LATE EOCENE IMPACT MICROSPHERULES: STRATIGRAPHY,AGE AND GEOCHEMISTRY

Recent discoveries of microtektite and related crystal bearing microspherule layers in deep-sea sediments of the west equatorial Pacific DSDP Sites 292, 315A and 462, off-shore New Jersey in Site 612 and in southern Spain have confirmed the presence of at least three microspherule layers in Late Eocene sediments. Moreover, these discoveries have extended the North American strewn field from the Caribbean and Gulf of Mexico region to the northwest Atlantic, and have established a third strewn field in western equatorial Pacific and Indian Ocean which may extend to the Mediterranean. Stratigraphically the oldest microspherule layer occurs in the planktonic foraminifer Globigerapsis semiinvoluta Zone about 0.5 m.y. prior to the closely spaced crystal bearing microspherule layer and North American microtektite layer in the Globorotalia cerroazulensis Zone. Major element composition of the G. semiinvoluta Zone layer and the crystal bearing microspherule layer overlap, but there is a clear trend towards higher Al₂O₃ and FeO values in SiO₂ equivalent microspherules of the latter layer. The G. semiinvoluta Zone microspherules also contain a higher percentage of non-crystalline spherules (microtektites) than the crystal bearing microspherule layer, but lower than the North American microtektite layer. Excess iridium due to an abrupt increase in supply is associated with the middle crystal bearing microspherule layer and to a lesser extent with the other two layers. But, Ir excess due to concentration as a result of carbonate loss was also observed at two sites (462, 612). The three late Eocene microspherule layers do not precisely coincide with planktonic foraminiferal species extinctions, but a major faunal assemblage change is associated with the G. semiinvoluta Zone layer. Abundant pyrite is present in the North American microtektite layer of DSDP Site 612 suggesting reducing conditions possibly due to a sudden influx of biologic matter (dead bodies) to the ocean floor, and the crystal bearing microspherule layer coincides with five radiolarian extinctions. All three microspherule layers are associated with decreased carbonate possibly due to sudden productivity changes, increased dissolution as a result of sea-level and climate fluctuations, or the impact events.     

不同频段上极移,日长变化与大气角动量变化激发结果的比较

本文用ＪＭＡ大气角动量序列计算了对极移和日长变化的激发量（ｍ＇１、ｍ＇２和ｍ＇３），并分三个频段与天文观测得到的地球自转参数序列（ｍ１、ｍ２和ｍ３）进行了比较．结果表明：在钱德勒和季节性频段上，大气运动确实是固体地球自转变化的主要激发源．     

CHEMICAL VARIATIONS AMONG L-GROUP CHONDRITES,III. MAJOR ELEMENT VARIATION IN L6 CHONDRITES

Bulk chemical and mineral analyses of five L6 chondrites of shock facies d to f bring the number of L6 falls analyzed by Jarosewich to 20 and enable us: 1) to examine the chemical effects of shock melting in chondrites of the same petrologic type that presumably sample a limited stratigraphic range in their parent body; and 2) to seek depth-related chemical variations by comparing the compositions of L3 and melt-free L6 chondrites. The mean Fe/Mg, Si/Mg, S/Mg and Ni/Mg ratios of melt-free L6 chondrites (shock facies a to c) are virtually identical to those of L3 chondrites, suggesting that L-group material had the same bulk composition early (L6) and late (L3) in the accretion of the parent body. Wider variations of S/Mg and Ni/Mg in L6 chondrites may signify that L6 material was less well mixed than L3, or that some mobilization of metal and troilite occurred at shock intensities (facies c) too low to melt silicates. L6 chondrites that experienced shock melting of silicates (facies d to f) show wide variations of Fe/Mg, Si/Mg, S/Mg and Ni/Mg. It appears that most of the major element variation in the L-group is tertiary (shock-related) rather than primary (nebular, accretionary) or secondary (metamorphic). There is some evidence that L-group chondrites comprise two subgroups with different Fe/S ratios, but these subgroups are now poorly defined and their significance is unknown.     

THE MAJOR ELEMENT CHEMISTRY OF LIBYAN DESERT GLASS AND THE MINERALOGY OF ITS PRECURSOR

Pieces of high-silica, natural glass (Libyan Desert Glass), found on the desert surface of western Egypt, have been treated as an enigma for 50 years although it is virtually certain they are similar to tektites in being impact-derived products. New major element analyses of four Libyan Desert Glass specimens agree extremely well with the only other recent analysis and demonstrate that the original bulk analyses reported by Spencer (1939) are in error. The five modern analyses define a very tight chemical range for SiO₂ (97.38–98.25 wt %), Al₂O₃ (1.16–2.26 wt %), total Fe (0.15–0.60 wt % as Fe₂O₃) and TiO₂ (0.13–0.19 wt %). Measurable MgO (0.04–0.20 wt %) was found in one specimen. No other elements are present in greater than trace amounts. Microprobe analyses show that Al, Fe and Ti are all positively correlated with one another and are almost ubiquitously distributed throughout the glass. They must also have been so distributed in the LDG precursor material as mechanical mixing and elemental diffusion in the short-lived melt were limited. In contrast, Mg is sharply restricted in occurrence and correlates only with Fe, strongly suggesting a precursor Mg-Fe oxide or silicate mineral present as rare, discrete grains. Aside from rare accessory minerals, the parent material was a sand or sandstone composed of quartz grains coated with a mixture of kaolinite, hematite and anatase. This conclusion is based solely on the elemental distribution in the glass but is buttressed by the occurrence of both sand and sandstone, in southwestern Egypt, with the requisite mineralogy. However, mineralogic identity need not, in general, translate to a chemical match and it is entirely possible that the specific sand or sandstone facies involved in the glass formation no longer exists after 28 million years. Consequently, it may well be that evidence other than chemical comparisons will be needed to identify the presently unknown parent crater.     

太阳常数的变化与“快异”黑子群

本文对1980年SMM(峰年卫星)/ACRIM日射计观测的太阳常数与伴有快速变化的异常磁结构黑子群面积进行了比较分析,结果表明:当日面出现伴有快速变化的异常磁结构黑子群时,太阳常数减少。另一方面,当结构简单的“剩余”黑子群占优势时,太阳常数值轻微地增加。     

印、澳板块边界分布与GPS资料用于对2004年Sumatra-Andaman岛地壳破裂测地学机制的再讨论

基于作者此前对印度尼西亚苏门答腊-安达曼岛地震测地学机制的讨论，进一步根据McCloskey等人应用的INDI板块与AUST板块之间大致呈SW（≈S5）-NE（≈N2）走向的板块边界划分模型，利用GPS站速度资料，估计了沿整个苏门答腊-尼科巴岛地壳破裂带INDI板块与Burma／SEEURA板块之间的板块聚敛率，并与基于NUVEL-1A模型的INDI、AUST板块边界的分布所估计的AUST板块与Burma／SEEURA板块问聚敛率结果作了比较；根据2004年印尼苏门答腊-安达曼岛地壳破裂特征，对该地壳破裂的测地学机制作了进一步讨论，认为基于NUVEL-1A模型的INDI、AUST板块边界分布条件下地壳破裂的测地学机制讨论和2004年主震由AUST板块运动触发的结论可能更符合2004年地壳破裂事件的观测结论。目前的Burma微板块的欧拉旋转参数可能不适合用于对该次地壳破裂事件的测地学估计和机制的讨论；INDI、AUST板块间的相对运动是这两者板块间造山带的主要动力源。