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At least 1,333 records · Page 74Linked to original sources
USGS1972

Oceanic ridges and transform faults: Their intersection angles and resistance to plate motion

The persistent near-orthogonal pattern formed by oceanic ridges and transform faults defies explanation in terms of rigid plates because it probably depends on the energy associated with deformation. For passive spreading, it is likely that the ridges and transforms adjust to a configuration offering minimum resistance to plate separation. This leads to a simple geometric model which yields conditions for the occurrence of transform faults and an aid to interpretation of structural patterns in the sea floor. Under reasonable assumptions, it is much more difficult for diverging plates to spread a kilometer of ridge than to slip a kilometer of transform fault, and the patterns observed at spreading centers might extend to lithospheric depths. Under these conditions, the resisting force at spreading centers could play a significant role in the dynamics of plate-tectonic systems.

Earth and Planetary Science Letters↗
USGS1972

The geologic setting of the Luna 16 landing site

The Luna 16 landing site is similar in its geologic setting to Apollos 11 and 12. All three sites are located on basaltic mare fill which occurs mostly within multi-ring basins formed by impact earlier in the moon's history. A regolith developed by impact bombardment is present at each of these sites. The regolith is composed mostly of locally derived volcanic material, but also contains exotic fine fragments that have been ballistically transported into the landing sites by large impact events which formed craters such as Langrenus and Copernicus. These exotic fragments probably consist mostly of earlier reworked multi-ring basin debris and, although not directly traceable to individual sources, they do represent a good statistical sample of the composition of most of the premare terrac regions.

Earth and Planetary Science Letters↗
USGS1972

Silicate melt inclusions and glasses in lunar soil fragments from the Luna 16 core sample

More than 2000 fragments were studied microscopically, and electron microprobe analyses were made of 39 selected areas, from a few square mm of polished surface, through 75- to 425-μm fragments of lunar soil from two samples of the Luna 16 core. The silicate melt inclusions and glasses differ in important details from those observed earlier in the Apollo samples. Melt inclusions in olivine contain epitaxially oriented daughter crystals, but also show a similar epitaxy around the outside of the crystals not observed in previous lunar samples. Melt inclusions in ilmenite suggest trapping at successive stages in a differentiation sequence. There is abundant evidence for late-stage silicate liquid immiscibility, with melt compositions similar but not identical to those from Apollo 11 and 12. A comparison of the alkali ratio of any given bulk rock analysis with that of its late-stage, high-silica melt shows gross differences for different rocks. This is pertinent to understanding late-stage differentiation processes. Glass fragments and spherules exhibit a wide range of crystallization textures, reflecting their wide range of compositions and cooling histories. No significant differences were found between the two portions of core examined (Zones A and D).

Earth and Planetary Science Letters↗
USGS1972

Lunar "dunite", "pyroxenite" and "anorthosite"

Monomineralic aggregates of olivine, clinopyroxene, orthopyroxene and plagioclase with granoblastic textures are widespread minor constituents of Apollo 14 breccias. Recrystallization is commonly incomplete within these aggregates, leaving relict material that clearly indicates single-mineral-grain sources for the aggregates. The aggregates are not, therefore, properly characterized by igneous rock names, nor can any conclusions regarding differentiation be drawn from them. Average sizes of the aggregates indicate source rocks with grain sizes mostly larger than 1 to 5 mm, a few clasts of which occur in the breccias; the proportions of the different types of aggregates suggest dominantly feldspathic source rocks.

Earth and Planetary Science Letters↗
USGS1972

Electron microprobe evaluation of terrestrial basalts for whole-rock K-Ar dating

Four basalt samples for whole-rock K-Ar dating were analyzed with an electron microprobe to locate potassium concentrations. Highest concentrations of potassium were found in those mineral phases which were the last to crystallize. The two reliable samples had potassium concentrated in fine-grained interstitial feldspar and along grain boundaries of earlier formed plagioclase crystals. The two unreliable samples had potassium concentrated in the glassy matrix, demonstrating the ineffectiveness of basaltic glass as a retainer of radiogenic argon. In selecting basalt samples for whole-rock K-Ar dating, particular emphasis should be placed on determining the nature and condition of the fine-grained interstitial phases.

Earth and Planetary Science Letters↗
USGS1972

Paleomagnetism of Midway Atoll lavas and northward movement of the Pacific plate

Two deep drill holes through the reef limestones of Midway Atoll penetrated 120 m and 19 m of basaltic lavas that were dated by the K Ar method at 18 my. Inclinations of natural remanent magnetization have been measured in 173 specimens cut from 57 core samples from 13 of the lava flows. The mean paleomagnetic inclination is27.6° ± 6.8°, corresponding to a paleolatitude of14.7° ± 4.2°. The present latitude of Midway is 28°, suggesting a northward component of motion of the Pacific plate of approximately 13° or 1400 km in the last 18 my. The paleolatitude of Midway is thus not significantly different from the present latitude (19°) of the active volcanic island of Hawaii. The paleomagnetic data from the Midway basalts thus support the hypothesis of Wilson and Morgan that volcanic heat sources are fixed with respect to the Earth's mantle below the asthenosphere and their apparent migration with time is due to plate motion.

Earth and Planetary Science Letters↗
USGS1972

Structural profile of the northwestern Caribbean

A seismic reflection and gravity profile across the continental margin of the Yucatan Peninsula, Yucatan Basin, Cayman Ridge, and Cayman Trough suggests that sediments in the Yucatan Basin consist of a thick succession of beds dominated by turbidites that overlie a thick but irregular sequence of beds, probably dominated by pelagic deposits. The so-called “Carib beds”, present elsewhere in the Caribbean, are not evident in the part of the basin crossed by this profile. The sedimentary section rests on a acoustic basement that probably represents the top of oceanic layer 2. A gravity model indicates that the crust beneath the Yucatan Basin is thin and therefore probably is oceanic in character. The crust thickens southward under the Cayman Ridge but thins again beneath the Cayman Trough. This local thickening is consistent with the suggestion that the Cayman Ridge is a rifted part of the Nicaraguan Rise.

Earth and Planetary Science Letters↗
USGS1973

K, Rb, Sr and 87Sr 86Sr in rocks from the Mid-Indian Oceanic Ridge

The 87 Sr/ 86 Sr ratios of Mid-Indian Oceanic Ridge (MIOR) basalts are nearly identical (0.7032 to 0.7035), with the exception of one more highly radiogenic sample (0.7043). These values are consistently higher than the strontium isotopic ratios of the ridge basalts from Atlantic and Pacific Oceans, suggesting that the source of the MIOR basalts was depleted in alkalies more recently and/or to a lesser degree than the basalts from other oceans.

Earth and Planetary Science Letters↗
USGS1973

Paleomagnetism of some Precambrian basaltic flows and red beds, Eastern Grand Canyon, Arizona

Lava flows and red sandstone beds near the middle of the Upper Precambrian Grand Canyon Series exhibit stable remanent magnetization. The beds are about 1000 m stratigraphically above rocks of the Grand Canyon Series for which paleomagnetic poles have been reported. All specimens were subjected to stepwise thermal (200°–700°C) or alternating field (25–5000 Oe) demagnetization for the determination of characteristic magnetization. The pole for two flows and an intercalated sandstone bed of the Cardenas Lavas of Ford, Breed and Mitchell (upper Unkar Group), is at 174.6W, 0.4N ( N = 10, K = 50, α 95 = 6.9°). The pole for a weathered zone developed across the Cardenas Lavas is at 167.8W, 49.4N ( N = 5, K = 79, α 95 = 8.6°). The pole for directly overlying sandstone of the Nankoweap Formation of Maxson is at 174.4E, 12.5N ( N = 6, K = 105, α 95 = 6.6°). These poles lie on or near, and appear to follow, part of an apparent polar wandering path recently proposed for the Precambrian of North America by Spall. If the fit is not accidental, little or no rotation has occurred between north-central Arizona and parts of the North American continent used to define the proposed path.

Arizona↗
USGS1973

Effect of sea water interaction on strontium isotope composition of deep-sea basalts

Analyses of rim-to-interior samples of fresh tholeiitic pillow basalts, deuterically altered holocrystalline basalts, and older, weathered tholeiitic basalts from the deep sea indicate that 87 Sr/ 86 Sr ratios of the older basalts are raised by low temperature interaction with strontium dissolved in sea water. 87 Sr/ 86 Sr correlates positively with H 2 O in these basalts; however, there is little detectable modification of the strontium isotope composition in rocks with H 2 O contents less than 1%. The isotope changes appear to be a function of relatively long-term, low-temperature weathering, rather than high-temperature or deuteric alteration. Strontium abundance and isotopic data for these rocks suggest that strontium content is only slightly modified by interaction with sea water, and it is a relatively insensitive indicator of marine alteration. Average Rb-Sr parameters for samples of apparently unaltered basalt are: Rb= 1.11ppm; Sr= 132ppm; 87 Sr/ 86 Sr= 0.70247.

Earth and Planetary Science Letters↗
USGS1973

18O 16O ratios in cherts associated with the saline lake deposits of East Africa

The cherts formed from sodium silicate precursors in East African saline, alkaline lakes have δ 18 O values ranging from 31.1 to 44.1. The δ 18 O values correlate in general with lake salinities as inferred from geologic evidence, indicating that most chert was formed from its precursor in contact with lake water trapped at the time of deposition. A few of the analyzed cherts probably formed in contact with dilute meteoric water. From the widely varying δ 18 O values we conclude that precursors were transformed to chert in fluids of widely varying salinity and a Na + /a H + ratio.

Earth and Planetary Science Letters↗
USGS1973

Uranium concentration and distribution in six peridotite inclusions of probable mantle origin

Fission-track activation was used to investigate uranium concentration and distribution in peridotite inclusions in alkali basalt from six localities. Whole-rock uranium concentrations range from 24 to 82 ng/g (1ng= 10 −9 g). Most of the uranium is uniformly distributed in the major silicate phases — olivine, orthopyroxene, and clinopyroxene. Chromian spinels may be classified into two groups on the basis of their uranium content, having either less than 10 ng/g or 100–150 ng/g U. In one sample accessory hydrous phases, phlogopite and hornblende, contain 130 and 300 ng/g U, respectively. The contact between the inclusion and the host basalt is usually quite sharp. Glassy or microcrystalline veinlets found in some samples contain more than 1μg/g(1 μg= 10 −6 g). Very little uranium is associated with microcrystals of apatite. Our results agree with some earlier investigators, who have concluded that suboceanic peridotites contain too little uranium to account for normal oceanic heat flow by conduction alone.

Earth and Planetary Science Letters↗
USGS1973

Lead concentration and isotopic composition in five peridotite inclusions of probable mantle origin

The lead content of five whole-rock peridotite inclusions (four lherzolites and one harzburgite) in alkali basalt ranges from 82 to 570 ppb (parts per billion). Approximately 30–60 ppb of this amount can be accounted for by analyzed major silicate minerals (olivine ≤ 10 ppb; enstatite 5–28 ppb; chrome diopside ∼400 ppb). Through a series of acid leaching experiments, the remainder of the lead is shown to be quite labile and to reside in either glassy or microcrystalline veinlets or accessory mineral phases, such as apatite and mica. The lead isotopic composition of the peridotites ( 206 Pb/ 204 Pb= 18.01–18.90; 207 Pb/ 204 Pb= 15.52–15.61; 208 Pb/ 204 Pb= 37.80–38.86) lies within the range of values defined by many modern volcanic rocks and, in particular, is essentially coextensive with the abyssal tholeiite field. In all but one instance, isotopic differences were found between the peridotite and its host alkali basalt. Two of the peridotites clearly demonstrated internal isotopic heterogeneity between leachable and residual fractions that could not simply be due to contamination by the host basalt. However, there is no evidence that these ultramafic rocks form some layer in the mantle with isotopic characteristics fundamentally different from those of the magma sources of volcanic rocks.

Earth and Planetary Science Letters↗
USGS1973

Age of Kōko Seamount, Emperor Seamount chain

K— Ar ages obtained by the conventional isotope-dilution and the 40 Ar/ 39 Ar techniques on two sanidine trachytes, four basalts, and a phonolite dredged from the top of Kōko Seamount, 300 km north of the Hawaiian-Emperor bend, show that the seamount is 46.4 ± 1.1 my old. These data indicate that the volcanoes in the Hawaiian-Emperor chain continue to increase in age to the west and north beyond Midway Atoll, as predicted by the melting-spot hypothesis for the origin of the chain, and that the rate of volcanic migration along the chain was nonlinear between the time of formation of the island of Hawaii and Kōko Seamount.

Earth and Planetary Science Letters↗
USGS1973

87Sr/86Sr ratios in basalts from islands in the Indian Ocean

87 Sr/ 86 Sr ratios of basalts from islands in the Indian Ocean (0.7040) are higher than those of basalts dredged from the Mid-Indian Ocean Ridge (0.7034). The sources of the island basalts have apparently not been in equilibrium with the source of the ridge basalts for roughly 10 9 years. Both ridge and island basalts in the Indian Ocean are higher in 87 Sr/ 86 Sr than are rocks from similar settings in the eastern Pacific.

Earth and Planetary Science Letters↗
USGS1974

Heat flow from eastern Panama and northwestern Colombia

Heat flows were determined at 12 sites in four distinct areas between longitude 77° and 80°W in eastern Panama and northwestern Colombia. Evidently, most of the region is underlain by mafic oceanic crust so that the crustal radiogenic component of heat flow is very small (∼ 0.1 μcal cm −2 sec −1 ). Low heat-flow values (∼ 0.7 μcal cm −2 sec −1 ) in northwestern Colombia may reflect thermal transients associated with shallow subduction. The normal values (∼ 1) at about 78°W are consistent with the mean heat flow from the western Caribbean and the Gulf of Mexico. At 80°W, a fairly high value of 1.8 may define the easterly limit of thermal transients due to Cenozoic volcanic activity in Central America.

Earth and Planetary Science Letters↗
USGS1974

Episodic Cenozoic volcanism and tectonism in the Andes of Peru

Radiometric and geologic information indicate a complex history of Cenozoic volcanism and tectonism in the central Andes. K-Ar ages on silicic pyroclastic rocks demonstrate major volcanic activity in central and southern Peru, northern Chile, and adjacent areas during the Early and Middle Miocene, and provide additional evidence for volcanism during the Late Eocene. A provisional outline of tectonic and volcanic events in the Peruvian Andes during the Cenozoic includes: one or more pulses of igneous activity and intense deformation during the Paleocene and Eocene; a period of quiescence, lasting most of Oligocene time; reinception of tectonism and volcanism at the beginning of the Miocene; and a major pulse of deformation in the Middle Miocene accompanied and followed through the Pliocene by intense volcanism and plutonism. Reinception of igneous activity and tectonism at about the Oligocene-Miocene boundary, a feature recognized in other circum-Pacific regions, may reflect an increase in the rate of rotation of the Pacific plate relative to fixed or quasifixed mantle coordinates. Middle Miocene tectonism and latest Tertiary volcanism correlates with and probably is genetically related to the beginning of very rapid spreading at the East Pacific Rise.

Earth and Planetary Science Letters↗
USGS1974

Antiferromagnetic inclusions in lunar glass

The magnetic susceptibility of 11 glass spherules from the Apollo 15, 16, and 17 fines and two specimens of a relatively large glass spherical shell were studied as a function of temperature from room temperature to liquid helium temperatures. All but one specimen showed the presence of antiferromagnetic inclusions. Closely spaced temperature measurements of the magnetic susceptibility below 77 K on five of the specimens showed antiferromagnetic temperature transitions (Néel transitions). With the exception of ilmenite in one specimen, these transitions did not correspond to any transitions in known antiferromagnetic compounds.

Earth and Planetary Science Letters↗