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901.
Abstract The ages of a number of small fragments of lunar granophyre have been determined by the in situ U-Th-Pb isotopic analysis of zircon using a sensitive high mass-resolution ion microprobe (SHRIMP I). The zircon from lunar granophyre is characterized by consistently high U and Th contents (most 200–500 ppm and 100–300 ppm, respectively) compared to zircon from mafic lunar rocks. Some fragments of lunar granophyre are found to be as old as 4.32 Ga, supporting other evidence that the original lunar magma ocean crystallized completely within ~200 Ma of the formation of the Moon itself. Other fragments are as young as 3.88 Ga, which is much later than the time of formation of most of the lunar crust. The older lunar granophyres have rare-earth-element (REE) patterns that are similar to lunar KREEP, whilst the younger granophyres have bow-shaped REE patterns that feature a greater relative enrichment in the heavy REE. The wide range of ages of numerous lunar zircons, lunar granophyres and other rocks indicates that zircon-forming magmatism in the lunar highlands was most active prior to 4.3 Ga but continuous until at least 3.88 Ga. The U-Pb isotopic composition of much lunar zircon is near concordant, but the effects of isotopic disturbance as late as ~1.0 Ga are observed in some zircon, both within granophyre fragments recrystallized by reheating and within fragments in which the original delicate silica-K-feldspar granophyric intergrowth is well preserved. It is therefore essential to make multiple analyses of individual zircon grains, and preferably analyses of suites of zircons from lunar igneous rocks if they are to be dated reliably by the U-Pb method. It is possible that some of the younger lunar granophyres are the product of large-scale silicate-liquid immiscibility within late-stage differentiates, but this remains unproven until remnants of demonstrably cogenetic, Fe-rich, immiscible liquid are positively identified. 相似文献
902.
Randall Whitlock Charles F. Lewis James C. Clark Carleton B. Moore 《Meteoritics & planetary science》1991,26(2):169-170
Abstract— The Cerro los Calvos meteorite is a single stone of 68.5 g found in the Nuevo Mercurio strewn field of Zacatecas, Mexico (24°20′N, 102°8′W). It is an unusual H4 chondrite. Its olivine (Fa12.5) and orthopyroxene (Fs 11.7, Wo 0.8) are reduced relative to typical H chondrites. The La Banderia meteorite of 54.3 g from the same vicinity is an LL5 chondrite of shock classification e. 相似文献
903.
904.
Charles D. Evans 《Solar physics》1970,14(1):157-158
Conclusions At the 7 March, 1970 total eclipse, there were no significant differences between intensity levels between K-line and H-line, nor were there any finely structured features in the upper atmosphere of the sun in H larger than 2.5. The two H features close to the limb, though unexpected, do not appear to be remarkable.Operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation. 相似文献
905.
906.
907.
908.
Charles L. Hyder 《Solar physics》1967,2(3):267-284
The study of the disparitions brusques to flarelike brightening process in Paper I led to many specific characteristics that appear to agree with the observations. Therefore, I test the consequences of the model for active-region flares as well. As in Paper I, I find agreement between the predictions of the model and the available observational data.The class III flare of 28 August 1966 is discussed in detail.I also consider the possibility that the gravitational infall-chromospheric impact mechanism may be the source of energy for other chromospheric phenomena, such as the bright network seen in CaiiK2, plages, normal flares, etc.The available data certainly indicate that the infall-impact mechanism should be studied further, but no firm conclusions as to its overall importance can be drawn at the present time.The prominence destabilization mechanism and its significance to flare predictability is explored in a somewhat speculative manner. Changing multipole fields appear to yield the necessary space and time behaviors to fit the observed behavior of eruptive prominences.As in Paper I, the major conclusion is that the model looks good, but we need specific and detailed observations to test the model. 相似文献
909.
Charles F. Yoder 《Icarus》1982,49(3):327-346
The Martian satellites Phobos and Deimos move along nearly circular coplanar, stable orbits and have created surfaces older than ~ 109 years. The accretion hypothesis suggests that their primordial orbits were also very regular. However, tides raised on Mars and Phobos can substantially alter the semimajor axis a of Phobos' orbit over time. The effect of the Martian tidal torque alone on Phobos' orbit implies that the primordial e was ~0.1 to 0.2 about 4.6 × 109 years ago if the present observed e = 0.015 is naively interpreted as a tidally damped remnant. Significant tidal friction in Phobos reduces the time scale for Phobos to achieve a crossing orbit with Deimos to less than 109 years and permits the primodial e to approach unity. The consequences of orbital intersections cannot easily be resolved by assuming either a catastrophic origin for both satellites (namely, that both are fragments of a common parent body fractured by an impact) or that they were captured sequentially by Mars. Either hypothesis is difficult to accept, given that Deimos' orbit, which is only slightly affected by tides, is now so regular. An alternative scenario is proposed in this paper in which the observed e of Phobos results from several gravitational resonance excitations within the last 109 years, assuming tidal friction in Phobos has had only a small effect on its orbit. In facr, both the primordial e and the inclination i may have been much smaller than presently observed. The constraints imposed on tidal friction in Phobos by both the apparent age of Phobos' surface (> 109yrs) and the above scenario can be satisfied only of μQ > 1012dynes/cm2. Since the Q factor is ~102, the rigidity μ > 1010dynes/cm2. Thus Phobos should have substantial internal strength. 相似文献
910.