The production rate of Al from target elements in the Bruderheim chondrite

An 840-g specimen of the Bruderheim chondrite was subjected to magnetic and heavy-liquid mineral separation procedures, resulting in a number of chemically distinct samples. These samples were analyzed for cosmogenic Al²⁶ by non-destructive gamma-gamma coincidence counting. The observed Al²⁶ specific activities were correlated with the chemical composition of potential target elements by a weighted least-squares fitting technique. The calculated Al²⁶ production rates, in dpm per kilogram of target element, are: Al, 1130 ± 190; Si, 245 ± 31; S, 133 ± 11; Mg, 28 ± 30. Production rates from Ca and Ni + Fe were estimated to be 24 and 2.2 dpm/kg, respectively, from spallation systematics.     

裂陷盆地洼槽演化规律研究——以歧口凹陷西部为例

裂陷盆地中洼槽的演化规律对烃灶的分布及生烃能力具有明显控制作用。歧口凹陷在古近纪经历了两期两向的裂陷活动,洼槽结构较为复杂,裂陷Ⅰ幕,在NW-SE向伸展作用下,形成了多个NE-NNE向洼槽;至裂陷Ⅱ幕,受N-S向伸展作用,NE-NNE向断层活动性逐渐减弱,NE-NNE向洼槽开始萎缩。本文基于控洼断层的空间组合样式及传播方式、洼槽内层序界面的接触关系,建立洼槽演化的3种模式;结合典型地震剖面、地层厚度图、控洼断层断距—距离曲线等相关证据,识别出歧口凹陷洼槽的演化规律。其中,歧南洼槽属于“生长连接型洼槽演化模式”,板桥洼槽属于“固定长度型洼槽演化模式”,而歧北洼槽属于“侧列叠覆型洼槽演化模式”。     

Estimation of seismic hazard parameters in TERESA test areas

A general overview of some of the problems involved in earthquake catalogue handling is given as part of the works carried out into the ESC/SC8-TERESA project related with the seismic hazard assessment in two selected test areas: Sannio-Matese in Italy and the northern Rhine region (BGN). Furthermore, the necessary input data to be used in the calculation of seismic hazard has been obtained, including earthquake source zones and their seismic hazard parameters.The importance is pointed out of detailed analysis of seismic catalogues, mainly in relation to the use of aftershock information, the historical records of the region, and the possible temporal and spatial variation of seismicity, which could have an important influence on short-term hazard assessment.     

Chemical alteration of the oceanic crust

The alteration of the upper oceanic crust through sea water-basement rock interaction produces different and distinct alteration zones with increasing depth. The zonation generally shows a consistent worldwide pattern.The uppermost basement is influenced by oxidative sea water-basalt alteration at low temperatures and high water/rock ratios. With increasing crustal depth or even in a single pillow the temperatures rise, the water/rock ratios become lower, the redox and pH values normally decrease, and the oxidative zone is followed by a non-oxidative one. Below these zones the basement has suffered high-temperature alteration under reducing and acidic conditions.This results in chemical modifications of the rocks by the addition or leaching of certain elements, and has also consequences for the composition of sea water.From our own data and data presently available in the literature I summarize the mineralogical and geochemical characteristica of sea water alteration processes affecting the oceanic crust.

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Zusammenfassung Die chemische Veränderung der oberen ozeanischen Kruste durch Gestein/Meerwasser-Interaktionen produziert verschiedene und unterscheidbare Alterationszonen die von der Krustentiefe abhängig sind. Diese Zonierung zeigt weltweit vergleichbare Muster.Das oberste Basement wird von einer niedrig temperierten oxidativen Meerwasseralteration beeinflußt. Mit zunehmender Krustentiefe steigen die Reaktionstemperaturen an, die Redox- und pH-Werte werden kleiner, d.h. nichtoxidative Alterationsvorgänge dominieren. Unterhalb dieser Zonen wird die ozeanische Kruste überwiegend von reduzierenden und sauren Hydrothermallösungen alteriert.Im Verlauf der Seewasser/Basalt-Reaktionen werden die Gesteine mineralogisch und chemisch verändert, indem verschiedene Elemente aus den Basalten herausgelöst und wiederum andere aufgenommen werden. Diese Vorgänge haben auch für die Zusammensetzung des Meerwassers drastische Konsequenzen.Mit Hilfe publizierter aber vor allem mit eigenen Daten werden die mineralogischen und geochemischen Charakteristika der unterschiedlichen Alterationsprozesse zusammengefaßt dargestellt.

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Résumé L'altération de la partie supérieure de la croûte océanique sous l'action de l'eau de mer engendre plusieurs zones distinctes qui se succèdent de haut en bas. Ce zonage présente d'ordinaire une extension mondiale.La partie supérieure de la croûte est affectée d'une altération oxydante de basse température du basalte. Vers le bas, parfois même au sein d'un même pillow, la température s'élève, la quantité d'eau décroît, le potentiel redox et le pH diminuent, de sorte que la zone d'oxydation est suivie d'une zone non oxydante. En-dessous de ces deux zones, les roches ont subi une altération de haute température dans des conditions acides et réductrices.Il en résulte des changements de la composition chimique des roches par apport ou par lessivage de certains élements, ainsi qu'une modification de la composition de l'eau de mer.A partir des données de la littérature et surtout de mes propres observations, je présente une synthèse des caractères minéralogiques et géochimiques des divers processus de l'altération sous-marine.

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Early Cambrian reefs, reef complexes, and associated lithofacies of the Shackleton Limestone, Transantarctic Mountains

The Shackleton Limestone formed a carbonate platform that bordered part of the Greater Antarctic craton in middle and late Early Cambrian time. In the Holyoake Range of the central Transantarctic Mountains, this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans. Burrow-mottled lime mudstone, wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy. Thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes. The framework of these reefs was principally the product of micro-organisms, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they form less than 10% and are absent from some. On the basis of microbial composition, three reef types are recognized. The first type is a Renalcis boundstone that lacks archaeocyathans. Within these, abundant upward-directed thalii of Renalcis formed a framework that trapped fine-grained sediment. The second type, which forms the core of some larger reefs, is composed of stromatactis-bearing, microbial boundstone. The third, yet most common, reef type is variable in composition. It is characterized by the presence of abundant Epiphyton, but may include archaeocyathans, and the microbial microfossils Girvanella and Renalcis as well as cryptomicrobial clotted micrite. In this type of reef, frame-building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth-framework cavities, as well as rare metre-sized caverns. Within these spaces, Epiphyton and, less commonly Renalcis, encrusted framework elements, fine-grained sediments accumulated, and pervasive sea-floor cements were precipitated. Boundstone fabrics in the Shackleton Limestone are highly complex, with fabrics analogous to younger, more metazoan-rich reefs, as well as deep-water stromatactis-bearing mud-mounds. The Epiphyton-Girvanella-archaeocyathan frameworks and stromatactis-bearing boundstones, both of which seemingly first appeared in the middle Early Cambrian, are regarded as the precursors, in structure, composition, and preferred hydrologic setting, of the more extensive reefs and complex framework styles of later Phanerozoic time.     

Accretion rates and sediment accumulation in Rhode Island salt marshes

In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,²¹⁰Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr^?1 (0.25 to 0.60 cm yr^?1) based on the constant flux model, 0.40±0.15 cm yr^?1 (0.15 to 0.58 cm yr^?1) based on the constant activity model, and 0.44±0.11 cm yr^?1 (0.30 to 0.59 cm yr^?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess²¹⁰Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr^?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.     

The influence of marine inflows on the chemical composition of groundwater in small islands: the example of the Cyclades (Greece)

 Marine contamination of groundwater may be caused by seawater intrusion and by salt spray. The role of both processes was studied in the Cyclades archipelago on four small islands (45–195 km²) whose aquifers consist essentially of fractured, weathered metamorphic rocks. Annual rainfall ranges from 400 to 650 mm and precipitation has high total dissolved solids contents of 45–223 mg l^–1. The chemical characteristics of the groundwater, whose salinity is from 0.4 to 22 g l^–1, are strongly influenced by seawater intrusion. However, the effect of atmospheric input is shown in certain water sampling locations on high ground elevation where the dissolved chloride contents may attain 200 mg l^–1. Received: 14 November 1995 · Accepted: 9 September 1996     

Geochemistry of Tertiary tholeiites and picrites from Qeqertarssuaq (Disko Island) and Nuussuaq, West Greenland with implications for the mineral potential of comagmatic intrusions

Tertiary continental flood basalts on Qeqertarssuaq and Nuussuaq in West Greenland contain ～3?km of picrites and variably contaminated tholeiites. The picrites are in the Naujánguit member of the Vaïgat Formation and they have 7–29?wt% MgO, La/Sm?=?0.9–2.1, and ¹⁴³Nd/¹⁴⁴Nd?=?0.51263–0.51307. They appear to have crystallised from high-Mg parental magmas (14.4–16.4?wt% MgO) with isotope and trace element ratios similar to recent Icelandic picrites. Discrete horizons of tholeiites, including the Asûk and?Kûgánguaq, have elevated SiO₂ (50–58 wt%), La/Sm?=?3–7, ⁸⁷Sr/⁸⁶Sr?=?0.70550–0.71224, and low ¹⁴³Nd/¹⁴⁴Nd?=?0.51234–0.51174. These lavas have low Cu and Ni abundances (typically 10–50?ppm Ni or Cu), and in the case of the Asûk on Qeqertarssuaq, they contain droplets of native iron. The low Cu and Ni contents are attributed to scavenging by magmatic sulphides formed in response to crustal contamination of picritic magmas. Two contamination trends are recognised, one to a sediment end-member with high Th/Nb and Archaean model Nd ages, and the other to a meta-igneous component with high La/Sm, low Th/Nb and Rb/Nb, and Proterozoic source ages. Overall, ²⁰⁶Pb/²⁰⁴Pb varies from 16.47–21.68. Both contamination trends are associated with low Cu and Ni, and high SiO₂, and it is argued that the magmatic sulphides were triggered by the increases in silica, rather than simply by the introduction of additional crustal-derived sulphur. Geochemically, the Asûk and Kûgánguaq rocks resemble the most contaminated Nadezhdinsky lavas of the Siberian Trap, which are widely regarded as the source of the Ni and Cu mineralisation in the giant Noril'sk deposits. Mass balance considerations indicate that the parental liquids to the contaminated magmas contained sufficient Ni, Cu, S and platinum group elements to form substantial magmatic sulphide deposits. However, unlike the lavas at Noril'sk, the contaminated (low Cu and Ni) West Greenland basalts are in isolated units with no evidence for a gradual recovery in Ni and Cu abundances with height in the lava column. Comparison with Noril'sk suggests that although significant quantities of metals were scavenged by sulphides in West Greenland, the metal contents of the sulphides may not have been upgraded by continued interaction with subsequent magma batches.