Rare earth and other trace elements in historic azorean lavas

Rare earth element (REE) and other trace element compositions of 16 lavas from all historic and 2 prehistoric eruptions on 5 islands of the Azores Archipelago show notable intra-and inter-island differences. Fe enrichment and “compatible” element depletion due to fractional crystallization have been superimposed on variations established in the source area. Fractionation of La/Sm, U/Th, K/Na and “large ion lithophile” (LIL) element abundances are probably related to variable fusion of a source peridotite whose LIL element distribution cannot be exactly specified in view of its possible heterogeneity. Relative light-REE enrichment in basalt appears greatest on the “potassic” island São Miguel, the more sodic island Fayal and one lava from Pico, and least in basalts from the “sodic” islands Terceira, São Jorge and Pico. This variation is matched by most other LIL elements, although P shows unexpected enrichment in Terceira lavas, otherwise the least LIL element-enriched and most heavy-REE-enriched. Upper mantle phase chemistry is probably critical in establishing the patterns. In particular, P—REE covariance may reflect phase stabilities of apatite and (P-bearing) garnet in the upper mantle. Distribution patterns of REE in the historic lavas are similar to those of basalts from the Atlantic median rift at the crest of the Azores “platform”. Transition to light-REE-depleted rift-erupted basalts to the southwest is believed to be step-wise with increasing water depth, possibly indicating retention of a light-REE-rich phase in the residue from partial fusion as intersection of geotherm and peridotite solidus occur at lower pressures. The source mantle for the Azores basalts is probably light-REE- and LIL element-enriched but we find no evidence so far to suggest its emplacement by thermal “plume” activity.     

白云鄂博碳酸岩的方解石-白云石地质温度计

利用方解石-白云石地质温度计对白云鄂博地区碳酸岩的平衡温度进行了测定。出露于东矿下盘的白云岩质火山岩和出露于尖山的方解石-白云石型火山岩获得了较高的温度,分别为681℃和648℃。这些样品中的方解石呈二十微米左右晶形较完整的小片,被稍大粒度的白云石颗粒包裹,未受交代作用影响,推测这种碳酸岩在快速冷却的情况下保存下了其岩浆侵位时的成分特点,从而指示出接近碳酸岩浆侵位时的温度。但本区多数碳酸岩的平衡温度在400～500℃之间,有下列三种情况:(1)具有自形-半自形中粗粒粒状变晶结构的碳酸岩最后的平衡温度为415～496℃;(2)产自东矿的其余样品(火山岩),所测最后平衡温度为431～485℃,在测温的微区范围内可见极细粒白云石方解石与稀土等矿物共生的现象;(3)为交代重结晶结构的碳酸岩明显受到后期热液流体的交代,在流体的作用下共生方解石和白云石在成分上达到新的平衡,平衡温度为432～507℃。本文所分析的样品多数结果(371～507℃)与用白云石(方解石)和磁铁矿氧同位素温度计对白云鄂博碳酸岩的计算结果(360～546℃)十分一致。虽然有研究者对方解石-白云石温度计用于火成碳酸岩表示过质疑,但本文资料表明火成碳酸岩最后的平衡温度是可以运用方解石-白云石温度计法来计算的。     

The Stress Accumulation Method and the Pattern Informatics Index: Complementary Approaches to Earthquake Forecasting

In this work, we provide a joint study of the stress accumulation method (SAM) (King and Bowman, 2003) and the Pattern Informatics (PI) index (Tiampo et al., 2002b). We examine the theoretical underpinnings for the similarities between the two techniques, as well as the differences in their application. The SAM technique is employed to determine likely mechanisms for smaller areas of increased probability identified by the PI index, while a modified version of the PI index can be used to locate regions where the smaller magnitude associated with the anomaly is below the resolution of the SAM. Finally, we present three case studies from different regions of the San Andreas fault system to illustrate both their complementary nature, as well as the advantages to combining them in one synthesized analysis.     

The influence of shallow landslides on sediment supply: A flume-based investigation using sandy soil

The impact of rainfall-induced shallow landslides on hillslope sediment discharge is not well understood. This paper reports experimental measurements of sediment discharge after water-induced shallow landslides are triggered on sandy soil in a flume under simulated rainfall. The principal aim of the research was to investigate how varying soil depth affects the location and occurrence of shallow slope failures, as well as how it affects sediment yields downslope. Four experiments were conducted using the same sandy soil and a 30° and 10° compound slope configuration under average rainfall intensity of 50 mm h^− 1 for up to 390 min. Soil depths were set to 200, 300, 400 and 500 mm. Engineering and geotechnical properties of the soil were examined. Sediment discharge and runoff were collected from the flume outlet at 15 minute intervals. Changes in the soil slope profiles after landslides and soil physical properties resulted from soil armouring, under continuous rainfall were also recorded. Results showed that sediment yields at the flume outlet, before landslides occurred, were very low and limited to the finer soil particles as would be expected for a sandy soil. However subsequent variations in sediment discharge were strongly related to failure events and their proximity to the outlet. Sediment yield was also affected by the original soil depth; the greater the depth, the higher the sediment yields. Post-failure reductions in sediment discharge were observed and attributed to post-failure slope stabilization under continuing rainfall and extensive soil armouring near the flume outlet. The results provide a clear linkage between landslides and sediment discharge due to hydrological processes occurring in the hillslope. This knowledge is being used to develop a model to predict sediment discharges from hillslopes following shallow landslide events.     

A sedimentary facies model for glacial-age sediments in Baldwin Lake, Southern California

A combined sedimentological and high-resolution petrographic analysis was conducted on a glacial-age (20,000–65,000 cal yr BP) sediment core from Baldwin Lake, Southern California. The results of this research represent the most complete glacial-age, terrestrial climate record from Southern California to date. These results are used to characterize the different sediment types and to investigate the difference in depositional processes and environments between the core's three predominant sediment types: massive, semi-laminated, and laminated sediments. Massive sediments are commonly associated with a blocky texture and/or desiccation cracks, are organic-poor, have high magnetic susceptibility values, and are coarser-grained. Thin-sections from massive sediments reveal a homogenous sediment fabric. Sub-centimeter-scale laminated and centimeter-scale semi-laminated sediments are generally organic-rich, have low magnetic susceptibility values, and are finer grained. Thin-sections from laminated and semi-laminated sediments reveal diffuse sub-millimeter- to millimeter-scale laminae. This combination of sedimentological and high-resolution petrographic data enabled us to characterize four sediment facies, each related to specific depositional processes and environments: (1) a playa lake; (2) a perennial shallow lake; (3) an intermediate lake with variable lake level; and, (4) a perennial deep lake. At centennial-to millennial-timescales, lower lake levels are represented by deposition of massive to semi-laminated sediments in a playa to a perennial shallow lake environment. At similar timescales, higher lake levels are recorded by semi-laminated to laminated sediments deposited in an intermediate lake to a perennial deep lake environment. These results provide an additional sedimentological study for comparison to similar arid environment basins, and for comparison to existing regional paleoclimatic reconstructions.     

Petrogenesis of the Kirkpatrick Basalt,Solo Nunatak,northern Victoria Land,Antarctica, based on isotopic compositions of strontium,oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial ⁸⁷Sr/⁸⁶Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ¹⁸O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial ⁸⁷Sr/⁸⁶Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ³⁴S= -4.01 to +3.41‰ Variations in (δ³⁴S probably resulted from outgassing of SO₂ under varying oxygen fugacities. Laboratory for Isotope Geology and Geochemistry (Isotopia), Contribution No. 71     

The small ice cap instability in seasonal energy balance models

Results from a two-dimensional energy balance model with a realistic land-ocean distribution show that the small ice cap instability exists in the Southern Hemisphere, but not in the Northern Hemisphere. A series of experiments with a one-dimensional energy balance model with idealized geography are used to study the roles of the seasonal cycle and the land-ocean distribution. The results indicate that the seasonal cycle and land-ocean distribution can influence the strength of the albedo feedback, which is responsible for the small ice cap instability, through two factors: the temperature gradient and the amplitude of the seasonal cycle. The land-ocean distribution in the Southern Hemisphere favors the small ice cap instability, while the land-ocean distribution in the Northern Hemisphere does not. Because of the longitudinal variations of land-ocean distribution in the Northern Hemisphere, the behavior of ice lines in the Northern Hemisphere cannot be simulated and explained by the model with zonally symmetric land-ocean distribution. Model results suggest that the small ice cap instability may be a possible mechanism for the formation of the Antarctic icesheet. The model results cast doubt, however, on the role of the small ice cap instability in Northern Hemisphere glaciations. Offprint requests to: J Huang     

Multi-technique approach to characterise the effects of cryopreservation on larval development of the Pacific oyster (Crassostrea gigas)

Cryopreservation experiments were conducted on D-stage larvae of the Pacific oyster (Crassostrea gigas) to investigate the effects of two cryoprotectant solutions and three cooling rates on larval development from 1 to 22 days post-fertilisation. Cryoprotectant solutions were made up to final concentrations (after 1:1 dilution with larvae) of 10% ethylene glycol, 1% polyvinylpyrrolidone and either 0.2 or 0.4 M trehalose. Three cooling rates (0.5, 1 and 2 °C min^?1 between ?10 and ?35 °C post-holding) were tested in an orthogonal design with the two cryoprotectants. Results indicate that control larvae out-performed all cryopreservation treatments for survival, feeding consumption and shell length parameters. However, larvae exposed to 0.4 M trehalose did considerably better than those exposed to 0.2 M trehalose, regardless of cooling rate conditions. Scanning electron and light microscopy observations were used to assess larval morphology and organogenesis, indicating that treatments with surviving larvae were morphologically and developmentally similar to control larvae.     

Climate-induced sediment-palaeosol cycles in a Late Cretaceous dry aeolian sand sheet: Marília Formation (North-West Bauru Basin, Brazil)

Aeolian sand sheets, which are characterized by low relief surfaces that lack dunes, are common in arid and semi‐arid climatic settings. The surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects, or unstable such that it is affected by deflation or sedimentation. The Marília Formation (Late Cretaceous) may be interpreted as an ancient aeolian sand sheet area, where alternating phases of stability and instability of the accumulation surface have been recorded. Detailed field studies were carried out in several sections of the Marília Formation, where cyclic alternations of palaeosols and aeolian deposits were evident, using palaeopedological and facies analysis methods, supported in the laboratory by the analysis of rock samples, cut and polished in slabs, thin sections, scanning electron microscope images and X‐ray diffraction data from the clay minerals. The deposits comprise three lithofacies that, in order of abundance, are characterized by: (i) translatent wind‐ripple strata; (ii) flood deposits; and (iii) ephemeral river channel deposits. Palaeosols constitute, on average, 65% of the vertical succession. Three types of palaeosols (pedotypes) are recognized: (i) Aridisols; (ii) Entisols; and (iii) Vertisols. Erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits. The palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the Marília Formation to be defined as a flat area, dominated by aeolian sedimentation, with subordinate ephemeral river sedimentation, and characterized by a dry climatic setting with occasional rainfall. The climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development. A climate‐controlled model is proposed in which: (i) the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases; and (ii) the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation, thereby increasing the availability of sediment during the drier climatic phases. The ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment.