Birch's diagram: Some new observations

Birch's diagram plotting the hydrodynamic sound velocity versus density for several metals is used in many of his publications and a number of textbooks to demonstrate the chemical changes from the earth's mantle to the core. This diagram is thoroughly discussed in this paper on the basis of theory and the periodic property for the density of elements. Birch's conclusion that “even without information concerning chemical abundances, these relations indicate that the mantle is composed principally of light elements and the core of elements of the iron group” is not convincing in the view of the present study. A more detailed velocity-density plot for all the solid elements having their densities in the range 1.5 – 8 g/cm³ excludes any elements lighter (of lower atomic weight or number) than vanadium as likely candidates for core values.     

Chemical heterogeneity of the Archaean mantle,composition of the earth and mantle evolution

New rare earth element (REE) data for Archaean basalts and spinifex-textured peridotites (STP) show a range of La/Sm ratios (chondrite-normalized) from 0.36 to 3.5, with the bulk of the data in the range 0.7–1.3. This supports the hypothesis, based on Sr isotope initial ratios, that the Archaean mantle was chemically heterogeneous. We suggest that the bulk mantle source for Archaean basaltic magmas was close to an undepleted earth material. An average chemical composition of the Archaean mantle is estimated using chemical regularities observed in Archaean STP and high-magnesian basalts. TiO₂ and MgO data show an inverse correlation which intersects the MgO axis at about 50% MgO (Fo₉₂). TiO₂ abundance in the mantle source is measured on this plot by assigning anMgO= 38% for the mantle. Concentrations of other elements are also estimated and these data are then used to obtain a composition for the bulk earth. We suggest an earth model with about 1.35 times ordinary chondrite abundances of refractory lithophile elements and about 0.2 times carbonaceous type 1 chondrite abundances of moderately volatile elements (such as Na, Rb, K, Mn). P shows severe depletion in the model earth relative to carbonaceous chondrites, a feature either due to volatilization or core formation (preferred). Our data support the hypothesis of Ringwood that the source material for the earth is a carbonaceous chondrite-like material.The generation of mid-ocean ridge basalts (MORB) is examined in the light of the model earth composition and Al₂O₃/TiO₂, CaO/TiO₂ ratios. It is suggested that for primitive basalts, these values can be used to predict the residual phases in their source. Comparison of chemical characteristics of inferred sources for 2.7-b.y. Archaean basalts and modern “normal” MORB indicates that the MORB source is severely depleted in highly incompatible elements such as Cs, Ba, Rb, U, Th, K, La and Nb, but has comparable abundances of less incompatible elements such as Ti, Zr, Y, Yb. The cause of the depletion in the MORB source is examined in terms of crust formation and extraction of silica-undersaturated melts. The latter seems to be a more likely explanation, since the degree of enrichment of highly incompatible elements in the crust only accounts for up to 40% of their abundances in the bulk earth and cannot match the depletion pattern in normal MORB. A large volume of material, less depleted than the source for normal MORB must therefore exist in the mantle and can serve as the source for the ocean island basalts and “normal” MORB.Three different mantle evolution models are examined and each suggests that the mantle is stratified with respect to abundances of incompatible trace elements. We suggest that no satisfactory model is available to fully explain the spectrum of geochemical and geophysical data. In particular the Pb and Sr isotope data on oceanic basalts, the depletion patterns of MORB and the balance between lithophile abundances in the crust and mantle, are important geochemical constraints to mantle models. Further modelling of the mantle evolution will be dependent on firmer information on the role of subduction, mantle convection pattern, and basalt production through geologic time together with a better understanding of the nature of Archaean crustal genesis.     

Petrology of basalts from Loihi Seamount,Hawaii

Loihi Seamount is the southeasternmost active volcano of the Emperor-Hawaii linear volcanic chain. It comprises a spectrum of basalt compositional varieties including basanite, alkali basalt, transitional basalt and tholeiite. Samples from four dredge collections made on Scripps Institution of Oceanography Benthic Expedition in October 1982 are tholeiite. The samples include highly vesicular, olivine-rich basalt and dense glass-rich pillow fragments containing olivine and augite phenocrysts. Both quartz-normative and olivine-normative tholeiites are present. Minor and trace element data indicate relatively high abundances of low partition coefficient elements (e.g., Ti, K, P. Rb, Ba, Zr) and suggest that the samples were derived by relatively small to moderate extent of partial melting, of an undepleted mantle source. Olivine composition, MgO, Cr and Ni abundances, and Mg/(Mg+Fe), are typical of moderately fractionated to relatively unfractionated “primary” magmas. The variations in chemistry between samples cannot be adequately explained by low-pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated. Alternatively, a heterogeneous source with variable abundances of certain trace elements, or mixing of liquids, may have been involved. Data for ³He/⁴He, presented in a separate paper, implies a mantle plume origin for the helium composition of the Loihi samples. There is little variation in the helium isotope ratio for samples having different compositions and textures. The helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as Loihi alkalic basalts and the whole source region for Loihi may have a nearly uniform helium compositions even though other element abundances may be variable. Complex petrologic processes including variable melting, fractional crystallization and magma mixing may have blurred original helium isotopic signatures.     

Thorium isotopes as analogues for “particle-reactive” pollutants in coastal marine environments

Experiments using radioactive tracers in microcosms of 150 l and 13 m³ volumes, which are designed to mimic Narragansett Bay, indicate that Th isotopes are good analogues for studying the removal behavior of “particle-reactive” pollutants such as Am, Pb, Po, Hg and Cr(III) in coastal environments. The removal of Th isotopes and Fe has been found to be closely linked in microcosms and Narragansett Bay. In addition, the settling velocities of tracer microspheres in the experimental tanks were found to be faster in spring and summer than expected from Stokes' law and similar to those of²³⁴Th-containing particles in Narragansett Bay [5]. It is concluded that aggregation of particles is important in accelerating the removal of surface-reactive elements during the warm season.     

Compressibility,core dynamics and the subseismic wave equation

It is shown that in the dynamics of a deep fluid of planetary scale such as the Earth's core, compressibility, stratification and self-gravitation are all important as well as rotation. The existing proof of Cowling's theorem prohibiting non-stationary axisymmetric dynamos, and the application of the Proudman-Taylor theorem to core flows, both based on the assumption of solenoidal flow, need to be reconsidered. For sufficiently small (subacoustic) frequencies or reciprocal time scales, an approximation which neglects the effect of flow pressure on the density is valid. We call this the “subseismic approximation” and show that it leads to a new second-order partial differential equation in a single scalar variable describing the low frequency dynamical behaviour. The new “subseismic wave equation” allows a direct connection to be made between the various possible physical regimes of core structure and its dynamics.     

An outline of the geology and geochemistry,and the possible petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec-New Zealand island arc

The Pleistocene-Recent volcanism of this arc extends nearly linearly NNE from northern New Zealand for some 2800 km. Along its western margin lies an active marginal basin (Lau Basin and Havre Trough) which has its southern termination in the Taupo volcanic zone (TVZ, New Zealand). The New Zealand arc segment is developed within a continental crust, whereas the Tonga-Kermadec segments are developed on a ridge system within the oceanic basin. Submarine morphology suggests that the Kermadec volcanoes represent a less advanced stage of evolution relative to those of Tonga.Magmas erupted within the TVZ are dominantly rhyolitic (≈16,000 km³) with subordinate andesites and rare high-alumina tholeiites and dacites. The Kermadec Islands are dominated by tholeiites and basaltic andesites, with subordinate andesites and dacites. The Tongan Islands are dominated by basaltic andesites, with locally developed andesites and dacites. These Tonga-Kermadec lavas are characterised by subcalcic groundmass clinopyroxenes, whereas the younger group of TVZ andesites contain groundmass hypersthene and augite.Geochemically, the TVZ andesites are systematically enriched (relative to those of Tonga-Kermadec) in “incompatible” elements (e.g. K, Rb, Cs, Ba, light REE, U, Th, Zr, Pb), are less Fe-enriched, and contain more radiogenic Sr and Pb (excepting certain ²⁰⁷Pb/²⁰⁴Pb compositions). The evidence points to crustal equilibration of the TVZ andesites prior to eruption.A complete overlap of major and trace element chemistry (including TiO₂) is observed between the Kermadec-TVZ tholeiites and basaltic andesites, and the ocean floor tholeiites of the Lau Basin. Compared to the Tongan lavas, those of the Kermadecs exhibit a greater degree of chemical variability, also reflected in the greater heterogeneity in their Pb isotopic compositions. Moreover, many of the Tonga-Kermadec basaltic andesites exhibit more depleted “incompatible” trace element abundances than the Kermadec and TVZ tholeiites.The “primary” magmas of this arc are interpreted to be of basaltic andesite type, derived from Benioff zone melting (essentially anhydrous), but extensively modified by low-pressure crystal fractionation processes. The Kermadec tholeiites are explained as products of relatively shallow upper mantle partial fusion induced during the earlier stages of diapiric rise of Benioff zone-derived magmas, which are sufficiently hot to intersect the peridotite solidus. This should result in the production and intermixing of a series of magmas extending from olivine tholeiite to basaltic andesite composition. The voluminous rhyolites of TVZ are interpreted as the products of crustal fusion involving Mesozoic sediments.     

Lewisian gneiss geochemistry and Archaean crustal development models

The geochemistry of Lewisian amphibolite-facies gneisses from northwest Scotland is described with particular reference to the rare earth elements (REE) and compared with the geochemistry of Lewisian granulite-facies gneisses. The results show that there are no significant differences between “Laxfordian” amphibolite-facies and “Scourian” granulite-facies gneisses in terms of REE and other immobile trace elements (at equivalent silica levels), although the mobile radioactive heat-producing elements, K, Rb, Th, U, are significantly lower in the granulites. In both types the basic gneisses have moderately fractionated REE patterns while the intermediate and acid gneisses have strongly fractionated REE patterns with low heavy REE abundances and decreasing levels of total REE with increasing SiO₂. The most silicic gneisses develop large positive europium anomalies.These gross chemical similarities between gneisses from intermediate (amphibolite-facies) and lower (granulite-facies) crustal levels constrain models for the evolution of the Archaean crust. The depletion of K, Rb, Cs, Th and U in granulites, but not other incompatible trace elements cannot be explained by magmatic processes. The positive Eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation. Fractionation of radioactive heat-producing elements from other trace elements is a result of granulite-facies metamorphism with these elements being removed by an active fluid phase. The apparent lack of partial melting in lower crustal granulites suggests a model for Archaean crustal growth largely through underplating by primary tonalitic magmas.     

Early Archaean rocks and geochemical evolution of the earth's crust

Remnants of Early Archaean rocks (>FX3000 m.y. old) are reported from most continents. A critical review of the radiometric data shows that few of these are well authenticated and most are very limited in extent. The oldest are predominantly plutonic gneisses of tonalitic-to-granitic composition (e.g., the basement gneisses of West Greenland, Labrador, Rhodesia and South Africa). In all cases there are inclusions of meta-volcanic and sedimentary rocks with greenstone belt affinities which probably represent crust into which the igneous parents of the gneisses were intruded.The trace element chemistry of these very old rocks is reviewed in an attempt to establish the mechanism of formation of early crust and place constraints on the chemical evolution of the earth's mantle. “Mantle-type” Sr isotope compositions show that the sialic members of both early gneisses and greenstone belts were not derived from much older crustal differentiates, either at 3800 or at 2800 m.y. ago. However, trace element ratios such as K/Rb and Sr/Ba, and rare earth element abundances, are not consistent with direct derivation of the plutonic suite from the upper mantle and also rule out a common parentage for the tonalites and granites. An origin by partial melting of metamorphosed juvenile crust with a composition range equivalent to that represented by the greenstone belts is preferred. Tonalites resulted from high-pressure melting of mafic garnet-amphibolite and at least some of the granites from low-pressure melting of more felsic (possibly even sedimentary) material.The trace element chemistry of the greenstone belt volcanics is thought to characterize the composition of early mantle melts, although the best preserved and best documented cases are about 500–1000 m.y. younger than the oldest known gneisses. The dominant type is tholeiite with low incompatible element contents and light-depleted or essentially flat rare earth patterns, features even more marked in the ultramafic komatiites which represent large degrees of melting. More evolved calc-alkaline rocks with relative incompatible and light rare earth element enrichment are also important. With the exception of the ultramafic lavas, all these types can be matched by the chemistry of present-day oceanic volcanism.It is concluded that the range of trace element variations in the earth's mantle was comparable in early Archaean times to that at the present. This is supported by mass balance calculations for the lithophile elements which have been preferentially extracted into the crust. Thus the isotope and trace element evidence of the oldest rocks argues against primary differentiation of the crust either during accretion of the earth or during its first 500 m.y. as a solid body. Crust formation has probably occurred continuously, although worldwide evidence for magmatism at around 2800 m.y. ago probably marks a particularly active period.     

Estimation of source function and medium response function by autoregressive method

By modelling seismograms as “low” and “high” order autoregressive (AR) processes, the source function and the medium response function are separated from a single channel seismogram. Akaike's final prediction error is used as a statistic to select the appropriate “low” and “high” AR order of the process. Case studies of synthetic data show that the recovered source and reflectivity functions compare very well with the input functions. Using this method, arrivals of the surface reflected P phases of five explosions from the Soviet region and of two earthquakes from Kamchatka, recorded at Gauribidanur Seismic Array (GBA), India, are identified. Certain features of the source and source region of these events are also inferred.     

Trace elements in shergottite meteorites: Implications for the origins of planets

The average concentrations of 19 siderophile and volatile elements in shergottite meteorites differ from those in terrestrial basalts by less than a factor of ten. This observation undermines claims that the abundances of siderophile and volatile elements in the Earth's upper mantle are uniquely terrestrial. Claims that similarities in the Moon's siderophile element pattern imply a terrestrial origin for the Moon are also weakened. The implication that basalt source regions on the asteroidal parent body of the shergottites resembled the terrestrial upper mantle constrains models of planetary formation and evolution. Heterogeneous accretion models may explain many of the similarities between these planets. Alternatively, separation of sulfide from basaltic magmas or their source regions on the Earth and the shergottite parent body may explain some of these similarities.     

Magsat anomaly field inversion for the U.S.

Long-wavelength anomalies in the total magnetic field measured by Magsat over the United States and adjacent areas are inverted to an equivalent surface layer magnetization distribution. The model is based on an equal-area dipole grid at the Earth's surface. Model resolution, defined as the closest dipole spacing giving a solution having physical significance, is about 220 km for Magsat data in the elevation range 300–550 km. The magnetization contours correlate well with large-scale tectonic provinces. A higher-resolution (200 km) model based on relatively noise-free synthetic “pseudodata” is also presented. An excellent inverse correlation between apparent magnetization and heat flow in the western U.S. is demonstrated. A new regional heat flow map derived indirectly from Magsat data shows nearly all the important thermal anomalies evidenced in previous published maps. Notably, the map predicts high heat flow in Nebraska and the Dakotas, suggesting the presence of a “blind” geothermal area of regional extent.     

A note on short-term core-mantle coupling,geomagnetic secular variation impulses,and potential magnetic field invariants as Lagrangian tracers of core motions

This note summarizes recent studies of atmospheric excitation of short-term changes in the length of the day and polar motion which set useful limits on the timescales associated with angular momentum transfer between the Earth's core and mantle. It also speculates about the nature of the recently-discovered phenomenon of “impulses” or “jerks” in the geomagnetic secular variation, proposing that they might be manifestations of “loop” instability of the magnetic field within the core. Finally, it outlines novel properties of high magnetic Reynolds number flows that bear on the inverse problem of deducing core motions from geomagnetic secular variation data.     

Twenty-five years with OBS sensors: The good,the bad,and the ugly

Since their development for investigating sand transport in the surf zone more than 25 years ago, optical backscatter sensors (OBSs) have been in continual use for a wide variety of scientific, engineering, and environmental-monitoring applications. Many OBS designs have evolved in that time but they all work in the same simple way. A water sample is illuminated by a light source and photodetectors in the sensor convert the light scattered from the sample to photocurrent. The amount of photocurrent depends mainly on the illuminated area of the particles, and because particle volume is proportional to area, the photocurrent provides an indirect estimate of suspended-sediment concentration. The relationship between OBS signal and sediment concentration is almost linear for many suspended sediments and this is the most important “good” feature for OBS users. Many other factors influence OBS measurements, including: (1) particle size, shape, composition, and aggregation/flocculation; (2) dissolved light-absorbing matter; (3) bubbles; and (4) chemical and biological fouling. Some of these factors can be troublesome and can result in poor data quality or ambiguous results; these are “bad” ones. And a few of the same factors can have truly “ugly” consequences when their effects are ignored. Unknown time variation in particle size or aggregation/flocculation and fouling will nearly always result in inaccurate OBSs data.     

Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean

The distribution of “ash” (the non-combustible fraction of marine suspended matter) and concentrations of particulate Al, Ca, Fe, Cr, Ni, Cu, Sr and²³⁴Th in surface waters and of²¹⁰Pb,²³⁰Th and²³⁴Th in two vertical profiles (385–4400 m) of the Indian Ocean are reported.The ash concentrations in surface waters follow the primary productivity pattern, with higher abundances in samples south of 40°S and lower concentrations in the equatorial and subtropical regions. Opaline silica and CaCO₃ are the dominant components of the ash in samples from >40°S and from 7°N to 39°S, respectively. Aluminosilicates are only a minor constituent of the surface particulate matter. The metal/Al ratios in the surface particles are significantly higher compared to their corresponding crustal ratios for all the metals analyzed in this work. Comparison of enrichment factors between marine aerosols, plankton and surface oceanic particles, seem to indicate that this high metal/Al ratio in surface particles most likely arises from their involvement in marine biogeochemical cycles. Particulate²³⁴Th activity in surface waters parallels the ash abundance implying that its scavenging efficiency from surface waters depends on the particulate concentration.The particulate²³⁰Th and²¹⁰Pb concentration profiles increase monotonously with depth. It is difficult to ascribe this increase to a process other than the in-situ vertical scavenging of²³⁰Th and²¹⁰Pb from the water column by settling particles. The mean settling velocities of particles calculated from the particulate²³⁰Th data using a one-dimensional settling model is about 2 × 10^?3 cm/s. The settling velocity computed from the particulate²³⁰Th profiles does not appear to be compatible with the particulate²¹⁰Pb depth profiles; one possible explanation to account for the disparity would be that²³⁰Th and²¹⁰Pb are scavenged by different size populations of particles.On the whole, the geographic distribution of particulate matter, their composition and settling velocities in the Atlantic, Pacific and Indian Oceans are similar indicating that they are controlled by quite similar processes in the marine hydrosphere.     

SYMPOSIUM ON EVAPORATION / SYMPOSIUM SUR L'ÉVAPORATION

SUMMARY

The geographical transposition of the results obtained from representative and experimental watersheds is a matter of regressions based upon geomorphological caracteristics of these watersheds. Therefore, the lumped model for the transformation of precipitations into runoff, generally used for small sized basins, must be simplified and standardized in order to be defined, in any case, by the same parameters. Moreover, the elements of the hydrograph, especially the base time, are to be determined by a very strict processus providing with “consistent” estimates. These new necessary conditions lead to see again the methods used till now for scientific purposes in operating the data from small watersheds, to the so-called “standard hydrograph”.     

The effect of interstitial gaseous pressure on the thermal conductivity of a simulated Apollo 12 lunar soil sample

The thermal conductivity of a simulated Apollo 12 lunar soil sample was measured with a needle probe under vacuum. The result showed that the sample, with bulk densities of 1.70–1.85 g cm^?3 held in a vertical cylinder (2.54 cm in diameter and 6.99 cm long) has a thermal conductivity ranging from 8.8 to 10.9 mW m^?1 K^?1. This is comparable to the lunar regolith's thermal conductivity as determined in situ. Besides the dense packing of the soil particles, an enhanced intergranular thermal contact, due to the self-compression of the sample, is necessary to raise the sample's thermal conductivity from the level of loose soil (< 5 mW m^?1 K^?1) to that of the lunar regolith deeper than 35 cm (～ 10 mW m^?1 K^?1). A model of the lunar regolith, a thin layer of loose soil resting on a compacted self-compressed substratum, is consistent with the lunar regolith's surface structure as deduced from an observation of the lunar surface's brightness temperature. Martian regolith surface structure is similar, except that its surface layer may be missing in places because of aeolian activity. Measurements of thermal conductivity under simulated martian surface conditions showed that the thermal properties of loose and compacted soils agreed with the two peak values of the martian surface's thermal inertia as observed from “Viking” orbiters, suggesting that drifted loose soil and exposed compacted soil are responsible for the bimodal distribution of the martian surface's thermal inertia near zero elevation. For compacted soil exposed to the martian surface to have the same thermal conductivity as that buried under the surface layer, a cohesion of the soil particles must be assumed.     

Thermodynamics of the oxygen isotope fractionation involving plagioclase

The thermodynamic relationship between the oxygen isotope fractionation properties of plagioclase and its composition has been derived by treating plagioclase as a “reciprocal solution” consisting of independent cationic and anionic substitutions, namely (NaAl)⁵⁺?(CaSi)⁵⁺and¹⁸O?¹⁶O. The results show that the logarithm of the oxygen isotope fractionation factor, α, between plagioclase and a coexisting phase varies linearly with the anorthite content of plagioclase. The proportionality constant is given by the oxygen isotope fractionation factor between albite and anorthite, and has been derived from the experimental data of two groups of workers, O'Neil and Taylor [2] and Matsuhisa et al. [3], on the isotopic fractionation between each plagioclase end-member and aqueous solutions. It is found that O'Neil and Taylor's data on isotopic exchange of plagioclase end-members with only 2–3 M chloride solution, rather than with both pure water and the chloride solution, lead essentially to zero intercept of the ln α(Ab-An) vs. 1/T² relation, in accord with Bottinga and Javoy's [10] conclusion about the oxygen isotope fractionation between two anhydrous silicates at T<500°C.     

Variability of sunlike stars

Stars with the same temperature, luminosity, chemical composition, and activity as the Sun may be designated “sunlike stars.” The variability of several dozen reasonably sunlike stars has been monitored regularly since 1966 in chromospheric Ca II HK emission, and since 1984 in the visual continuum; larger stellar samples have been observed less comprehensively. Similar solar time series exist. A comparison of solar variability with its stellar analogs indicates that the Sun's current behavior is not unusual among sunlike stars, although the amplitude of the Sun's cyclic variation, measured photometrically, may be smaller than that of its stellar analogs. If the ensemble characteristics of a sample of sunlike stars are representative of the Sun's possible behavior, then such samples may provide insight about states of solar activity, such as Maunder Minima, which have not been observed on the Sun during the contemporary era of space-based measurements. Future progress in solar–stellar research will likely require studies of (1) rigorously selected samples of sunlike field stars, and (2) larger samples from stellar clusters, observed using 4-m class telescopes equipped with multi-object spectrographs and CCD photometers.     

A stable isotope study of the Garda lake, northern Italy: Its hydrological balance

Discontinuous measurements of the isotopic composition of surface water samples of the Garda lake carried out between 1998 and 2006 showed almost constant δ¹⁸O, δD and d-excess values through time. During 2006 and 2007 monthly vertical profiles of water samples were collected in the northernmost section of the lake, not far from the main inflow (Sarca river) to check whether there was any detectable influence from this inflowing river and whether there was a vertical isotopic stratification of the lake water. The isotopic measurement of water samples from the vertical profiles yielded isotopic values which were almost equal to those obtained from surface waters showing no detectable effect of the inflowing river water and no isotopic vertical stratification. The attempt to evaluate the evaporation rate of lake water by means of current models was totally unsuccessful. Despite the marked summer warming of the surface layer no isotopic fractionation related to evaporation processes could be detected. This anomalous behaviour may be related to the large amount of spring and summer precipitation characteristic of this area. The water balance of the lake calculated according to the amount of the inflowing water (Sarca river water plus rain water on the lake plus 20% of the precipitations on the whole catchment basin) and to the amount of outflowing water (Mincio river) showed a large imbalance, the river outflow alone resulting on average, during the last decade, at least double the inflow. To explain this imbalance of the lake, a large recharge by concealed groundwater is suggested: its isotopic composition should be quite close to the mean isotopic composition of precipitations over that area. This would be in agreement with the almost constant isotopic composition of both surface and deep waters and with the lack of vertical isotopic stratification. A few measurements of the tritium concentration carried out on lake water show values that are considerably higher than modern tritium values either in precipitation or in the Sarca river water: these results are in good agreement with the hypothesis of a recharge of the lake by deep aquifers.     

Investigations on cosmic-ray-produced nuclides in iron meteorites, 3. Exposure ages,meteoroid sizes and sample depths determined by mass spectrometric analyses of potassium and rare gases

Three physical quantities define the essentials of the cosmic ray exposure history of a sample of an iron meteorite: (1) the cosmic ray exposure age T, (2) the pre-atmospheric “size” S of the irradiated body, and (3) the location, i.e. the “depth” D, of the samples within the body. To establish these quantities for a given sample three independent quantities must be determined experimentally. In the present work T is ascertained by the ⁴¹K/⁴⁰K method and the ⁴He and ²¹Ne concentrations (C₄ and C₂₁) are measured by the isotope dilution method. Signer and Nier's evaluation of the rare gas distribution in the meteorite Grant and the measured exposure age for this meteorite provide the relationships allowing to ascertain for any meteorite the quantities S and D from the ²¹Ne production rate (P₂₁ = C₂₁/T) and the ⁴He/²¹Ne ratio.Earlier measurements have provided data on the isotopic composition of potassium in 74 different iron meteorites. New rare gas measurements are reported for some 40 samples. Results on the age, size and depth are obtained for almost 60 samples. These data suggest that Signer and Nier's model is well suited for describing not only the rare gas distribution in a single selected meteorite (Grant) but also the exposure histories of the great majority of all irons. For a few samples, however, secondary breakups of the meteoroid and a two- or multiple-stage irradiation must be invoked. Further measurements are proposed for testing and, possibly, refining the still somewhat uncertain relationships between the abundances of cosmogenic nuclides and the quantities T, S, and D in very large meteorites.Histograms are presented showing the age distributions for irons of different chemical groups and of different size ranges.The feasibility and the relative merits of other methods for the determination of T, S, and D are discussed.