摘要:

Detailed optical studies of the textural features of several hundred diamonds and associated phases in three specimens of the Canyon Diablo meteorite and one specimen of the Novo Urei achondrite were made. Diamonds in the Canyon Diablo are associated exclusively with troilite, graphite, and schreibersite, and they occur both in shock‐heated and unreheated specimens. Diamonds in the Novo Urei occur in pods and lenses interstitial to slightly deformed olivine and pyroxene crystals. The diamonds, once formed, were shattered and partially graphitized. X‐ray and electron microscope studies show that the diamond bodies, which range in size up to 2 mm in Canyon Diablo, are now composed of 0.5‐μ subindividuals with nearly random orientations. The evidence obtained from these studies indicates that the diamonds probably precipitated as a stable phase from a late‐freezing residuum composed primarily of iron sulfide, iron phosphide, and carbon, under high gravitational pressures. The diamonds were subsequently shattered, distorted, and partially graphitized, possibly from a pressure drop due to the disruption of the large parent body. The physical evidence against the theory that the diamonds originated by high temperature and pressures due to shock is co

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02403

年代:1964

数据来源: WILEY

摘要:

Several more or less unsatisfactory theories have been proposed for the origin of lunar sinuous rills such as Schröter's Valley. This paper presents a new explanation of the formation of these rills, namely, that they are valleys eroded by nuées ardentes. Characteristics of the rills, including form and association, are cited in support of this theory. Supporting evidence is found in the similarity of the rills to furrows eroded by nuées ardentes on the ear

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02423

年代:1964

数据来源: WILEY

摘要:

Simultaneous records of electrostatic field, field change, and radiation at 420 and 850 Mc/s were obtained for lightning flashes from 10 to 30 km distant. At these frequencies, the principal source of radiation appears to be associated with the development of streamers in the breakdown process. Strong radiation is associated with the stepped and dart leaders and with theKchanges in both the intracloud and cloud‐to‐ground flashes. The return stroke does not always produce detectable radiation at these frequencies; in about 50 per cent of the cases the radiation is absent or appears only after a delay of 60 to 100 μsec after the onset of the return stroke. This delay is attributed to the absence of radiation associated with breakdown streamers until the return stroke reaches the top of the channel within the cloud. A cessation of the dart leader radiation of 50 to 150 μsec before the return stroke also suggests that the radiation occurring during the dart leader phase is produced primarily within the

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02431

年代:1964

数据来源: WILEY

摘要:

Laboratory model vortex experiments and theoretical investigations were conducted to determine whether the unusual electrical phenomena which sometimes accompany a tornado might play an important role in the life cycle of the tornado. The electrodynamic accelerations of ions appear unimportant in either the formative or mature stages. Heat released by lightning may serve to initiate updrafts, but the effectiveness in tornado formation is difficult to evaluate in the presence of the latent heat available in a convectively unstable atmosphere. Laboratory experiments indicate that a more or less continuous electrical discharge in the center of a vigorous aerodynamic vortex will inhibit rather than augment the vortex circulation.

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02435

年代:1964

数据来源: WILEY

摘要:

We have sampled dust deposits from five Pacific Coast volcanoes to search for and analyze microscopic volcanic spherules that might possibly be similar to the supposed meteoritic spherules found in polar ice sediments. In the size range 10 to 100 μ, approximately 2 × 10−5of volcanic particles are perfect or nearly perfect spherules, and 2 × 10−3of them are rough magnetic spheroids. Chemical analysis, using an electron‐probe microanalyzer, shows them to be made up typically of some 0.4 Fe, 0.2 Si, 0.1 Al, 0.03 Ti, and 0.01 K. In composition they are similar to only a few of the polar glacier particles analyzed by us. In addition, we find that the numerical ratio of spherules to irregular particles for the volcanic dust is so different from that for the ice sediments that a volcanic origin for the latter seems impossible. Therefore, a meteoroidal origin for the arctic and antarctic spherules is still the most reasonable hy

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02449

年代:1964

数据来源: WILEY

摘要:

A spinner type rock remanent magnetometer is described. The instrument measures specimens having moments from 2 × 10−6emu upward, and measures moments from 10−5emu with precisions of 1° in direction and 5 per cent in magnitude. The lower limit for measurement corresponds to an intensity of magnetization of 5 × 10−8emu cm−3for the largest size of specimen which can be accommodated. A discussion is given of the factors which limit the sensitivity of a spinner magnetometer. Annulment of the geomagnetic field at the rotor is shown to be desirable when rocks having low remanence and high susceptibility are to b

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02455

年代:1964

数据来源: WILEY

摘要:

Thirty sites have been sampled in dolerites which intrude the Precambrian shield and the Umkondo system of eastern Southern Rhodesia. The natural remanent magnetization is random at sixteen of the twenty‐five sites used in the final analysis. In many cases it has been necessary to resort to the use of high alternating fields to successfully remove later components of magnetization, which are thought to be the result of isothermal remanent magnetization due to lightning currents. The directions of magnetization after treatment in alternating fields form two distinct groups about 90° apart, which indicates that these dolerites represent two quite distinct magmatic episodes. These directions are given byD= 292°,I= +51° (pole position 7°N, 20°W, A95= 7½°), andD= 182½°,I= −10½° (pole position 65°N, 42½°E, A95= 6°). The former group, which intrudes the Rhodesian Precambrian shield, are termed theMashonaland dolerites, and the latter, which are associated with the Umkondo system, are termed theUmkondo dolerites. Paleomagnetic evidence is presented to show that the Mashonaland dolerites have intruded the Umkondo dolerites, which are therefore the older of the two sets of intrusions. Potassium‐argon age determinations on some of the samples of Mashonaland dolerite used in this study indicate that this group of intrusions is at least 1600 m.y. old. This would indicate that the Umkondo dolerites and therefore the Umkondo system itself a

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02465

年代:1964

数据来源: WILEY

摘要:

Samples have been collected from twelve sites in red horizons of lower Permian (Ecca) age in southwest Tanganyika. Two directions of magnetization are observed, one steeply inclined with north‐seeking poles down and the other with south‐seeking poles down in the direction of the earth's present magnetic field. Specimens from all the sites were subjected to both alternating‐field and thermal demagnetization. These laboratory studies show that the component which is north‐seeking down with steep inclination is the stable direction of magnetization. It is suggested that the component aligned along the earth's present field is due to the growth of hematite (CRM) in this field. After treatment at temperatures of 300°C, specimens from nine of the twelve sites gave well‐grouped directions of magnetization. On the assumption of a geocentric axial dipole field, the mean pole position is at 38.2°S, 64.9°E (A95= 16°). This pole position places southern and eastern Africa in temperate or cold‐temperate climatic zones, which is in accord with the available paleoclimatic evidence. The polarity (reversed) of the Ecca of southwest Tanganyika is in accord with all other Permian rocks that have previous

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02477

年代:1964

数据来源: WILEY

摘要:

Oriented cores have been drilled at seven sites in ijolite exposures of the Shawa carbonatite complex, Southern Rhodesia. The ijolite has been dated at 209±16 m.y. from Rb → Sr decay. The directions of natural remanent magnetization are not consistent between sites. The ijolite proved difficult to treat by partial demagnetization in alternating fields, as instability of the direction of remanent magnetization tended to develop with demagnetizing fields only slightly larger than those required to remove isothermal magnetizations. Nevertheless, consistent directions of remanent magnetization were secured from five of the seven sites, after alternating‐field treatment. On the assumption of a geocentric dipole field, the mean south magnetic pole position is 64.2°S, 85.6°E, with radius of 95 per cent confidence 14.1°. This is in excellent agreement with other Mesozoic pole positions from

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02489

年代:1964

数据来源: WILEY

摘要:

During August 1963 fifteen sites were sampled in the Red Sandstone formation (upper Triassic) of the Zambezi Valley, Northern Rhodesia, covering a stratigraphic interval of more than 300 meters. Six of these sites proved to be measurable. Specimens were subjected to alternating‐field and thermal demagnetization, which demonstrated that the direction of magnetization was stable. The pole position calculated from the six sites is 68°S, 50½°E (δp = 5°, δm = 6½°). This pole position is in accord with other Mesozoic results fr

ISSN:0148-0227    

DOI:10.1029/JZ069i012p02495

年代:1964

数据来源: WILEY