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21.
Noriko T. Kita Qing‐Zhu Yin Glenn J. MacPherson Takayuki Ushikubo Benjamin Jacobsen Kazuhide Nagashima Erika Kurahashi Alexander N. Krot Stein B. Jacobsen 《Meteoritics & planetary science》2013,48(8):1383-1400
High‐precision bulk aluminum‐magnesium isotope measurements of calcium‐aluminum‐rich inclusions (CAIs) from CV carbonaceous chondrites in several laboratories define a bulk 26Al‐26Mg isochron with an inferred initial 26Al/27Al ratio of approximately 5.25 × 10?5, named the canonical ratio. Nonigneous CV CAIs yield well‐defined internal 26Al‐26Mg isochrons consistent with the canonical value. These observations indicate that the canonical 26Al/27Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI‐forming region. The internal isochrons of igneous CV CAIs show a range of inferred initial 26Al/27Al ratios, (4.2–5.2) × 10?5, indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial 26Al/27Al ratios is also obtained from the internal isochrons of many CAIs (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAIs and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CVs, are significantly depleted in 26Al. At least some of these refractory objects may have formed prior to injection of 26Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium‐isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of 26Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial 26Al/27Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAIs with the canonical 26Al/27Al ratio. 相似文献
22.
V. Debaille Q.-Z. Yin A.D. Brandon B. Jacobsen 《Earth and Planetary Science Letters》2008,269(1-2):186-199
Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled 176Lu–176Hf and 147Sm–143Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive 176Hfi from + 46.2 to + 50.4 and 143Ndi from + 36.2 to + 39.1. Another, termed as enriched, has negative 176Hfi = − 16.5 to − 13.2 and 143Ndi = − 7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive 176Hfi = + 30.0 to + 33.4 and 143Ndi = + 16.9. Together, they describe mixing curves between 176Hf/177Hf, 143Nd/144Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd)source < (Sm/Nd)sample, but (Lu/Hf)source > (Lu/Hf)sample. This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars. 相似文献
23.
Sensitivity Kernels for Time-Distance Inversion 总被引:1,自引:0,他引:1
Jensen Jesper Munk Jacobsen Bo Holm Christensen-Dalsgaard Jørgen 《Solar physics》2000,192(1-2):231-239
Inversion of local-area helioseismic time-distance data has so far only been done in the ray approximation (Kosovichev, 1996). Since this is a high-frequency approximation its applicability can be questioned for the solar case. Bogdan (1997) showed that for a simple solar model the localized wave packets do follow, but are not confined to, the ray path. We use an approximation based on the first Fresnel zone that has been developed in geophysics by Snieder and Lomax (1996) to go beyond the ray approximation in the inversions. We have calculated sensitivity kernels using both approximations. To test them we use a finite-difference forward modeling of the whole wave field in an acoustic medium reminiscent of the Sun. We use the finite-difference modeling to calculate sensitivity kernels for the full wave field and compare this with the other kernels. The results show that the Fresnel-zone-based kernels are in good agreement with the sensitivity obtained from the modeling. Thus these new kernels represent a significant step forward in the inversion of time-distance data. 相似文献
24.
H. Htún M.R. Payne G. Beaugrand P.C. Reid A.B. Sand H. Drange B. Hansen J.A. Jacobsen D. Bloch 《Progress in Oceanography》2009,80(3-4):149-162
Pronounced changes in fauna, extending from the English Channel in the south to the Barents Sea in the north-east and off Greenland in the north-west, have occurred in the late 1920s, the late 1960s and again in the late 1990s. We attribute these events to exchanges of subarctic and subtropical water masses in the north-eastern North Atlantic Ocean, associated with changes in the strength and extent of the subpolar gyre. These exchanges lead to variations in the influence exerted by the subarctic or Lusitanian biomes on the intermediate faunistic zone in the north-eastern Atlantic. This strong and persistent bottom-up bio-physical link is demonstrated using a numerical ocean general circulation model and data on four trophically connected levels in the food chain – phytoplankton, zooplankton, blue whiting, and pilot whales. The plankton data give a unique basin-scale depiction of these changes, and a long pilot whale record from the Faroe Islands offers an exceptional temporal perspective over three centuries. Recent advances in simulating the dynamics of the subpolar gyre suggests a potential for predicting the distribution of the main faunistic zones in the north-eastern Atlantic a few years into the future, which might facilitate a more rational management of the commercially important fisheries in this region. 相似文献
25.
Wilfred H. Theakstone & Frank M. Jacobsen 《Geografiska Annaler: Series A, Physical Geography》1997,79(4):201-214
Twentieth-century changes in Norwegian glaciers have been pronounced, but the different geometries and dynamics of the glaciers have caused different responses to similar climatic changes. Close to the Arctic Circle, all the glaciers of Svartisen, the largest ice-covered area of northern Scandinavia, have retreated since the beginning of the century. However, several of the smaller glaciers which end at relatively high altitude have experienced both periods of advance and periods of retreat since the mid-1960s. The mass balance of Engabreen, the largest of the West Svartisen glaciers, was positive in 21 of the 27 years to 1995–96. The sizes of most of the glaciers of the Okstindan area, 60 km south-east of Svartisen, have also decreased throughout the twentieth century, but Corneliussens Bre, a small glacier at the eastern side of the massif, has been advancing since 1970. The areas supplying some of the southern glaciers of Okstindan have been reduced as a result of changes in ice thickness at high altitude. Studies of glacier change are aided by the use of digital terrain models (DTMs). Triangular irregular network DTMs of the surface and bed topography of the largest of the Okstindan glaciers, Austre Okstindbreen, have been used in studies of mass-balance variations and changing surface flow patterns between 1976 and 1995. 相似文献
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Werner Jacobsen 《International Journal of Earth Sciences》1940,31(3-4):255-284
Ohne Zusammenfassung 相似文献
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In the inner magnetosphere of Saturn, the plasma density and drift velocity are high enough, and the photoelectron current low enough, for a Langmuir probe to produce useful data on ion parameters. Plasma density and velocity are found by analyzing the current due to collected ions and emitted photoelectrons for a negative probe potential. In order to correctly analyze the data, the current caused by photoelectrons emitted from the probe must be known. For a spherical probe at negative bias this should be a constant current, but for Cassini's probe it varies with attitude. A likely cause of this is a leakage current from the stub to the probe. The plasma drift velocities derived from Langmuir probe measurements did not agree with those found by the Cassini plasma spectrometer in the inner magnetosphere, but did so elsewhere. A possible solution to this is a two-component plasma where the components have different drift velocities. 相似文献