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41.
Twelve apatite samples from igneous and metamorphic rocks from the Black Forest and igneous rocks from the Kaiserstuhl were analysed for their REE content. The ΣREE range from 0.116 to 1.69 wt.%; the lowest values were found in the metamorphic rocks. All apatites from the various parent rocks show a general enrichment of the lighter rare earths over the heavier and their chondrite-normalized rare earth patterns exhibit a more or less pronounced negative Ce anomaly. This Ce depletion is accompanied by relatively low La and Pr values. In addition, the apatites from igneous rocks from the Black Forest show a marked negative Eu anomaly. An explanation is offered for the simultaneous occurrence of a negative Ce and a negative Eu anomaly in one investigated rock system. The negative Ce anomaly is attributed to the occurrence of phases enriched in Ce (e.g., monazite, allanite) which crystallized prior to or simultaneously with apatite. The negative Eu anomaly in these magmatic apatites most probably is caused by discrimination of Eu2+ from the apatite lattice. The result of this discrimination is a selective Eu enrichment in the later crystallizing feldspars, plagioclase and orthoclase. At least in this case, the positive Eu anomaly in feldspars is not a reliable indicator of low oxygen fugacity during their crystallization; the Eu depletion of the earlier crystallized apatites is preferable for this purpose. 相似文献
42.
Harald Bhnel 《Journal of South American Earth Sciences》1999,12(6):121
Three sites from Cretaceous limestone and Jurassic sandstone in northern Oaxaca, Mexico, were studied paleomagnetically. Thermal demagnetization isolated site-mean remanence directions which differ significantly from the recent geomagnetic field. The paleopole for the Albian–Cenomanian Morelos formation is indistinguishable from the corresponding reference pole for stable North America, indicating tectonic stability of the Mixteca terrane since the Cretaceous. Rock magnetic properties and a positive reversal test for the Bajocian Tecomazuchil sandstone suggest that the remanence could be of primary origin, although no fold test could be applied. The Tecomazuchil paleopole is rotated 10°±5° clockwise and displaced 24°±5° towards the study area, with respect to the reference pole for stable North America. Similar values were found for the Toarcien–Aalenian Rosario Formation, with 35°±6° clockwise rotation and 33°±6° latitudinal translation. These data support a post-Bajocian southward translation of the Mixteca terrane by around 25°, which was completed in mid-Cretaceous time. 相似文献
43.
44.
Giancarlo Bellucci Joern Helbert Francesca Altieri Dennis Reiss Jean-Pierre Bibring Stephan van Gasselt Harald Hoffmann Yves Langevin Gerhard Neukum Franois Poulet 《Icarus》2007,192(2):361-377
In this paper we report about a small region on the northern scarp of Olympus Mons showing an increase of the 3 μm hydration band in the OMEGA spectra, together with low superficial temperatures. Although water ice clouds can occurs on the flank of big martian volcanoes, radiative transfer modeling indicates that atmospheric water ice alone cannot justify the shape of the observed band. A fit of the 1.9–3 μm absorption features is obtained by hypothesizing that the study region consists of a mixture of dust and water ice covered by an optically thin (τ=0.08 at 3 μm) layer of dust. Thermal modeling also suggests that water ice in this region may be stable during most of the martian year due to the saturation of the atmosphere. If water ice is responsible for the observed spectral behavior, it might consist of a number of ice or snow patches possibly deposited in small depressions. 相似文献
45.
46.
Marc Peters Harald Strauss James Farquhar Charlotte Ockert Benjamin Eickmann Cristiane L. Jost 《Chemical Geology》2010,269(3-4):180-196
The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H2S from different SMAR sites range from + 1.5 to + 8.9‰ in δ34S and from + 0.001 to + 0.051‰ in Δ33S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ34S and Δ33S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ34S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ34S values between ? 24.5 and + 6.5‰ and Δ33S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ33S = ? 0.008‰ reveal lower Δ33S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ34S and Δ33S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ33S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ34S were identical. 相似文献
47.
Large‐scale simulations of flow in deformable porous media require efficient iterative methods for solving the involved systems of linear algebraic equations. Construction of efficient iterative methods is particularly challenging in problems with large jumps in material properties, which is often the case in geological applications, such as basin evolution at regional scales. The success of iterative methods for this type of problems depends strongly on finding effective preconditioners. This paper investigates how the block‐structured matrix system arising from single‐phase flow in elastic porous media should be preconditioned, in particular for highly discontinuous permeability and significant jumps in elastic properties. The most promising preconditioner combines algebraic multigrid with a Schur complement‐based exact block decomposition. The paper compares numerous block preconditioners with the aim of providing guidelines on how to formulate efficient preconditioners. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
48.
Four Quaternary volcanic ash zones in the southern Norwegian Sea have been investigated in core P57-7 from the Iceland Plateau. Both the geochemical composition and morphological variation of each ash layer have been studied. The four volcanic ash zones appear in the light oxygen isotope stages 1, 5, 7 and 11. The ash zones are composed of transparent platy grains, light brown transparent grains, brown blocky and black blocky grains and white/transparent pumice, and each zone shows a distinct stratigraphic evolution. The geochemical results show a mixture of basaltic and silicic grains in each ash zone, and that each zone contains grains from more than one eruption. The geochemical investigations strongly suggest that all the ashes are derived from Iceland. The youngest ash zone includes two layers, which based on their geochemical composition and stratigraphic position are correlated with the 14C-dated Vedde Ash (10 600 yr BP) and Saksunarvatn Ash (9 100 yr BP). Possible sources on Iceland for these layers are discussed. 相似文献
49.
Nils-Olof Svensson 《地学学报》1991,3(4):369-378
Comparison of the land uplift pattern for the last 10,300 years, shown by studies of raised shorelines of the Baltic Ice Lake, with the pattern of present-day land uplift of Fennoscandia, shows that significant regional changes of uplift pattern have taken place. Some of these changes seem to be related to a halt in ice retreat during the Younger Dryas cold stage. It is also probable that some observed anomalies in the present uplift are not of glacio-isostatic origin, but are possibly related to structures in the lower lithosphere and upper mantle or large scale tectonics. 相似文献
50.
Three-frequency BDS precise point positioning ambiguity resolution based on raw observables 总被引:1,自引:0,他引:1
All BeiDou navigation satellite system (BDS) satellites are transmitting signals on three frequencies, which brings new opportunity and challenges for high-accuracy precise point positioning (PPP) with ambiguity resolution (AR). This paper proposes an effective uncalibrated phase delay (UPD) estimation and AR strategy which is based on a raw PPP model. First, triple-frequency raw PPP models are developed. The observation model and stochastic model are designed and extended to accommodate the third frequency. Then, the UPD is parameterized in raw frequency form while estimating with the high-precision and low-noise integer linear combination of float ambiguity which are derived by ambiguity decorrelation. Third, with UPD corrected, the LAMBDA method is used for resolving full or partial ambiguities which can be fixed. This method can be easily and flexibly extended for dual-, triple- or even more frequency. To verify the effectiveness and performance of triple-frequency PPP AR, tests with real BDS data from 90 stations lasting for 21 days were performed in static mode. Data were processed with three strategies: BDS triple-frequency ambiguity-float PPP, BDS triple-frequency PPP with dual-frequency (B1/B2) and three-frequency AR, respectively. Numerous experiment results showed that compared with the ambiguity-float solution, the performance in terms of convergence time and positioning biases can be significantly improved by AR. Among three groups of solutions, the triple-frequency PPP AR achieved the best performance. Compared with dual-frequency AR, additional the third frequency could apparently improve the position estimations during the initialization phase and under constraint environments when the dual-frequency PPP AR is limited by few satellite numbers. 相似文献