On the possibility of obtaining relative paleointensities from lake sediments

Relative paleointensities are obtained from a 6-m sediment core from Lake St. Croix, Minnesota, spanning the time range from 445 to 1740 years B.P. To normalize the natural remanent magnetization (NRM) for variations in the magnetic content, a laboratory-induced remanence is chosen, whose alternating field (AF) demagnetization curves most closely resemble the NRM demagnetization curves. By plotting the ratio of the NRM to the normalizing remanence versus AF demagnetizing field, H_AF, for samples of the same sediment horizon, as well as for samples from different horizons, estimates are obtained for expected uncertainties in the relative paleointensities. For the Lake St. Croix sediments the anhysteretic remanence (ARM) demagnetization curves are very similar to those of the NRM's, and ARM is therefore used as the normalization parameter. Because the sediment exhibits homogeneous remanence properties throughout, and H_AF = 100Oe is the optimum “cleaning” field for the entire core, NRM₁₀₀/ARM₁₀₀ is evaluated to represent the fluctuations of the relative paleointensity. Our relative paleointensity data exhibit the same general features as obtained from archeomagnetic studies. The intensity increases as one goes back in time with a peak near 800 years B.P., representing an increase in the intensity of up to 60%. Apparent periodicities in the intensity of 300–400 years are observed.     

Mineral age patterns in ca. 3700 my old rocks from West Greenland

KAr,⁴⁰Ar³⁹Ar and RbSr dates are reported for minerals from the ca. 3700 my-old Amîtsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. KAr dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2250–2750 my) than biotites. One biotite from Isua gives an impossibly high KAr date of 4940 my.⁴⁰Ar³⁹Ar mineral dates are in close agreement with conventional KAr dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated.Rb-Sr measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic⁸⁷Sr at about 1600–1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south.The evidence suggests that those KAr biotite dates greater than about 2700–2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600–1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600–1700 my event.KAr mineral dates and an Rb-Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Qo?rqut granite, indicate an age of emplacement of 2580 ± 30 my.     

Despinning of the earth rotation in the geological past and geomagnetic paleointensities

Length of day (l.o.d.) values deduced from fossils and tidal deposits suggest that the despinning rate was, on the average, about 5 times smaller during the Proterozoic than during the Phanerozoic and, moreover, that between 250 and 100 million years ago, there was a slight non-linear variation super-imposed on the overall linear trend of the Earth's rotation rate. To explain these observational facts, it has recently been argued that formation of the inner structure of the Earth (mass redistribution within the mantle and/or core formation) had not been fully completed before the Proterozoic, and that the decrease of the inertia moment associated with the evolving terrestrial interior compensated to some extent the rotational effects of tidal friction. There is an another plausible explanation to account for the difference of despinning rates during the Proterozoic and Phanerozoic, namely: the distribution of the continents had been significantly different during these epochs and the world ocean had been much shallower in the Proterozoic than in the Phanerozoic. We used published data for the Phanerozoic, Proterozoic and Archean in order to check whether there had been significant long-term changes of geomagnetic intensity. Our results are based on robust statistical analysis; they indicate that during a time interval coinciding roughly with the Mesozoic, the geomagnetic dipole moment underwent a minimum in a quite similar way as the l.o.d. data. For the Proterozoic (2500–570 million years ago) and the late Archean (3000–2500 million years ago), it is very difficult to draw a conclusion concerning the variation in time of the intensity of the geomagnetic field: the data set we used is incomplete and the statistical scatter is larger than the derived mean value. Nevertheless, we tentatively conclude that the values of the average geomagnetic moment were approximately the same in the Phanerozoic and in the Proterozoic+late Archean, and that there is no significant long-term change in the geomagnetic intensity detectable before the Phanerozoic.     

Neodymium isotopic study of Baffin Bay water: sources of REE from very old terranes

Water samples were collected from Baffin Bay and surrounding areas in order to evaluate this region as a potential source of Nd from old continental material to Atlantic water. The isotopic data ranged from ε_Nd(0) = −9.0 to −26 with most of the data around ε_Nd(0) = −20 compared with values of North Atlantic Deep Water (NADW) with ε_Nd(0) = −13.5. The concentration of Nd in Baffin Bay waters was as high as 6 × 10⁻¹² g/g compared with 2.5 × 10⁻¹² g/g for NADW. The combination of low ε_Nd and high Nd concentration indicates that Baffin Bay may be a significant source of Nd from very old crustal material. A simple box model was used to evaluate the contribution to the Nd budget of NADW and it was concluded that a substantial fraction of the Nd from ancient crustal sources that is required to maintain the isotopic composition of NADW could be supplied by Baffin Bay outflow.     

中国大陆科学钻探井(CCSD)超高压变质岩岩性的识别

通过对中国大陆科学钻探井先导孔（CCSD-PH）测井资料的研究,并结合全井段的取芯资料,总结出了单一变质岩岩性在声、电、核三方面的测井响应,建立了岩性－测井响应数据库,并提取出了多种特征参数.应用聚类分析和逐步判别的方法,结合不同岩性的测井响应和多种特征参数,进行岩性的识别,以达到岩性剖面重组的目的.最后将岩性识别结果与取心资料进行对比,按厚度统计,达到了80％以上的符合率,取得了良好的应用效果.     

Problems in the dating of volcanic rocks by the potassium-argon method

The potassium-argon method is attractive for dating volcanics since it can be applied to rocks of Pleistocene age and older, thus encompassing important periods of general volcanic activity. However it has been found that dates obtained on whole rocks and on included minerals frequently show gross discordances. In order to establish this dating method in this application an attempt has been made to trace the sources of the anomalies. To illustrate these efforts, dating results from a rhyodacite of Mauna Kuwale, Oahu, Hawaii, are reported. Determinations on several minerals and the whole rock of this ridge give a concordant age of 2.3 ± 0.3 million years, excluding some few results on minerals which show old age anomalies. It has been noted that xenoliths in certain Hawaiian volcanics contain fluid inclusions which show evidence of formation at depth. We have found that gas released from such inclusions by crushing contains radiogenic argon, and that the constituent minerals give very old potassium-argon ages (circa 800 million years). Similar gaseous inclusions have been noted in a variety of other lava phenocrysts, and their presence in a dated sample may produce an anomalous old age. In the minerals from Mauna Kuwale sporadic occurrences of inclusions have been noted in biotites and hornblendes, and crushing of the mineral releases the excess radiogenic argon. The determination of the age of such a material would give an old age, and thus account for the anomalies found. For meaningful dating of volcanics by the potassium argon method it is concluded that phenocryst-containing materials should be examined for fluid inclusion content, and samples which contain these should be rejected.     

Microfractures in rocks from two geothermal areas

Core samples from the Dunes, California, and Raft River, Idaho, geothermal areas show diagenesis superimposed on episodic fracturing and fracture sealing. The minerals that fill fractures show significant temporal variations. Sealed fractures can act as barriers to fluid flow. Sealed fractures often mark boundaries between regions of significantly different physical properties. The fracture porosities measured on several samples are less than 0.1%. This low value indicates that fractures are effectively sealed or that fracturing is confined to the relatively few large fractures visible within the samples. Fracture sealing and low fracture porosity imply that only the most recently formed fractures are open to fluids.     

Hydrogen generation from mantle source rocks in Oman

Hydrogen gas, associated with Ca²⁺ OH-rich alkaline groundwaters (pH = 10–12), is at present emanating from ultramafic rocks of the Oman ophiolite. Isotopic and chemical evidence indicates that hydrogen is formed by low-temperature redox reactions in a closed groundwater environment. This is normally a cryptic process in the hydrosphere but is fortuitously revealed by the unusual hydrogeological conditions in Oman where atmospheric oxygen is totally assimilated. It is suggested that hydrogen generation in mantle source rocks at depth and in the early Archaean surface environment may be more widespread than has hitherto been realised.     

Remagnetization in tertiary rocks from the Faeroe Islands

Palaeomagnetic investigations of highly oxidized interbasaltic horizons and associated Lower Tertiary lavas from the Faeroe Islads suggest a long history of magnetization. The remanence directions are composed mostly of two deviating directions of opposite polarities aligned along an axis which palaeomagnetically appears to be younger than that of the original field. This, in addition to the observation that both the baking and baked rocks in most cases show almost identical directional trends upon demagnetization, questions the general validity of the baked contact test. It is suggested that to a major extent post-baking processes, such as low-temperature oxidation and precipitation of ferric oxides, have remagnetized the lavas as well as the tuff layers. Only two of the seven interbasaltic horizons investigated appear to have retained the ‘primary’ magnetization. It is concluded that besides a high bulk oxidation state (induced prior to, or as a result of baking) a “hornfelsic” nature of the baked sediments may be a necessary requirement for establishing primary palaeomagnetic directions in many volcanic regions.     

Uranium in volcanic rocks from the central Andes

The distribution of U in volcanic rocks from two transects across the Central Andes (latitudes 27°–28° S and 16°–18° S, respectively) differs from that of K. For a given SiO₂, content of the rocks, K systematically increases with the distance from the trench, while the highest U abundances are found in the rocks overlying the thickest segments of the continental crust, which are situated in the middle parts of the transects. It is suggested that this variation of U reflects crustal contamination.     

Palaeomagnetism of middle Ordovician volcanic rocks from Quebec

This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (d_m = 14°, d_p = 7°) after thermal treatment and 164°E, 19°N (d_m = 11°, d_p = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe.     

Potassium-argon radioages of karroo volcanic rocks from Lesotho

The geology of Lesotho is relatively simple: the overall structure being that of a large shallow basin of Karroo sediments and volcanics. The rocks analysed in this study were collected in December 1966. The principal objectives of the study were (a) to date the inception of Karroo volcanism and (b) to arrive at an estimate of the time-span represented by the lava section along the Bushman’s Pass road east of Maseru. The date at which volcanism began in Lesotho is important because of recent discoveries of early mammalian fossils in underlying beds; in relation to the Phanerozoic Time-Scale and to the K-Ar age pattern found in the Karroo dolerites of South Africa byMcDougall (1963). Cox andHornung (1966) have suggested that the fractionation stage reached by Karroo magmas may depend upon either the height of the magma column or the time that elapsed since the beginning of the volcanic episode. An estimate of the time-span of volcanism along the Bushman’s Pass section is of interest because of the extensive palaeomagnetic work done on these rocks at the Bernard Price Institute of Geophysical Research in Johannesburg. The paper contains the results of triplicate conventional total degassing whole rock K-Ar age determinations on 8 Drakensberg lavas and on 8 Karroo dolerite sills and dykes. The analysed rocks are described petrographically and the age pattern obtained from them is discussed in relation to the age and petrological information available from other Karroo igneous rocks. It is concluded that Karroo volcanism began in Lesotho around 187 m.y. and that « Karroo » intrusive activity continued intermittently until at least 155 m.y. ago. Some possible geological and petrological implications of these conclusions are outlined.     

Strontium isotope ratios in volcanic rocks from the South Sandwich Islands

⁸⁷Sr/⁸⁶Sr ratios in the island-arc tholeiite series from the South Sandwich Islands cluster about a value of 0.7040. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The uniformity of the⁸⁷Sr/⁸⁶Sr values is consistent with the fractional crystallization relationship previously proposed for this suite. Though higher than values reported for ocean-floor tholeiites they are not significantly different from ratios reported for calc-alkaline island-arcs.     

Strontium isotope composition in volcanic rocks from the Northeast Japan arc

In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. ⁸⁷Sr/⁸⁶Sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the Northeast Japan arc area. The results reveal that the ratios change systematically in space. Decreasing ⁸⁷Sr/⁸⁶Sr ratios across the arc were confirmed over a wide area of Northeast Japan. In the same direction, increases in both Rb and Sr contents were also found. The regular trends are considered to be a strong constraint for elucidation of subduction-originated magma genesis at the Eurasia plate vs. Pacific plate boundary. In the northern region of the Northeast Japan arc, ⁸⁷Sr/⁸⁶Sr ratios in volcanic rocks along the volcanic front were almost constant (0.7038–0.7045) and slightly higher than those from the Izu-Ogasawara arc (0.7032–0.7038). This suggests that “interactions” between the Eurasia plate and the Pacific plate, and those between the Philippine Sea plate and the Pacific plate are slightly different. The southern region of the Northeast Japan arc, where the direction of the volcanic front bends from southward to westward, showed anomalously high ⁸⁷Sr/⁸⁶Sr ratios, reaching to 0.7077. This region coincides with the triple junction of the Eurasia, Pacific and Philippine Sea plates, suggesting “anomalous interaction” at the triple junction.     

Elastic parameters of rocks from well logging in near surface sediments

Acoustic full waveforms recorded in wells are the simplest way to get the velocity of P, S, and Stoneley waves in situ. Processing and interpretation of acoustic full waveforms in hard formations does not generate problems with identification packets of waves and calculation of their slowness and arrivals, and determination of the elastic parameter of rocks. But in shallow intervals of wells, in soft formations, some difficulties arise with proper evaluation of the S-wave velocity due to the lack of refracted S wave in case when its velocity is lower than the velocity of mud. Dynamic approach to selection of a proper value of semblance to determine the proper slowness and arrival is presented. Correlation between the results obtained from the proposed approach and the theoretical modeling is a measure of the correctness of the method.     

Petrogenetic implications of uranium abundances in volcanic rocks from southern Peru

The content of uranium of andesitic rocks of southern Peru varies according to the distance from the Peru-Chile trench, as has been observed for other trace elements geochemically associated with K. The highest contents found in these rocks may exceed the estimated U content of the upper crust. This enrichment may be related to a variable degree of partial melting and/or crustal contamination of the same source material.     

Estimation of the rocks statistical parameters from traveltime measurements

In this paper the method for estimating the statistical parameters of the medium from traveltime measurements of refracted waves is applied to study the statistical characteristics of crystalline rocks at the Multifunctional Station Faido (Gotthard Base Tunnel, Switzerland). The method is based on the geometrical optics (GO) approximation. A covariance function for traveltime fluctuations has been obtained by considering quasihomogeneous fluctuations of sound velocity in a plain-stratified medium. Strongly anisometric (having unequal dimensions in different directions) random inhomogeneities were embedded in this medium. To estimate the statistical parameters around the tunnel, the traveltime fluctuations are calculated. It is assumed that each observation of traveltime-distance relation for a given shot-receiver group corresponds to a particular realization of a medium statistical ensemble. By calculating the variance and the zero cross intervals of the first derivative of traveltime fluctuations, the standard deviation of the velocity fluctuations and the characteristic horizontal scale of the inhomogeneities are estimated. Although the method allows to obtain the characteristic lengths of the inhomogeneities in vertical as well as in horizontal direction, the limited offset of the field data made it only possible to measure the latter. The estimated horizontal characteristic scale is about 13 m, which is reasonably close to the direct geological measurements in the studied region, where quartz lenses are dominant among the inhomogeneities. The standard deviation of the velocity is estimated as 4.5%, which might be caused by the fractured structure around the tunnel and also by the fault zone near the study area.     

Origin of volcanic rocks from Stromboli (Italy)

Quaternary volcanic rocks of Stromboli (Italy) can be divided into older calc-alkaline and younger shoshonitic series. The SiO₂ contents of the rocks range from 50% to 61% but the majority of them are basalts. The rocks show systematic variations in chemical composition which correlate with the volcanic stratigraphy, such that, at a given SiO₂ content, K and other incompatible elements such as REE increase with decreasing age. In addition, the La/Yb ratio increases while the K/Rb, K/Ba, Zr/Ce and Zr/Nb ratios decrease towards the top of the volcanic pile. On the other hand, the abundances of transition elements, V, Co, Sc and Zn, like most major elements are broadly similar in comparable rocks of different ages. It is suggested that the parent magmas were derived by partial melting from upper mantle peridotite enriched in incompatible elements by fluids released from the descending oceanic lithosphere. The temporal chemical variations may probably be related to the lengths of time during which fluids were in contact with the upper mantle source.