Flood and Shield Basalts from Ethiopia: Magmas from the African Superswell

The Ethiopian plateau is made up of several distinct volcaniccentres of different ages and magmatic affinities. In the NE,a thick sequence of 30 Ma flood basalts is overlain by the 30Ma Simien shield volcano. The flood basalts and most of thisshield volcano, except for a thin veneer of alkali basalt, aretholeiitic. In the centre of the province, a far thinner sequenceof flood basalt is overlain by the 22 Ma Choke and Guguftu shieldvolcanoes. Like the underlying flood basalts, these shieldsare composed of alkaline lavas. A third type of magma, whichalso erupted at 30 Ma, is more magnesian, alkaline and stronglyenriched in incompatible trace elements. Eruption of this magmawas confined to the NE of the province, a region where the lavaflows are steeply tilted as a result of deformation contemporaneouswith their emplacement. Younger shields (e.g. Mt Guna, 10·7Ma) are composed of Si-undersaturated lavas. The three maintypes of magma have very different major and trace element characteristicsranging from compositions low in incompatible elements in thetholeiites [e.g. 10 ppm La at 7 wt % MgO (=La₇), La/Yb = 4·2],moderate in the alkali basalts (La₇ = 24, La/Yb = 9·2),and very high in the magnesian alkaline magmas (La₇ = 43, La/Yb= 17). Although their Nd and Sr isotope compositions are similar,Pb isotopic compositions vary considerably; ²⁰⁶Pb/²⁰⁴Pb variesin the range of     

DETERMINATION OF RARE EARTH ELEMENTS IN SIXTEEN SILICATE REFERENCE SAMPLES BY ICP-MS AFTER TM ADDITION AND ION EXCHANGE SEPARATION

This study reports the results of thirteen rare earth elements (REE) in sixteen geochemical reference samples. The analytical procedure involved dissolution of a whole rock or mineral separate, spiking with Tm, and separation of the REE using a simple ion-exchange chromatography procedure. The resulting REE solutions were analysed by ICP-MS. The results are compared with literature values. The agreement between our data and recommended or ID-TIMS values is very favorable. The precision of the technique is better than 5% (2) for all the REE.     

A METHOD FOR THE DETERMINATION OF THE THERMAL CONDUCTIVITY OF SANDSTONES USING A VARIABLE STATE APPROACH*

Common problems encountered during the determination of the thermal conductivities of tight sandstones are rock specimen-to-heat source contact and complicated sample preparation. An experimental technique using an electromagnetic heat source solves the direct contact problem between the heat source and the sample. Also, it simplifies the sample preparation and reduces the measurement time. A CO₂ laser operating in a pulsed mode is used as a heat source with about 500 W output power. Thus, heat losses due to radiation and air convection are negligible. Unpolished penny-sized samples of tight sandstones are irradiated on one side and the temperature is measured on the opposite side. The temperature is recorded with 12-bit accuracy by a digital data acquisition system. Carbon black is used to give the samples a uniform absorption. The transient temperature data are acquired, processed, and interpreted with interactive computer programs. Transients for each sample can be stacked, to improve the signal-to-noise ratio, and normalized. The thermal conductivity is calculated from the rise portion of the transient using a ridge-regression type generalized linear inversion scheme. As many as 20 samples per hour can be measured with this simple but expensive set-up. In the long run, this means a reduction of laboratory expenses. In addition, the resolution of this method is superior to other variable-state measurement methods due to the real time controlled data acquisition and the numerical interpretation.     

A transmission electron microscope study of iron-nickel carbides in the matrix of the Semarkona unequilibrated ordinary chondrite

Abstract— The fine-grained matrix of the Semarkona unequilibrated ordinary chondrite contains an assemblage of Fe carbides that are associated with oxides, sulfides, carbonates, and hydrated silicates. The carbides consist of fine-scale epitactic intergrowths of cohenite (Fe₃C) and Hägg carbide (Fe₅C₂) that formed as by-products of gas-solid reactions on the Semarkona parent body. The carbide intergrowths resulted from prograde reactions at moderate temperatures in the presence of a highly reducing carbon-bearing gas. The carburization occurred prior to the aqueous alteration episode that produced the Fe-rich phyllosilicates throughout the interchondrule matrix and the thin magnetite rims surrounding the carbide grains. The occurrence of the carbide intergrowths places an upper limit of <500 °C on the maximum postaccretional thermal processing that occurred on the Semarkona parent body.     

Experimental aqueous alteration of cometary dust

Abstract– Recent spacecraft missions to comets have reopened a long‐standing debate about the histories and origins of cometary materials. Comets contain mixtures of anhydrous minerals and ices seemingly unaffected by planetary processes, yet there are indications of a hydrated silicate component. We have performed aqueous alteration experiments on anhydrous interplanetary dust particles (IDPs) that likely derived from comets. Hydrated silicates rapidly formed from submicrometer amorphous silicates within the IDPs at room temperature in mildly alkaline solution. Hydrated silicates may thus form in the near‐surface regions of comets if liquid water is ever present. Our findings provide insight into origins of cometary IDPs containing both anhydrous and hydrated minerals and help reconcile the seemingly inconsistent observations of hydrated silicates from the Stardust and Deep Impact missions.     

HYDROLOGICAL RESPONSE OF SMALL WATERSHEDS FOLLOWING THE SOUTHERN CALIFORNIA PAINTED CAVE FIRE OF JUNE 1990

Following the Painted Cave Fire of 25 June 1990 in Santa Barbara, California which burned 1214 ha, an emergency watershed protection plan was implemented consisting of stream clearing, grade stabilizers and construction of debris basins. Research was initiated focusing on hydrological response and channel morphology changes on two branches of Maria Ygnacio Creek, the main drainage of the burned area. Research results support the hypothesis that the response of small drainage basins in chaparral ecosystems to wildfire is complex and flushing of sediment by fluvial processes is more likely than by high magnitude debris flows. During the winter of 1990–1991, 35–66 cm of rainfall and intensities up to 10 cm per hour for a five-minute period were recorded with a seasonal total of 100% of average (normal) rainfall (average=63 cm/year). During the winter of 1991–1992, 48–74 cm of rainfall and intensities up to 8 cm per hour were recorded with a seasonal total of 115% of normal. Even though there was moderate rainfall on barren, saturated soils, no major debris flows occurred in burned areas. The winter of 1992–1993 recorded total precipitation of about 170% of normal, annual average intensities were relatively low and again no debris flows were observed. The response to winter storms in the first three years following the fire was a moderate but spectacular flushing of sediment, most of which was derived from the hillslopes upstream of the debris basins. The first significant storm and stream flow of the 1990–1991 winter was transport-limited resulting in large volumes of sediment being deposited in the channel of Maria Ygnacio Creek; the second storm and stream flow was sediment-limited and the channel scoured. Debris basins trapped about 23 000 m³, the majority coming from the storm of 17–20 March 1991. Sediment transported downstream during the three winters following the fire and not trapped in the debris basins was eventually flushed to the estuarine reaches of the creeks below the burn area, where approximately 108 000 m³ accumulated. Changes in stream morphology following the fire were dramatic as pools filled with sediment which greatly smoothed longitudinal and cross-sectional profiles. Major changes in channel morphology occur following a fire as sediment derived from the hillslope is temporarily stored in channels within the burned area. However, this sediment may quickly move downstream of the burned region, where it may accumulate reducing channel capacity and increasing the flood hazard. Ecological consequences of wildfire to the riparian zone of streams in the chaparral environment are virtually unknown, but must be significant as the majority of sediment (particularly gravel necessary for fish and other aquatic organisms) entering the system does so in response to fires. © 1997 John Wiley & Sons, Ltd.