Collisional records in LL-chondrites

Abstract One-third of all the LL-chondrites have exposure ages of ～15 Ma and were exposed to cosmic rays following a collisional break-up. The probability that the 15-Ma peak represents a random signal is calculated to be less than 2%. Considerably lower probabilities are obtained if only LL5s or subgroups of high ⁴⁰Ar retention are used. Furthermore, we show that the peak shape agrees with statistical constraints obtained from multiple analyses of samples from the St. Severin LL6-chondrite. The frequency in and out of the 15-Ma peak varies significantly for different petrographic LL-types. The radiogenic ⁴⁰Ar retention systematics (most LL-chondrites retained ⁴⁰Ar_rad) shows that no substantial heat pulse resulted in the 15-Ma collisional event. Interestingly, smaller exposure age clusters at ～28 Ma and ～40 Ma match up well with clusters in the histogram of L-chondrites. The distribution of LL-exposure ages is not consistent with that expected for a quasi-continuous injection of LL material into a resonance zone of the asteroid belt. The near absence of exposure ages shorter than 8 Ma either indicates a lack of recent collisional events or considerably longer transfer times than inferred from dynamical considerations.     

Southwest Scandinavia, 40–15 kyr BP: palaeogeography and environmental change

Twelve palaeogeographical reconstructions illustrate environmental changes at the southwest rim of the Scandinavian Ice Sheet 40–15 kyr BP. Synchronised land, sea and glacier configurations are based on the lithostratigraphy of tills and intertill sediments. Dating is provided by optically stimulated luminescence and calibrated accelerator mass spectrometry radiocarbon. An interstadial sequence ca. 40–30 kyr BP with boreo‐arctic proglacial fjords and subarctic flora and occasional glaciation in the Baltic was succeeded by a Last Glacial Maximum sequence ca. 30–20 kyr BP, with the closure of fjords and subsequent ice streams in glacial lake basins in Kattegat and the Baltic. Steadily flowing ice from Sweden bordered the Norwegian Channel Ice Stream. A deglaciation sequence ca. 20–15 kyr BP indicates the transgression of arctic waters, retreat of the Swedish ice and advance of Baltic ice streams succeeded by a return to interstadial conditions. When ameliorated ice‐free conditions prevailed in maritime regions, glaciers advanced through the Baltic and when interstadial regimes dominated the Baltic, glaciers expanded off the Norwegian coast. The largest glacier extent was reached in the North Sea around 29 kyr BP, about 22 kyr BP in Denmark and ca. 18 kyr BP in the Baltic. Our model provides new data for future numerical and qualitative landform‐based models. Copyright © 2003 John Wiley & Sons, Ltd.     

Chondrule-like objects and brown glasses in howardites

Abstract— Chondrule-like objects and brown glasses were analyzed in the howardites, Bununu, Malvern, Monticello, Pavlovka, and Yamato 7308. The objects are very similar to chondrules in ordinary and carbonaceous chondrites. Like the brown glasses the chondrule-like objects could have been produced by impact melting that left some crystalline nuclei, followed by a slower cooling rate than for the glasses. Alternatively, these objects are chondrules implanted from chondrite impactors. They are, however, without rims or any adhering matrix. The brown glasses appear to represent melting of average regolithic surface material, except for Monticello and Y7308, both of which have some siliceous glasses. The siliceous glasses could not have been produced by vapor fractionation but by melting of differentiated lithologies such as fayalitic granites. Impact mechanics indicates that howardites with abundant brown glasses came from an asteroid larger than Vesta (>400 km radius), upon which impacts occurred at relative velocities of up to 5 km/s. Howardites with little or no brown glasses came from a smaller parent body. We conclude that at least two parent bodies are likely sources for the basaltic achondrites.     

Pulled array continuous electrical sounding with an additional inductive source: an experimental design study

Electrical and electromagnetic profiling methods are used extensively in environmental geophysical investigations for many different purposes. The pulled array continuous electrical sounding (PACES) method, where a tail of electrodes is towed behind a small vehicle while continuously and simultaneously measuring several electrode configurations, has been used extensively for mapping the vulnerability of aquifers in Denmark. Measurements are taken every 1 m, and 10–15 km of profile can be achieved in one day.
This paper presents a theoretical study of the resolution capabilities of PACES measurements as they are now performed, and an experimental design study for including an inductive source in the measuring equipment. The joint interpretation of the galvanic data set of ordinary PACES measurements with inductive data from a horizontal magnetic dipole source will enhance the resolution capabilities of the data set significantly. The study is carried out as an analysis of the uncertainty of the model parameters of one-dimensional three-layer models using the estimation error variances of the inversion problem.
The results indicate that the addition of only two frequency data from a magnetic dipole source will substantially improve the resolution of the subsurface resistivity structure. The improvement is model dependent, but reduction in the relative error of model parameters by an order of magnitude is observed.     

Experimental and Economic Assessment of Two Surfactant Formulations for Source Zone Remediation at a Former Dry Cleaning Facility

Treatability tests and cost analyses were conducted to provide objective criteria for selection of a surfactant formulation to be used for surfactant enhanced aquifer remediation (SEAR) of a tetrachloroethene (PCE)-contaminated site in Oscoda, Michigan. Two surfactant formulations, 4% Tween 80 + 500 mg/L CaCl₂ and 8% Aerosol MA/IPA +15,000 mg/L NaCl + 1000 mg/L CaCl₂, were considered based on their capacity to solubilize PCE and prior use in SEAR applications. Results of a two-dimensional aquifer cell experiment indicated that 53% of the released PCE was recovered after flushing with approximately 8 pore volumes of 4% Tween 80. In contrast, only 3 pore volumes of 8% Aerosol MA/IPA solution were required to recover 78% of the PCE from the two-dimensional aquifer cell, although the greater recovery of PCE was attributed, in large part, to the higher concentration of Aerosol MA. However, mobilization of PCE as free product was observed during the 8% Aerosol MA/IPA flood, which was consistent with total trapping number (N_T) calculations. At the pilot-scale, SEAR treatment costs were estimated to be $222,000 and $244,000 for 4% Tween 80 and 8% Aerosol MA/IPA, respectively, which compared favorably to the estimated pump-and-treat cost of $316,000. Projected full-scale costs, based on a line-drive flushing system, were $382,000 for 4% Tween 80 and $443,000 for 8% Aerosol MA/IPA. In contrast, full-scale pump-and-treat costs were estimated to be $1,167,000. Surfactant recycling was shown to be logistically and economically infeasible at the pilot scale, and provided only a minimal cost benefit for 4% Tween 80 at the full scale. Based on the similarities in solubilization capacity and treatment cost, but substantially lower risk of PCE displacement, Tween 80 was recommended over Aerosol MA/IPA for pilot-scale testing of SEAR.