broad range of Pb total concentrations in the Hohberg botryoidal hematites (150–2700 ppm), by comparison with virtually identical Pb total concentrations found in cogenetic hematites from four different vein systems in the Schwarzwald. The Th/U ratios of the hematites vary by a factor of ∼700 (0.01–7.25) mainly as a result of the range of Th concentrations (0.1–74 ppm). The incorporation of substantial amounts of Th in hematite is thought to result from increased Th transport mobility, possibly caused by association with colloids and/or complexing in the ore solution. δ 18 O analyses of a ∼160 Ma old cogenetic hematite–quartz pair indicate that precipitation occurred from a fluid of magmatic origin at a temperature of ∼180°C. The regional temperature peak in the basement at the present erosional surface during the Jurassic (∼130°C) provides a lower temperature limit of hematite formation. This implies that hematite emplacement occurred at temperatures roughly ranging between ∼130 and ∼180°C. The Jurassic continuous hydrothermal activity suggests that the pre-rifting European craton is characterized by local thermal fluxes during a long time span.