ISSN:0094-8276    

DOI:10.1029/GL005i006p00417

年代:1978

数据来源: WILEY

摘要:

Rifts occur in diverse tectonic environments that result from the continuous two‐dimensional evolution of the multi‐plate mosaic of the Earth and also from the interaction between the mantle processes and the overlying lithosphere. Most rifts have associated volcanics that are mainly basaltic. In continental rifts these basalts are predominantly alkaline and the relative timing of rifting and volcanism is variable; however, despite this variation there seem to be two basic types of rifting‐volcanism relative timing: in one, volcanism and usually local doming predates major rift formation whereas, in the other, rifts form first and volcanism (? and doming) follow thereafter. These two basic types of rifting‐volcanism relative timing may be related to two basic modes of rifting. In the first the mantle plays an active role, convection ‘plumes’ dome up and crack the lithosphere, whereas, in the second, the horizontal movements of plates give rise to extension of the lithosphere and induce rifting. In this latter case, the mantle is passive. Numerous local conditions complicate this simple pattern and result in extremely complicated rifting‐volcanism relationships that make geophysical/geochemical modelling difficult. Petrologic/geochemical studies of rift volcanics do not provide unique solutions for our understanding of rift environments. Detailed stratigraphic/structural analysis of individual rifts are still the best methods for

ISSN:0094-8276    

DOI:10.1029/GL005i006p00419

年代:1978

数据来源: WILEY

摘要:

An integrated petrologic and geochemical study of basalts recovered in Legs 45 and 46 (DSDP) has yielded evidence that these moderately evolved basalts are mixtures of primitive, mantle‐derived tholeiites with more evolved magmas. The hybrid nature of these rocks is recognized on the basis of disequilibrium mineralogy. Plagioclase phenocrysts exhibit substantial diversity in composition and zoning patterns including both normally and reversely zoned grains and abundant textural evidence of resorption. Many olivine and plagioclase phenocrysts are too refractory (e.g., Fo90−88) to be in equilibrium with liquids of the host basalt composition and are in fact of a composition consistent with crystallization from a primitive, mantle‐derived basalt liquid. An estimate of this primitive melt, presumed to be parental to the observed lavas, has been derived from melt inclusions trapped in the olivine phenocrysts. These are characterized by high Ca/Al and low TiO2, two chemical features typical of primitive ocean floor basalts recovered elsewhere in the Atlantic.We suggest that subvolcanic magma chambers beneath mid‐ocean ridges receive periodic injections of this primitive melt and its attendant phenocrysts which mix with fractionated chamber‐bound magmas, resulting in observed moderately evolved lavas. Chemical modeling of ocean floor basalt differentiation assuming a combination of magma mixing and crystal fractionation alleviates some apparent anomalies encountered in previous studies (Rhodes et a

ISSN:0094-8276    

DOI:10.1029/GL005i006p00423

年代:1978

数据来源: WILEY

摘要:

Studies of ophiolitic complexes and thermal and mechanical speculations indicate a highly complex history of basaltic magmas at mid‐ocean ridges. A thin molten region about 3 km wide probably exists at the top of the magma chamber. Primitive material enters the uppermost part of the chamber from below. Fractionated material is removed by the upward extrusion of dikes and settling of crystals to form a mush which probably fills the bulk of the chamber. The roof of the magma chamber is probably cooler than the melting temperature of primitive basalt. Secondary melts are probably produced most voluminously in the cumulate mush and also at the distal ends of the magma chamber and the bottom portions of dikes which cannot immediately solidify. The volume of the uppermost molten region can in principle be determined from chemical heterogeneities in basalts over a limited area and time. For mechanical reasons, the volume of additions of material from below is comparable to the volume of crustal dikes. Weaknesses along the line of dike intrusion and limited plating of material onto the roof of the chamber may restrict dike intrusion to the exceeding narrow zone inferred from ophioltic complexe

ISSN:0094-8276    

DOI:10.1029/GL005i006p00426

年代:1978

数据来源: WILEY

摘要:

Size can be used as a criterion to select 18 large (>1 cm) samples from among 148 melt‐rock fragments of all sizes. This selection provides a suite of large samples which represent the important chemical variants among highland melt rocks; each large sample has enough material for a number of sample‐destructive studies, as well as for future reference. Cluster analysis of the total data base of 148 highland melt rocks shows six distinct groups: anorthosite, gabbroic anorthosite, anorthositic gabbro (“highland basalt”), low‐K Fra Mauro, intermediate‐K Fra Mauro, and high‐K. Large samples are available for four of the melt‐rock groups (gabbroic anorthosite, anorthositic gabbro, low‐K Fra Mauro, and intermediate‐K Fra Mauro). This sample selection reveals two sub‐groups of anorthositic gabbro (one anorthite‐poor with negative Eu anomaly and one anorthite‐rich without Eu anomaly). There is a sharp distinction between those Apollo 16 melt rocks and glasses which have both been classified

ISSN:0094-8276    

DOI:10.1029/GL005i006p00429

年代:1978

数据来源: WILEY

摘要:

Recent studies have greatly expanded our knowledge of lunar mare basalts. Since 1976 there has been a revision of the Apollo 12 low‐Ti mare basalt suite and the discovery of a new very low‐Ti (VLT:<1% TiO2) basalt suite at Apollo 17 and in the new Soviet samples from Mare Crisium (LUNA 24). Current studies suggest that the VLT basalts may be in some way related to the enigmatic “green glasses” which are found in the soils from every lunar landing site. Telescopic studies of spectral reflectance and crater systematics show that basalts of varying Ti content were extruded throughout the history of mare volcanism. These new discoveries indicate that mare basalts can no longer be classified into the two simple groups of older, high‐Ti basalts and younger, low

ISSN:0094-8276    

DOI:10.1029/GL005i006p00433

年代:1978

数据来源: WILEY

摘要:

Statistical relations have been determined between geometry, volume, slope, and age for 26 circum‐Pacific composite (strato) volcanoes. General trends in eruption characteristics, repose periods, flow lengths, and petrology are also documented. Few examples of the earliest stages of composite volcano activity are known, perhaps because these small volcanoes are indistinguishable from cinder cones. If cinder cones evolve into composite volcanoes a fundamental change in morphometry, eruption style, and petrology occurs at a basal diameter of 2 km.Composite volcanoes (stratovolcanoes), composed of layered deposits of pyroclastics and lavas, are the characteristic volcanic landform at sub‐ducting plate margins, and are the most abundant type of large volcano on the Earth's surface. Composite volcano morphology results from repeated eruptions of pyroclastics and relatively short lava flows from a central vent. By comparison, pyroclastics are insignificant and lava flows tend to be much longer (10‐100 km) for shield volcanoes. Most composite cones are formed of andesites or basaltic andesites, but some are composed of basalts (Fuji, Fuego, Izalco); thus petrology may be less important in determining the morphology of composite cones than eruption style, as is true for shield volcanoes (Wood, 1977a). In this study, observations of eruption characteristics and chemical/petrological trends for a number of composite cones were synthesized with newly determined measurements of cone morphology to document statistically the evolution of composite volcanoes and to examine their relation to cinder cones. Additionally, such quantitative descriptions of terrestrial volcanoes provide basic data for comparisons with volcanic structures on Mars and the

ISSN:0094-8276    

DOI:10.1029/GL005i006p00437

年代:1978

数据来源: WILEY

摘要:

The solidus for peridotite‐H2O‐CO2is a divariant surface traversed by univariant lines that locate the intersections of subsolidus divariant surfaces for carbonation or hydration reactions occurring in the presence of H2O‐CO2mixtures. Vapor phase compositions are normally buffered to these lines; the buffering capacity of carbonates is much greater than that of amphibole and phlogopite. Near the buffered curve for the solidus of partly carbonated peridotite, extending to higher pressures and lower temperatures from an invariant point near 26 kb‐1200°C, there is a temperature maximum on the peridotite‐vapor solidus. On the CO2side of the maximum, above 26 kb, CO2/H2O is greater in liquid than in vapor, and liquids are SiO2‐poor; on the H2O side of this maximum (including all pressures below 26 kb), H2O/CO2is greater in liquid than in vapor, and liquids change from forsterite‐normative to quartz‐normative with increasing H2O/CO2in vapor. Even traces of H2O and CO2, in minerals or vapor, lower mantle solidus temperatures through hundreds of degrees compared with the volat

ISSN:0094-8276    

DOI:10.1029/GL005i006p00440

年代:1978

数据来源: WILEY

摘要:

A well established data base exists for the oxidation‐reduction (redox) states of basalts on the Earth and on the Moon; the former equilibrate along the fayalite‐magnetite‐quartz buffer curve, and the latter crystallize in the field of metallic iron stability, below the iron wustite buffer. Preferred accumulation of volatiles and the disproportionation of water into hydrogen and oxygen in thick terrestrial lavas and in ponded lava lakes results in high states of oxidation equivalent to that of hematite stability. At the other extreme, lunar basalts exhibit the effects of subsolidus reduction and estimates yield T = 700‐1000°C and fO2= 10−16to 10−23atms. Based on a number of cosmogenic properties, models for planetary interiors, and the sequence of condensation with heliocentric distance from the protosun, estimates for the redox states of inner solar system planetary basalts yield the following results: Basalts on Mercury and the Moon crystallize below the iron‐wustite buffer curve; Venusian basalts are more oxidized than those on Mercury, less oxidized than those on Earth, and crystallization within the field of wustite stability is suggested; basalts on Earth are dominantly in the field of magnetite stability in close proximity to the fayalite‐magnetite‐quartz buffer curve; Martian basalts are estimated to crystallize in the upper regions of magnetite stability and well into the hematite field of stability expressed in terms of temperature a

ISSN:0094-8276    

DOI:10.1029/GL005i006p00443

年代:1978

数据来源: WILEY

摘要:

It is probable that elemental carbon exists in the source region of a basaltic magma and is suspended in the magma during ascent. As carbon has a large redox capacity, it is probably in control of the magmatic fO2from the source region to the deep crustal environment. Isothermally carbon becomes more reducing with decreasing pressure, and thus reduces the host magma upon ascent. If a magma is anhydrous (e.g., lunar basalts), the reduction by carbon continues through the extrusive phase and the relative fO2decreases rapidly until buffered by the precipitation of a metallic phase. If a magma is hydrous (e.g., terrestrial basalts), reduction by carbon is eventually superceded by oxidation due to the loss of H2, which is generated by the reaction of C with H2O and also by the thermal dissociation of H2O, and ferric iron is produced. Cumulus crystallization of ferrous silicates also contributes to the oxidation of magma. The relative fO2of a hydrous magma initially decreases as the magma ascends from the source region, and then increases until magnetite crystallization curbs the rising trend of the relative fO2.

ISSN:0094-8276    

DOI:10.1029/GL005i006p00447

年代:1978

数据来源: WILEY