Cosmic-ray exposure ages of diogenites and the recent collisional history of the howardite,eucrite and diogenite parent body/bodies

Abstract— We determined the cosmic-ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of ³He, ²¹Ne and ³⁸Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic ²²Ne/²¹Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured ¹⁰Be and ²⁶AI concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages cluster at 21–25 Ma and four at 35–42 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20–25 Ma and 10 at 35–42 Ma. The shape of the two peaks is consistent with single impact events, and random number statistics show that they are statistically significant at the 99% level. Altogether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to explain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2–4 km in diameter for Vesta and of 60–300 m for the 100× smaller Vesta-derived “vestoids.” Based on the HED exposure-age distribution, the size distribution of the main-belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta.     

Allan Hills 88019: An Antarctic H-chondrite with a very long terrestrial age

Abstract— We have measured the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al and ³⁶Cl (half-lives 1.51 Ma, 716 ka, and 300 ka, respectively) in two different laboratories by accelerator mass spectrometry (AMS) techniques, as well as concentrations and isotopic compositions of stable He, Ne and Ar in the Antarctic H-chondrite Allan Hills (ALH) 88019. In addition, nuclear track densities were measured. From these results, it is concluded that the meteoroid ALH 88019 had a preatmospheric radius of (20 ± 5) cm and a shielding depth for the analyzed samples of between 4 and 8 cm. Using calculated and experimentally determined production rates of cosmogenic nuclides, an exposure age of ～40 Ma is obtained from cosmogenic ²¹Ne and ³⁸Ar. The extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 ± 0.4 Ma. A similarly long terrestrial age was found so far only for the Antarctic L-chondrite Lewis Cliff (LEW) 86360. Such long ages establish one boundary condition for the history of meteorites in Antarctica.     

EVALUATION OF GRIDDED PRECIPITATION FOR NORWAY USING GLACIER MASS‐BALANCE MEASUREMENTS

The service seNorge ( http://senorge.no ) provides gridded temperature and precipitation for mainland Norway. The products are provided as interpolated station measurements on a 1 × 1 km grid. Precipitation gauges are predominantly located at lower elevations such as coastal areas and valleys. Therefore, there are large uncertainties in extrapolating precipitation data to higher altitudes, both due to sparsity of observations as well as the large spatial variability of precipitation in mountainous regions. Using gridded temperature and precipitation data from seNorge, surface mass balance was modeled for five Norwegian glaciers of different size and climate conditions. The model accounts for melting of snow and ice by applying a degree‐day approach and considers refreezing assuming a snow depth depended storage. Calculated values are compared to point measurements of glacier winter mass balance. On average for each glacier, modeled and measured surface mass‐balance evolutions agree well, but results at individual stake locations show large variability. Two types of problems were identified: first, grid data were not able to capture spatial mass balance variability at smaller glaciers. Second, a significant increase in the bias between model and observations with altitude for one glacier suggested that orographic enhancement of precipitation was not appropriately captured by the gridded interpolation.     

Quantitative aspects of Mn-activated cathodoluminescence of natural and synthetic aragonite

The cathodoluminescence analyses of the trigonal carbonates calcite and dolomite have been applied intensively in sedimentary petrology for a long time and the properties of these minerals are well-known, but much less attention has been paid to aragonite. In this study, the cathodoluminescence behaviour and the trace element composition of natural and synthetic aragonite have been studied employing trace element analyses (proton induced X-ray emission) and luminescence spectroscopy. Aragonite doped with Mn²⁺ has been synthesized in a NH₄⁺–Mg²⁺–Ca²⁺–Cl⁻ solution in contact with a CO₂–H₂O–NH₃ atmosphere. The low effective distribution coefficients indicate a rapid growth of the crystals of millimetre size which occurred within hours or days. The natural aragonite samples contain Mn, Fe and Sr in different concentrations. The Mn-bearing aragonites exhibit a bright green luminescence which is caused by a strong emission band at 575 nm with a half-width of about 84 nm. The luminescence intensity shows a strong positive correlation with Mn in aragonite when Fe and Mn do not exceed 2000 p.p.m. The intensity is depressed if the concentration of these elements exceeds the critical values. In the shell of a recent Unio sp., the luminescence intensity deviates from the linear correlation, although the trace element contents are not too high; this is probably an effect of quenching by organic material between the crystallites of the biogenic aragonite.     

Stromatolites in the Late Ordovician Eureka Quartzite: implications for microbial growth and preservation in siliciclastic settings

Well-preserved siliciclastic domal stromatolites, up to 2 m wide and 1·5 m high, are found in a 10 to 15 m thick interval within the Late Ordovician Eureka Quartzite of Southern Nevada and Eastern California, USA. These stromatolites appear as either isolated features or patchy clusters that contain more than 70% by volume quartz grains; their association with planar, trough and herringbone cross-bedding suggests that they were formed in an upper shoreface environment with high hydraulic energy. In this environment, sand bars or dunes may have provided localized shelter for initial microbial mat colonization. Biostabilization and early lithification of microbial mats effectively prevented erosion during tidal flushing and storm surges, and the prevalence of translucent quartz sand grains permitted light penetration into the sediment, leading to thick microbial mat accretion and the formation of domal stromatolites. Decimetre-scale to metre-scale stromatolite domes may have served as localized shelter and nucleation sites for further microbial mat colonization, forming patchy stromatolite clusters. Enrichment of iron minerals, including pyrite and hematite, within dark internal laminae of the stromatolites indicates anaerobic mineralization of microbial mats. The occurrence of stromatolites in the Eureka Quartzite provides an example of microbial growth in highly stressed, siliciclastic sedimentary environments, in which microbial communities may have been able to create microenvironments promoting early cementation/lithification essential for the growth and preservation of siliciclastic stromatolites.     

LIKELIHOOD TECHNIQUES FOR INTERLABORATORY CALIBRATION IN THE NATIONAL STREAM SURVEY

Techniques for testing for and estimating relative bias between two laboratories are developed andapplied to a survey of the chemistry of streams in the United States.The design of the quality assuranceprogram allows estimation of linear corrections for bias as well as testing of the hypothesis of linearity,Designs of this type are useful,but improvements are suggested.     

Towards a National Landslide Information Base for New Zealand

Landslides constitute a problem as a natural hazard, as a factor in land development decisions, and as a major process in the depletion of New Zealand's soil resource. There are compelling social, economic, and legislative reasons for obtaining and accessing information on landslide activity. As a way of improving a currently weak information base, recommendations are given for establishing an information system appropriate to specific management functions.     

Dacite Genesis via both Slab Melting and Differentiation: Petrogenesis of La Yeguada Volcanic Complex, Panama

La Yeguada volcanioc complex (LYVC) is one of many major volcanoesthat represent the extension of the Central American arc inwestern Panama and that have resulted from current oblique subductionsouth of Panama. There are two major phases of calc-alkalinevolcanic activity at LYVC based on mapping and K-Ar radiometricdates. The first phase began at {small tilde} 13 Ma and ceasedat {small tilde} 7?5 Ma. This sequence, termed the old group,consists of basalts to rhyolites with typical arc mineralogies(OL, CPX, PL, MGT, and OPX). The samples have similar radiogenicSr and Nd values and appear to be related by fractional crystallizationwith assimilation and/or magma mixing involved in the differentiation.The parental basalts were probably derived from the metasomatizedmantle wedge via melting induced by fluids released from thesubducted lithosphere. There was an apparent period of minor volcanic activity from7–5 to 2–5 Ma (only one documented sample from thisperiod). The second phase (<2?5 Ma), termed the young group,consists only of dacites but with very different mineralogies(PL, MGT, AM, BI, with no PX) and geochemistries (e.g., highSr and low Y and HREE) compared with the old-group dacites (andandesites and rhyolites). The dacites cannot be related to theold group by various petrogenetic modeling techniques. Thesehigh-Al dacites have the characteristics of magmas derived fromthe partial melting of the subducted oceanic lithosphere witha hornblende eclogite residuum. This has been substantiatedby geochemical modeling. Samples similar to the young-group dacites in other arcs havebeen termed adakites and arc associated with the subductionof young hot crust which may explain why the slab melts. ThePanama basin has extremely high heat flow values, comparablewith those of the Galapagos ridge system. The change from normalarc volcanism to adakites suggests that the subducted oceaniccrust became hotter as time progressed. The subduction of anoceanic ridge or new ridge development along the Sandra Riftin the Panama basin can explain the change in volcanism withtime but more geophysical data are needed.