摘要:

Although ureilites have been considered bizarre meteorites, our analysis of their properties suggests that they formed by simple, well‐known processes. We conclude that: (1) ureilites are the sole rock type in a major fraction of their parent body; (2) they are partial melt residues and not cumulates; (3) removal of 90% of the Al in the basalts would largely stop radioactive heating if26Al was the heat source, ensuring that large volumes of ureilite‐like material were generated; (4) they formed in a large chondritic body that had a heterogeneous O isotopic composition; (5) slow diffusion of O in solid and molten silicates ensured that partial melting did not homogenize the O isotopes of the residues; (6) the puzzling absence of the basaltic melts that accompanied ureilite formation probably results from ejection into space by explosive volcanism. Together, these conclusions provide a simple, plausible origin for ureilites that accounts for most of their properties: ureilites are residues from the partial melting of isotopically heterogeneous chondritic material. We suggest that other meteorite parent bodies and planetesimals may have experienced similar histories, and that ureilites provide important constraints on the accretionary and igneous histories of planets and astero

ISSN:0094-8276    

DOI:10.1029/93GL00474

年代:1993

数据来源: WILEY

摘要:

We have used near‐infrared reflectance spectra and Earth‐based radar data to investigate the composition and origin of the various geologic units northwest of Humorum basin as well as the stratigraphy of the Humorum pre‐impact target site. The results of our spectral analysis indicate that at least a portion of the inner, mare‐bounding ring is composed of pure anorthosite. Other highlands units in the region are dominated by noritic anorthosite. The anorthosites on the inner ring may have been derived from a layer of anorthosite that exists at depth beneath a more pyroxene‐rich unit. Both Gassendi G and F craters expose mare material from beneath a highlands‐rich surface unit that was emplaced as a result of the Letronne, Gassendi, and other impact events. This ancient basalt unit was emplaced after the formation of Humorum basin but prior to the Orien

ISSN:0094-8276    

DOI:10.1029/92GL02921

年代:1993

数据来源: WILEY

摘要:

It is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust. At the East Pacific Rise this neutral buoyancy level lies only 100–400 m below the sea floor, significantly shallower than the depths (>1–2 km) of the magma bodies imaged in multichannel reflection data, suggesting that other factors must control the collection of melt in these reservoirs. The apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis, not the buoyancy of melt, is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid‐ocean r

ISSN:0094-8276    

DOI:10.1029/93GL00295

年代:1993

数据来源: WILEY

摘要:

A series of 1503 induced seismic events ranging in magnitude from −2 to 1 as computed based on a locally calibrated seismic moment scale were located within a well‐defined volume at depth. The seismicity analysis was carried out over 2 months and included events recorded before, during, and following the after‐shock period of amN2.6 rockburst. A non‐similar pattern was clearly apparent for both cumulative and non‐cumulative frequency‐magnitude distributions, showing a relative enhancement of seismicity between magnitudes −0.5 and 0. This supports the existence of non‐similar scaling at these magnitudes. The time variation of theb‐value, as computed for a limited self‐similar magnitude domain (−1.1 to −0.4), did not appear to be related to the rockburst. Instead, it is suggested that the rockburst perturbed the local stress field and consequently the distribution of the very small events. No correlations were found betwe

ISSN:0094-8276    

DOI:10.1029/93GL00426

年代:1993

数据来源: WILEY

摘要:

Infrared remote sensing provides a means for radiometry of hot volcanic features. Previously, two spectral band measurements have been combined to solve simple 2‐component models of lava surface temperatures. However, pixel dimensions often exceed the widths of lava bodies. In such cases, 3‐component models are more realistic. Here, a procedure for solving such a model from three infrared observations is presented. It is then demonstrated using Thematic Mapper Simulator data recorded over Mount Etna, Italy, in 1991. By constraining temperatures and pixel‐filling fractions of the three different thermal components in this way, improved estimates of radiant emittance can be obtained. The techniques described could be applied to satellite data of volcanic, and other, hot spots (including f

ISSN:0094-8276    

DOI:10.1029/93GL00500

年代:1993

数据来源: WILEY

摘要:

We propose that the broad spectrum of interannual variability in the tropics with a peak around four years results from an interaction between the linear low frequency oscillatory mode of the coupled system and the nonlinear higher frequency modes of the system. We illustrate this using a conceptual model where the higher frequency modes, represented by a nonlinear low‐order model, are coupled to a low frequency linear oscillato

ISSN:0094-8276    

DOI:10.1029/93GL00206

年代:1993

数据来源: WILEY

摘要:

Preliminary investigations show regional differences in atmospheric14C activity archived in tree‐ring cellulose may reflect oceanic and atmospheric shifts associated with ENSO (El Niño‐Southern Oscillation) events. Much of the shared variation between sites in Washington and Arizona can be attributed to global forcings such as anthropogenic14C‐depleted CO2emissions and modulation of14C production. The contrasted residual variation between the Washington and Arizona data sets correlates with the annual Southern Oscillation Index (SOI) of the previous year. The spatial differences between Δ14C data sets may provide a proxy indicator of ENSO phe

ISSN:0094-8276    

DOI:10.1029/93GL00498

年代:1993

数据来源: WILEY

摘要:

Proxy records of biogenic sulfur gas obtained from ice cores suggest that variability in marine biogenic sulfur emissions may reflect changes in climate [Saigne and Legrand, 1987; Legrand et al., 1988, Legrand et al., 1991; Anderson and Charlson, 1991]. Increased sea‐ice extent has previously been proposed as one cause of relatively high methanesulfonic acid (MSA) in glacial‐age ice core samples [Gibson et al., 1990]. We have analyzed MSA, one of the oxidation products of the biogenic sulfur gas dimethylsulfide [Hatakeyama et al., 1985], from snowpit samples recovered from a coastal site in Southern Victoria Land, Antarctica. Time series of MSA correlate significantly with the longest continuous record available of Southern Ocean sea‐ice extent (two decades) [Jacka,

ISSN:0094-8276    

DOI:10.1029/93GL00499

年代:1993

数据来源: WILEY

摘要:

Since the middle of July 1991, approximately one month after the explosive eruptions of Mt. Pinatubo in the Philippines, Bishop's ring phenomena were occasionally observed over Japan under favorable conditions of clear skies. On the basis of the single‐scattering approximation, the chromaticity of Bishop's rings was simulated. It was found that Bishop's rings are observable only when a sufficient amount of aerosols exist that have an unusually narrow mono‐modal size distribution and a mode radius between 0.57μm and 0.63μm, with an effective variance of less than

ISSN:0094-8276    

DOI:10.1029/93GL00512

年代:1993

数据来源: WILEY

摘要:

An aircraft equipped with a new video camera designed to count ice crystals flew into the Hokuriku winter cumulus clouds which developed over the Japan sea during typical Monsoon weather. On one day we encountered cumulus clouds which formed many small ice crystals. There, a very high concentration of ice crystals in the size of 60/100 μm in diameter was measured when both large and small graupel were observed, while medium‐sized graupel were missing. It was proposed that ice crystals were shattered in the collision of large and small graupel, which have different ice surfaces formed by different mod

ISSN:0094-8276    

DOI:10.1029/93GL00613

年代:1993

数据来源: WILEY