The Puerto Lápice eucrite

Abstract— Puerto Lápice is a new eucrite fall (Castilla‐La Mancha, Spain, 10 May 2007). In this paper, we report its detailed petrography, magnetic characterization, mineral and bulk chemistry, oxygen and noble gas isotope systematics, and radionuclide data. Study of four thin sections from two different specimens reveal that the meteorite is brecciated in nature, and it contains basaltic and granulitic clasts, as well as recrystallized impact melt and breccia fragments. Shock veins are ubiquitous and show evidence of at least three different shock events. Bulk chemical analyses suggest that Puerto Lápice belongs to the main group of basaltic eucrites, although it has a significantly higher Cr content. Oxygen isotopes also confirm that the meteorite is a normal member of the HED suite. Noble gas abundances show typical patterns, with dominant cosmogenic and radiogenic contributions, and indicate an average exposure age of 19 ± 2 Ma, and a Pu‐fission Xe age well within typical eucrite values. Cosmogenic radionuclides suggest a preatmospheric size of about 20–30 cm in diameter.     

An analysis of Apollo lunar soil samples 12070,889, 12030,187, and 12070,891: Basaltic diversity at the Apollo 12 landing site and implications for classification of small‐sized lunar samples

Lunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size <0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.     

Contributions to atmospheric methane by natural seepages on the U.K. continental shelf [Mar. Geol. 137 (1997) 165–189]

Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane.

Potential gas source rocks include Quaternary and Tertiary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf.

Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction.

Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble.

The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km². This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.     

Comparison of the effects of unidirectional and sign‐alternating temperature gradients on the sintering of ice spheres

To simplify the complex snow structures that occur in nature, polycrystalline ice spheres were produced and arranged vertically to model the sintering process. By controlling the temperatures on both the top and bottom of the ice sphere array, the effect of upward and downward vapor transfers was examined. The evolution of the neck areas between ice spheres was observed using X‐ray computed microtomography. As frequently observed under the basal part of a snow layer and previous experiments of snow temperature gradient metamorphism, depth hoar structures were formed along neck areas and their formation was found to be directly related to the vapor transfer direction. To model the temperature gradient inversion that can be induced in nature by daily cycles of radiative heating and cooling, we also performed sign‐alternating temperature gradient experiments on the ice sphere arrays. The morphological evolution of the neck and the associated vapor transfer were examined through image analysis and 2D modeling. The final microstructures of the neck area turned out to be a symmetrical distribution of ice protrusions bridging neighboring ice spheres.     

Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions

Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km³ (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H₂O, CO₂ and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H₂O and CO₂ in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H₂O and CO₂ in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions

Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 °C in air along [100] with activities of SiO₂, MgO and Al₂O₃ (a_SiO2, a_MgO and a_Al2O3) buffered. At low a_SiO2, the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high a_SiO2 and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100–1350 °C. Experiments at high a_SiO2 and 1400 °C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 °C, fo + melt. The resulting diffusion profiles were analysed by LA–ICP–MS in scanning mode. Diffusion profiles in the high a_SiO2 experiments were generally several hundred microns in length, but diffusion at low a_SiO2 was three orders of magnitude slower than in high a_SiO2 experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (a_Al2O3)^1/2 and (a_SiO2)^3/4, pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al _4/3 ³⁺ vac_2/3SiO₄, whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl₂O₄. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high a_SiO2, even faster than Fe–Mg interdiffusion.     

Elevated marine deposits in Bermuda record a late Quaternary megatsunami

Deposits of coral-bearing, marine shell conglomerate exposed at elevations higher than 20 m above present-day mean sea level (MSL) in Bermuda and the Bahamas have previously been interpreted as relict intertidal deposits formed during marine isotope stage (MIS) 11, ca. 360–420 ka before present. On the strength of this evidence, a sea level highstand more than 20 m higher than present-day MSL was inferred for the MIS 11 interglacial, despite a lack of clear supporting evidence in the oxygen-isotope records of deep-sea sediment cores. We have critically re-examined the elevated marine deposits in Bermuda, and find their geological setting, sedimentary relations, and microfaunal assemblages to be inconsistent with intertidal deposition over an extended period. Rather, these deposits, which comprise a poorly sorted mixture of reef, lagoon and shoreline sediments, appear to have been carried tens of meters inside karst caves, presumably by large waves, at some time earlier than ca. 310–360 ka before present (MIS 9–11). We hypothesize that these deposits are the result of a large tsunami during the mid-Pleistocene, in which Bermuda was impacted by a wave set that carried sediments from the surrounding reef platform and nearshore waters over the eolianite atoll. Likely causes for such a megatsunami are the flank collapse of an Atlantic island volcano, such as the roughly synchronous Julan or Orotava submarine landslides in the Canary Islands, or a giant submarine landslide on the Atlantic continental margin.     

Passive L-band microwave soil moisture retrieval error arising from topography in otherwise uniform scenes

Soil moisture is an important component of the water cycle and will be measured for the first time on a global scale by a dedicated passive L-band microwave radiometer that is planned for launch in 2008. Here, the contribution of topography to the error budget is examined for a vegetated scene with uniform microwave emission. Dual-polarization brightness temperature curves were generated over a range of look angles for 1-D scenes with simple geometrical features, and the soil moisture was retrieved assuming a flat surface. The errors were small for the scenarios considered. Theoretical errors were tested for realistic topography with a DEM transect of a mountainous region, and were found to be comparable. Knowledge of the mean slope from high-resolution DEM data can be used to improve the accuracy of the retrieval.