K. Nishiizumi

Chemical isolation of quartz for measurement of in-situ-produced cosmogenic nuclides

Abstract An isolation method relying totally on chemical steps was developed to separate large quantities (10–200 g) of clean mono-minerallic quartz samples from a variety of terrestrial rocks and soils for the purpose of measuring 10Be ( t 1 2 = 1.5 Myr ) and 26Al ( t 1 2 = 0.705 Myr ) produced by cosmic rays in situ in the quartz phase. The procedure consists of grinding the sample, heating it in HCl, and treating it with a series of leaches using a dilute HF/HNO3 mixture in a heated ultrasonic tank. The purified quartz was also used for the measurements of in-situ -cosmic-ray-produced 21Ne and 14C ( t 1 2 = 5730 myr ). The method is applicable to any problem requiring purified quartz on a large scale.

Cosmic ray produced 10Be and 26Al in Antarctic rocks: exposure and erosion history

We have measured cosmic ray produced (t1/2 = 1.5 million years) 10Be and (t1/2 = 0.705 million years) 26Al in purified quartz fractions of selected rock samples from Antarctic mountains. From these data we calculate (1) mean erosion rates, for the limiting case of steady-state surface exposure to cosmic rays, and (2) minimum exposure ages, for the limiting case of no erosion. Calculated mean erosion rates are very low, on the order of a few times 10−5 cm/yr; we believe the sampling to be sufficient to generalize this result to exposed bedrock in Antarctica. In favorable cases it is possible to distinguish between the limiting cases: steady-state erosion seems a better description in such cases. Most samples, including some taken a few meters above the present ice level, seem to have been exposed for millions of years, without major episodes of burial or abrasion by ice.

Cosmic ray exposure ages of chondrites, pre-irradiation and constancy of cosmic ray flux in the past

A systematic calibration of the production rate of one specific cosmic-ray-produced nuclide in chondrites, that of21Ne, was achieved by using four independent methods:P21(1.11) = 0.507 ± 0.039, 0.302 ± 0.013, 0.312 ± 0.017and0.292 ± 0.019 (in units of 10−8 cm3 STP/g My) based on26Al-age,53Mn-age,81Kr-83Kr and22Na-22Ne methods, respectively. These production rates are all normalized to a shielding parameter ratio22Ne/21Ne= 1.11 and to the chemical composition of L chondrites. The results obtained by the latter three methods are in good agreement, but they disagree in a systematic way with the26Al-age calibration. Based on these results, we recommend a valueP21(1.11) = 0.31 and a production rate equation:P21 = 4.845 P21 (1.11) F[21.77(22Ne/21Ne) − 19.32]−, whereF = 1.00 for L and LL, andF = 0.93 for H chondrites, for the calculation of cosmic ray exposure ages on the basis of Ne concentrations. In an attempt to assess possible causes for this discrepancy, we discuss the26Al half-life measurements, we evaluate effects resulting from pre-irradiation of meteorites, and we discuss the evidence regarding the constancy of the cosmic ray flux in the past, in the light of some recent astronomical observations.

Update on terrestrial ages of Antarctic meteorites

Terrestrial ages of Antarctic meteorites are one of the few parameters that will help us to understand the meteorite concentration mechanism on blue-ice fields. Traditionally, terrestrial ages were determined on the basis of {sup 36}Cl in the metal phase, which has an uncertainty of about 70 ky. For young meteorites (< 40 ky), the terrestrial age is usually and most accurately determined using {sup 14}C in the stone phase. In recent years two methods have been developed which are independent of shielding effects, the {sup 10}Be-{sup 36}Cl/{sup 10}Be method and the {sup 41}Ca/{sup 36}Cl method. These methods have reduced the typical uncertainties in terrestrial ages by a factor of 2, to about 30 ky. The {sup 10}Be-{sup 36}Cl/{sup 10}Be method is quite dependent on the exposure age, which is unknown for most Antarctic meteorites. The authors therefore also attempt to use the relation between {sup 26}Al and {sup 36}Cl/{sup 26}Al to derive a terrestrial age less dependent on the exposure age. The authors have measured the concentrations of cosmogenic {sup 10}Be, {sup 26}Al and {sup 36}Cl in the metal phase of {approximately} 70 Antarctic meteorites, from more than 10 different ice-fields, including many new ones. They then discuss the trends in terrestrial ages of meteorites from different ice-fields.

Development of 36Cl standards for AMS

Abstract Large-quantity dilutions of the National Institute of Standards and Technology (NIST) 36Cl standard (SRM 4943) to 36 Cl Cl ratios of 5.000 × 10−13, 1.600 × 10−12, 5.003 × 10−12 and 1.000 × 10−11 have been prepared for AMS standards. Dilutions were made with an estimated uncertainty better than 1% and were then checked in three separate runs using the AMS facility at the University of Rochester. The results show excellent agreement between the gravimetric dilution factors and AMS measurements.

10Be and36Cl depth profiles in an Apollo 15 drill core

Cosmic-ray-produced10Be (t1/2 = 1.6 × 106 years) and36Cl (t1/2 = 3.0 × 105 years) have been measured in the Apollo 15 long core for study of galactic cosmic ray production profiles using tandem accelerator mass spectrometry. From these experiments, the half-attenuation length for10Be production and36Cl production were calculated to be 120 g/cm2 and 132 g/cm2 (150–400 g/cm2 region). The measured half-attenuation length for10Be is slightly longer than that predicted by the Reedy-Arnold theoretical model. The flatter and somewhat deeper maximum seen in the36Cl profile compared to the10Be,26Al and53Mn profiles can be explained by production from secondary thermal neutrons on35Cl.

In situ10Be-26Al exposure ages at Meteor Crater, Arizona

A new method of dating the surface exposure of rocks from in situ production of 10Be and 26Al has been applied to determine the age of Meteor Crater, Arizona. A lower bound on the crater age of 49,200 ± 1,700 years has been obtained by this method.

Cosmic-ray-produced 21Ne in terrestrial quartz: the neon inventory of Sierra Nevada quartz separates

The study of cosmic-ray-produced radioactive and stable nuclides on the surface of the Earth can provide relevant geomorphological and glaciological information. At present, the cosmic ray production rates of stable 21Ne are not well known. This study attempts to remedy the situation by determining the production rate ratio of 21Ne and 26Al, P21P26, in quartz. 26Al concentrations and P26 rates have previously been investigated for quartz separates of Sierra Nevada rocks which were brought to the surface by glacial scouring during the Tioga period at the end of the last ice age [1]. We used splits of the same samples for our studies and found that Ne in these rocks represents a mixture of several components: trapped Ne, nucleogenic 21Ne and 22Ne produced by (α,n) reactions in oxygen and fluorine, respectively, as well as cosmic-ray-produced Ne, which is the component of interest in this study. The trapped component was substantially lost in one sample (W86-12) by crushing and by a density separation of the grain sizes 38–90 μm and 90–125 μm, permitting the resolution of the in situ produced 21Ne into cosmic-ray spallation and (α,n) produced components and the determination of a lower limit to P21P26. In a second sample (W86-8) one split contained small enough amounts of nucleogenic 21Ne to permit the determination of a reasonable upper limit to P21P26. The two ratio determinations are consistent within error limits and the value adopted, 0.65 ± 0.11 (2σ), agrees with ratios observed in extraterrestrial matter. Apparently, P21P26 is thus not very sensitive to the neutron spectrum. However, the observed production rate ratio is substantially larger than theoretical estimates for Si targets, reflecting poorly known neutron excitation functions. The above P21P26 value, coupled to the observed 26Al production rate [1], corresponds to a 21Ne production rate of P21 = 21 atoms g−1 a−1 in quartz or to P21 = 45 atoms (g Si)−1 a−1 (at sea level and high latitudes). This rate is based on an adopted exposure age of 11,000 yr for our quartz samples.

Cosmogenic nuclides in the Kirin chondrite

Abstract Radioactive and stable cosmogenic nuclides have been determined in the various fragments of the Kirin chondrite (H5). Experimental results obtained from our samples are described and compared with similar data obtained in other laboratories. Cosmogenic 54 Mn, 22 Na, 60 Co, 26 Al, 53 Mn, 40 K (in the metal phase), and light noble gases were measured. Based on these data, the irradiation history of this meteorite can be explained in terms of (1) a multi-stage exposure which involves a first-stage irradiation for 10 7 years and a second stage for about 4×10 5 years, and (2) depth effects in the productions of the nuclides in 4π(second stage) and 2π(first stage) geometries. These conclusions are consistent with those of our previous work which was based on a limited number of samples.

Measurements of 36Cl in Antarctic meteorites and Antarctic ice using a Van de Graaff accelerator

Abstract Cosmic-ray produced 36 Cl( t 1/2 = 3.0 × 10 5 years) has been measured in four Antarctic meteorites and one sample of Antarctic ice using a tandem Van de Graaff accelerator as an ultrasensitive mass spectrometer with the extremely low background level of 36 Cl/Cl −16 . Results from this ion counting technique (applied here to extraterrestrial materals for the first time) are used to support a two-stage irradiation model for the Yamato-7301 and Allan Hills-76008 meteorites and to show a long terrestrial age (0.7 ± 0.1 m.y.) for Allan Hills-77002. Yamato-7304 has a terrestrial age of less than 0.1 m.y. The 36 Cl content of the Antarctic ice sample from the Yamato Mountain area implies that the age of the ice cap at this site is less than one 36 Cl half-life.