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At least 1,315 records · Page 73Linked to original sources
USGS1970

The diffusion of ions in unconsolidated sediments

Diffusion in unconsolidated sediments generally proceeds at rates ranging from half to one twentieth of those applying to diffusion of ions and molecules in free solution. Diffusion rates are predictable with respect to porosity and path tortuosity in host sediments, and can be conveniently measured by determinations of electrical resistivity on bulk sediment samples. Net ion flux is further influenced by reactions of diffusing species with enclosing sediments, but such influences should not be confused with or lumped with diffusion processes.

Earth and Planetary Science Letters↗
USGS1970

Strontium isotopes in some Cenozoic lavas from Oregon and Washington

The 87 Sr/ 86 Sr ratios have been determined in 22 volcanic rocks from Oregon and Washington. The emphasis of the study was on andesites and dacites, but several basalts were included. Most of the samples have a very limited range in strontium isotopic composition and all fall within variations previously defined for oceanic basalts. There are no systematic differences between lavas erupted through the pre-Cenozoic basement and those erupted in the Columbia River Embayment. It is concluded that the role played by crustal rocks in the generation of these andesites and dacites is minor or nonexistent, but assimiliation of crustal material may have been a factor for some of the basalts of the Columbia River Group.

Oregon, Washington↗
USGS1970

Cummingtonite: A reversible, nonquenchable transition from P21/m to C2/m symmetry

Cummingtonite from the Gouverneur, New York, talc district (Sample No. U.S.N.M. 115046) with the composition, (Ca 0.36 Na 0.06 Mn 0.96 Mg 0.57 ) Mg 5 Si 8 O 22 (OH) 2 , unit cell parametersa = 9.550 ± 0.001, b = 18.007 ± 0.003, c = 5.298 ± 0.001Å, β = 102°39′ ± 1′, and space groupP2 1 /m, has been used in a high-temperature, single-crystal, X-ray diffraction study. When diffraction patterns are taken of the crystals heated in a sealed silica capillary to 45°C or higher, these reflections disappear, indicating a transition toC2/m symmetry. The transition appears to be analogous to theP2 1 /ctoC2/c transition between 700° and 800°C discovered recently in (Mg 0.30 Fe 0.70 ) SiO 3 clinopyroxene. In addition to the cummingtonite transition, we have also found a transition in pigeonite at higher temperatures. The transition occurs in this pigeonite (Gunflint Iron Formation, Minnesota, (Ca 0.04 Fe 0.78 Mg 0.18 )SiO 3 ) at about 670°C. This is a significantly lower temperature than was found for the Ca-poor clinopyroxene.

Earth and Planetary Science Letters↗
USGS1970

Middle Miocene hiatus in volcanic activity in the Great Basin area of the Western United States

A summary of potassium-argon dates shows that a high level of igneous activity in the Great Basin and adjacent regions during middle Tertiary time (40 to 20 my ago) was followed by a period of relative quiescence in middle Miocene time that lasted for several million years (from 20 to 17 my ago). Volcanism resumed 16 my ago mainly at the margins of the region and has continued to the present.

Arizona, California, Nevada, Oregon, Utah↗
USGS1971

Geomagnetic polarity epochs: age and duration of the olduvai normal polarity event

New data show that the Olduvai normal geomagnetic polarity event is represented in Olduvai Gorge, Tanzania, by rocks covering a time span of roughly from 0.1 to 0.2 my and is no older than 2.0 my. Hence the long normal polarity event of this age that is seen in deep-sea sediment cores and in magnetic profiles over oceanic ridges should be called the Olduvai event. The lava from which the Gilsàevent was defined may have been erupted during the Olduvai event and, if so, the term Gilsàshould now be abandoned. Many dated lavas that were originally assigned to the Olduvai event represent one or two much shorter normal polarity events that preceded the Olduvai event; these are herein named the Réunion normal polarity events. This revision brings the geomagnetic reversal time scale into conformity with the one implied by assumptions of uniform sedimentation rates on the ocean floor and uniform rates of sea-floor spreading.

Earth and Planetary Science Letters↗
USGS1971

Some Pb and Sr isotopic measurements on eclogites from the Roberts Victor mine, South Africa

Five nodules of eclogite, one nodule of garnet peridotite and one sample of kimberlite from the Roberts Victor mine were analyzed for concentrations of U, Th, Pb, Rb and Sr and isotopic compositions of Pb and Sr. In the eclogites, U content ranges from 0.09 to 0.26 ppm, Th from 0.35 to 1.1 ppm, Pb from 0.79 to 5.5 ppm, Rb from 2.1 to 28 ppm and Sr from 133 to 346 ppm; 206 Pb/ 204 Pb ratios range from 14.8 to 18.5, 207 Pb/ 204 Pb from 14.9 to 15.7, 208 Pb/ 204 Pb from 35.2 to 38.5. The garnet peridotite contains 0.22 ppm U, 0.97 ppm Th, 1.05 ppm Pb, 6.9 ppm Rb and 108 ppm Sr and the kimberlite contains 2.5 ppm U, 30 ppm Th, 37 ppm Pb, 113 ppm Rb and 2040 ppm Sr. The lead in the eclogites has two components, a lead pyroextractable at 1100–1200° and a non-pyroextractable residual lead. In three of the eclogites, which are to some extent altered, a proportion of the pyroextractable lead may be contaminating lead from the kimberlite, but an altered kyanite eclogite does not appear to be contaminated by this same kimberlite. The pyroextractable lead from a less altered eclogite contains a much larger proportion of 206 Pb. Compositions calculated for the residual leads vary greatly. In many of the pyroextraction runs the primary eclogitic phases disappeared and the new phases plagioclase, clinopyroxene and a magnetic iron compound were formed. Why part of the lead should have been retained by these new phases is not understood.

Earth and Planetary Science Letters↗
USGS1971

Kaersutite - A product of reaction between pargasite and basanite at Dish Hill, California

Paragasitic amphibole, occurring interstitially and as veins in peridotite inclusions in basanite, has reacted with the host basanite to form kaersutitic amphibole. The amphibole compositions vary with respect to distance from the edge of the xenolith; iron, titanium, and potassium contents are higher and magnesium, silicon, sodium, and chromium contents are lower closer to the basanite. Pargasite was exposed to the basanite when peridotite blocks broke open along amphibole veins during transport to the surface. Small amphibole fragments isolated in the basanite show the most reaction; compositional gradients in interstitial and vein amphibole are steep into peridotite inclusions where the amphibole was shielded from reaction. The compositions of amphiboles so modified have no direct bearing on high pressure fractionation trends if the amphibole is cognate, or on the bulk composition of the upper mantle if it is accidental.

California↗
USGS1971

A test of the 40Ar/39Ar age spectrum technique on some terrestrial materials

40 Ar/ 39 Ar age spectra were determined for 10 terrestrial rock and mineral samples whose geologic history is known from independent evidence. The spectra for six mineral and whole rock samples, including biotite, feldspar, hornblende, muscovite, and granodiorite, that have experienced post-crystallization heating did not reveal the age of crystallization in any obvious way. Minima in the spectra, however, give reasonable maximum ages for reheating and high-temperature maxima can be interpreted as minimum crystallization ages. High-temperature ages of microcline and albite that have not been reheated are approximately 10% younger than the known crystallization age. Apparently there are no domains in these feldspars that have retained radiogenic 40 Ar quantitatively. Spectra from two diabase samples that contain significant quantities of excess argon might mistakenly be interpreted as spectra from reheated samples and do not give the age of emplacement. The 40 Ar/ 39 Ar age spectrum technique may be a potentially valuable tool for the study of geologic areas with complex histories, but the interpretation of age spectra from terrestrial samples seems to be more difficult than suggested by some previous studies.

Earth and Planetary Science Letters↗
USGS1971

Thermoluminescence of Apollo 12 lunar samples

Thermoluminescence (TL) glow curve and decay characteristics of Apollo 12 fines and soil samples are similar to those from Apollo 11. Interpretation of the results from the core sample is difficult because of inadequate sample, spacing, but it appears that the part of the core below about 8 cm has been undisturbed for about 10 4 years whereas the part of the core above 10 cm may have been disturbed by recent surface activity. TL in the Apollo 12 samples is about twice that in the Apollo 11 samples, suggesting a lower mean daytime surface temperature of a few degrees at the Apollo 12 site.

Earth and Planetary Science Letters↗
USGS1971

On the origin of lunar soil 12033

The lunar soil 12033 is compositionally distinctly different from both the local rocks at the Apollo 12 site and other lunar samples. It must be a recent deposit, else it would not have retained its identity. It contains a meteoritic component which is about 0.59 times as large as that in the more typical Apollo 12 soils. The amount of meteoritic component may be either a fortuitous residuum from the object which produced the 12033 material as crater ejecta, or 12033 may consist of a mixture of an exotic component with the local soil in approximately 41:59 proportions. The available evidence favors the latter interpretation.

Earth and Planetary Science Letters↗
USGS1971

Paleomagnetism of San Cristobal Island, Galapagos

Isla San Cristobal, the most easterly of the Galapagos Islands, consists of two parts: a large volcano constitutes the southwest half of the island and an irregular apron of small cones and flows makes up the northeast half. As some of the younger flows on the flanks of the large volcano are reversely magnetized, the minimum age of the volcano is 0.7 my, which is the age of the Brunhes-Matuyama reversal boundary. The true age is probably several times greater. The cones and flows to the northeast are all normally magnetized. The between-site angular dispersion of virtual poles is 11.3° - a value consistent with mathematical models for the latitude dependence of geomagnetic secular variation.

Earth and Planetary Science Letters↗
USGS1971

Uranium-series dating of some pleistocene marine deposits in Southern California

Analyses of uranium isotopes and their long-lived daughter products showed evidence of uranium migration in most of the 22 fossil mollusk shell samples from marine terrace deposits in southern California. Two samples, however, remained an ideal closed system as indicated by concordant 230 Th/ 234 U and 231 Pa/ 235 U dates. The closed-system age of the lowest exposed marine terrace deposit along part of the mainland coast of southern California is 69 000 ± 10 000 yr, and correlation of this discontinuous deposit is afforded between the Newport Beach-Laguna Beach area and the Palos Verdes Hills. Some mollusk shell samples were dated by the open-system model; the remaining samples were found to be unsuitable for uranium-series dating.

California↗
USGS1971

40Ar/39Ar technique of KAr dating: A comparison with the conventional technique

K-Ar ages have been determined by the 40 Ar/ 39 Ar total fusion technique on 19 terrestrial samples whose conventional K-Ar ages range from 3.4 my to nearly 1700 my. Sample materials included biotite, muscovite, sanidine, adularia, plagioclase, hornblende, actinolite, alunite, dacite, and basalt. For 18 samples there are no significant differences at the 95% confidence level between the K Ar ages obtained by these two techniques; for one sample the difference is 4.3% and is statistically significant. For the neutron doses used in these experiments (≈4 × 10 18 nvt) it appears that corrections for interfering Ca- and K-derived Ar isotopes can be made without significant loss of precision for samples with K/Ca > 1 as young as about 5 × 10 5 yr, and for samples with K/Ca < 1 as young as about 10 7 yr. For younger samples the combination of large atmospheric Ar corrections and large corrections for Ca- and K-derived Ar may make the precision of the 40 Ar/ 39 Ar technique less than that of the conventional technique unless the irradiation parameters are adjusted to minimize these corrections.

Earth and Planetary Science Letters↗
USGS1972

Differentiation and volcanism in the lunar highlands: Photogeologic evidence and Apollo 16 implications

Materials of possible volcanic origin in the lunar highlands include (1) highland plains materials, (2) materials forming closely spaced hills in which summit furrows and chains of craters are common and (3) materials forming closely spaced hills (some of which parallel the lunar grid) on which summit furrows and chain craters are rare. The highland plains materials probably are basaltic lavas with less Fe and Ti than the mare plains materials. The two hilly units appear to consist of materials that, if volcanic, were more viscous in the molten state than any of the lunar plains units; thus these materials may be significantly enriched in felsic components. Most of the highland materials of possible volcanic origin formed after the Imbrium multi-ring basin but before mare material completed flooding parts of the moon; they therefore postdate accretion of the moon and may represent several episodes of premare volcanism.

Earth and Planetary Science Letters↗
USGS1972

Site distribution of iron in staurolite

The Mössbauer spectrum of staurolite does not necessarily indicate that ferrous iron is distributed over both the tetrahedral Fe site and one or more of the octahedral sites, as previously thought. Two doublets are present in the spectrum, but their quadrupole spliting and isomer shift are more consistent with both being due to tetrahedrally coordinated ferrous iron. Smith's X-ray work has shown that cations in the Fe site actually assume several slightly different positions, and iron in each of these positions may give rise to a different Mössbauer doublet.

Earth and Planetary Science Letters↗
USGS1972

Occurrence of chromian, hercynitic spinel ("pleonaste") in Apollo-14 samples and its petrologic implications

Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An 80 -An 94 ), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide. Similar pleonaste occurs in crystalline rock clasts, mainly with plagioclase; one clast (A) consists only of coarse olivine, plagioclase, and pleonaste, with granulated grain boundaries suggestive of deformation. From composition and texture, this clast is one possible candidate for the mafic cumulate counterpart of the “anorthositic” crust. Another clast (B), also made solely of olivine, plagioclase and pleonaste, is itself a breccia. These data suggest a two-stage brecciation process: 1) disruption (probably pre-Imbrian) of a deep-seated pleonaste-bearing source rock like A and reconsolidation to form a breccia without addition of pyroxene, ilmenite or other minerals; and 2) disruption of this breccia to yield breccia clast B which was then incorporated into the Fra Mauro formation.

Earth and Planetary Science Letters↗