Formation of Ureilites by Impact-Melting of Carbonaceous Chondritic Material

Abstract— Ureilites are modeled as impact-melt products of CV-chondrite-like material. This model is consistent with the brecciated nature and cumulate textures of ureilites, O-isotopic constraints (which indicate ureilite derivation from an isotopically heterogeneous body like the CV-chondrite parent), the high abundance of planetary-type noble gases, and the relatively high concentrations of siderophile and chalcophile elements (indicating incomplete separation of metal-sulfide from silicate). Each ureilite may have been derived from a separate cratering event.     

PETROLOGY OF NINE ORDINARY CHONDRITE FALLS FROM CHINA

Nine twentieth-century ordinary chondrite falls from China are described and classified. They include: Nantong (H6), Zaoyang (H5), Zhaodong (L4), Qidong (L-LL5), Raoyang (L6), Sheyang (L6), Guangnan (L6), Suizhou (L6) and Nan Yang Pao (L6). Kamacite in Qidong is rare and contains much more Co (15 mg/g) than is characteristic of L-group chondrites; Qidong may be a member of a chondrite group intermediate in its properties between L and LL. Zhaodong, Qidong, Raoyang, Sheyang and Suizhou have several olivine and/or low-Ca pyroxene grains with aberrant Fel(Fe + Mg) ratios; it is probable that these five chondrites are fragmental breccias. The lack of correlation between shock facies and occurrence of aberrant silicate grains suggests that breccia lithification caused only minimal shock effects in many meteorites. Alternatively, postshock annealing may have resulted in the recrystallization of shock-indicating phases, leading to assignment of a shock facies that is lower than that present immediately after the shock event.     

Extinction changes at Black Birch Astrometric Observatory due to volcanic aerosols from Mount Pinatubo

Nightly means for extinction were determined from the recorded real-time extinction values of observations made on the 7-inch Transit Circle Telescope at Black Birch Astrometric Observatory (BBAO). The residual effects of the 15 June 1991 Mount Pinatubo eruption in the Philippines increased the extinction by about 0.25 magnitudes per airmass.     

Benthic-pelagic coupling and export of organic carbon from a tropical Atlantic continental shelf—Sierra Leone

An approximate carbon budget for the Sierra Leone River Estuary and adjacent inner continental shelf indicates that phytoplankton production is balanced by grazers only in the wet season estuary; on the continental shelf, and in the dry season estuary, phytoplankton production exceeds demand of consumer populations by 70–90%. Regional production is dominated by the dry season estuary where diatom blooms support large population of the phytophagous clupeid fish Ethmalosa fimbriata. It is suggested that as well as what is used in bacterial respiration, not quantified in the study, important amounts of inshore organic carbon production are available for export, or burial, or both, either in inshore mudbanks or at the shelf edge by slumping. This carbon budget for a tropical continental shelf having neither coral-reefs nor coastal upwelling appears to correspond with what is emerging as a generic distinction between shelves and open ocean ecosystems: surplus production on shelves, balance between producers and consumers in the open ocean.     

The 1.7- to 4.2-μm spectrum of asteroid 1 Ceres: Evidence for structural water in clay minerals

A high-resolution Fourier spectrum (1.7–3.5 μm) and medium-resolution spectrophotometry (2.7–4.2 μm) were obtained for Asteroid 1 Ceres. The presence of the 3-μm absorption feature due to water of hydration was confirmed. The 3-μm feature is compared with the 3-μm bands due to water of hydration in clays and salts. It is concluded that the spectrum of Ceres shows a strong absorption at 2.7–2.8 μm due to structural OH groups in clay minerals. The dominant minerals on the surface of Ceres are therefore hydrated clay minerals structurally similar to terrestrial montmorillonites. There is also a narrow absorption feature at 3.1 μm which is attributable to a very small amount of water ice on Ceres. This is the first evidence for ice on the surface of an asteroid.     

The initiation of Waulsortian buildups in Western Ireland

The contact zone at the base of the Waulsortian (Upper Tournaisian) carbonate mud‐bank complex in western Ireland has been investigated at four localities spaced over a distance of 120 km. At all localities, a transition facies up to 3 m thick, characterized by several types of grumous (clotted and/or peloidal) carbonate muds, immediately underlies the Waulsortian facies. These muds show a developmental sequence provisionally interpreted as a necessary precursor to the formation of Waulsortian polymuds. Such pre‐bank precursors produced thin (a few decimetres) units of transition facies. The same mud types also persisted as an aureole around growing banks (mud‐mounds). Migration of the aureole during bank progradation produced thicker units of transition facies. The distribution of skeletal grain types in the Waulsortian banks, the transition facies and the ‘background’ argillaceous bioclastic limestones show two trends: one regional and one local. The regional trend is expressed by progressive north–south attenuation and, in some cases (for example, plurilocular foraminifera), the disappearance of organism groups. It parallels changes in Waulsortian Phases (defined by skeletal grain‐type assemblages) and is thought to indicate a southerly increase in water depth. The local trend, which occurs only in the two southern localities (deeper water), expresses differences between the skeletal grain content of the various lithofacies. These differences result partly from increased sensitivity to substrate texture by organism groups suffering southward attenuation (notably gastropods, hyalosteliid sponges, aoujgaliids, Earlandia and kirkbyacean ostracods) and partly from selective colonization, particularly of the transition facies, by tabulate corals and stick/ramose bryozoans. However, the developmental sequence of precursor carbonate muds is the same at all localities, indicating that the mud‐making process (probably microbial) was independent of water depth.