1,426 research outputs found
Oxygen isotope ratios in olivine from the Hawaii Scientific Drilling Project
Oxygen isotope ratios of olivine in 23 tholeiites from the Hawaii Scientific Drilling Project (HSDP) core (15 from Mauna Kea, 8 from Mauna Loa) and three samples of outcropping subaerial or dredged submarine Mauna Kea lavas have been measured by laser fluorination. The ÎŽ^(18)O values are 4.6â5.4 â°, confirming previous observations that some Hawaiian lavas are derived from sources with ÎŽ^(18)O values lower than typical upper mantle (ÎŽ^(18)Oolivine â 5.2±0.2 â°). The Mauna Kea-Mauna Loa transition marks a shift from ÎŽ^(18)O values lower than the mantle average in Mauna Kea olivines (âŒ4.8) to more typical mantle values in Mauna Loa olivines. Lavas containing olivines with ÎŽ^(18)O values similar to the typical upper mantle are associated with more âprimitiveâ or less depleted radiogenic isotope characteristics; i.e., with higher ^3He/^4He (>13 Ra), higher ^(87)Sr/^(86)Sr (>0.7036) and lower Ń_(Nd) (<6.5), and with ^(206)Pb/^(204)Pb ratios less than -18.3. These relationships indicate that the ÎŽ^(18)O values of the relatively enriched source components of the Hawaiian plume sampled by Mauna Loa lavas are comparable to (or greater than) the mantle average. This conclusion is supported by ÎŽ^(18)O values of olivine from other high ^3He/^4He islands, which are also comparable to the upper mantle average. The low ÎŽ^(18)O values in Hawaiian lavas are derived from a source having more MORB-like, or depleted, He, Nd, and Sr isotope ratios, but more radiogenic Pb than is seen in the Mauna Loa lavas Assimilation of ^(18)O-depleted lower oceanic crust from the underlying Pacific crust by hot, MgO-rich parental magmas or melting of older, recycled oceanic crust entrained in the Hawaiian plume are both possible sources of this ^(18)O-depleted, MORB-like component in Hawaiian magmas
Fast magnetoacoustic waves in a randomly structured solar corona
The propagation of fast magnetoacoustic waves in a randomly structured solar corona is considered in the linear and cold plasma limits. The random field is assumed to be static and associated with plasma density inhomogeneities only. A transcendental dispersion relation for the fast magnetoacoustic waves which propagate perpendicularly to the magnetic field is derived in the weak random field approximation. It is shown analytically that the fast magnetosonic waves experience acceleration, attenuation, and dispersion in comparison to the homogeneous case. These analytical findings are essentially confirmed by numerical simulations for a wide-spectrum pulse, except that the waves were found decelerated. It is concluded that the coronal Moreton waves can be applied to MHD seismology of the solar corona
Oxygen-isotope and trace element constraints on the origins of silica-rich melts in the subarc mantle
Peridotitic xenoliths in basaltic andesites from Batan island in the Luzon arc contain silica-rich (broadly dacitic) hydrous melt inclusions that were likely trapped when these rocks were within the upper mantle wedge underlying the arc. These melt inclusions have been previously interpreted to be slab-derived melts. We tested this hypothesis by analyzing the oxygen isotope compositions of these inclusions with an ion microprobe. The melt inclusions from Batan xenoliths have ÎŽ 18OVSMOW values of 6.45 ± 0.51â°. These values are consistent with the melts having been in oxygen isotope exchange equilibrium with average mantle peridotite at temperatures of â„875°C. We suggest the ÎŽ 18O values of Batan inclusions, as well as their major and trace element compositions, can be explained if they are low-degree melts (or differentiation products of such melts) of peridotites in the mantle wedge that had previously undergone extensive melt extraction followed by metasomatism by small amounts (several percent or less) of slab-derived components. A model based on the trace element contents of Batan inclusions suggests that this metasomatic agent was an aqueous fluid extracted from subducted basalts and had many characteristics similar to slab-derived components of the sources of arc-related basalts at Batan and elsewhere. Batan inclusions bear similarities to âadakites,â a class of arc-related lava widely considered to be slab-derived melts. Our results suggest the alternative interpretation that at least some adakite-like liquids might be generated from low-degree melting of metasomatized peridotites
Aquatic Vegetation, Largemouth Bass and Water Quality Responses to Low-Dose Fluridone Two Years Post Treatment
Whole-lake techniques are increasingly being used to selectively
remove exotic plants, including Eurasian watermilfoil
(
Myriophyllum spicatum
L.). Fluridone (1-methyl-3-phenyl-
5-[3-(trifluoromethyl)phenyl]-4(1
H
)-pyridinone), a systemic
whole-lake herbicide, is selective for Eurasian watermilfoil
within a narrow low concentration range. Because fluridone
applications have the potential for large effects on plant assemblages
and lake food webs, they should be evaluated at
the whole-lake scale. We examined effects of low-dose (5 to 8
ppb) fluridone applications by comparing submersed plant
assemblages, water quality and largemouth bass (
Micropterus
salmoides
) growth rates and diets between three reference
lakes and three treatment lakes one- and two-years post treatment.
In the treatment lakes, fluridone reduced Eurasian watermilfoil
cover without reducing native plant cover, although
the duration of Eurasian watermilfoil reduction varied among
treatment lakes. (PDF has 11 pages.
Stable isotopic characteristics and magmatic history of meta-igneous rocks, Adirondacks, NY
The Adirondack Highlands contain dominantly meta-igneous rocks
including the anorthosite-granite association and a wide variety of
unrelated orthogneisses from gabbroic to granitic composition. The
internal variation of oxygen isotopes within Adirondack meta-igneous
suites related to the Marcy anorthosite shows that elevated
ÎŽ^(18)O is a compositionally dependent characteristic of the protolith.
Analysis of ÎŽ^(18)O in previously unsampled compositions in this suite
has led to a stable isotopic, petrographic and major element
description of a continuous range of rock types from two-pyroxene,
oxide rich monzonitic gneiss to mesoperthite, quartz rich granite
gneiss within the Tupper and Saranac sheets on the NW margin of
the Marcy anorthosite. This expanded data set has allowed the
formulation of a semi-quantitative model, based both in
thermodynamics and mass balance, for the AFC magmatic evolution
of this suite. A sampling traverse of the Snowy Mountain dome
anorthosite and related halo of granitic gneiss in the southern
Adirondack Highlands shows the concentric spatial distribution of
ÎŽ^(18)O in granites around anorthosite, constrains the extent of
interaction between anorthosite and granite, and provides an
additional example of crustal interaction in the magmatic evolution
of this suite. This evidence, combined with previous work (Morrison
and Valley, 1988), documents extensive crustal interaction in region-wide
anorthosite-granite plutonism prior to the peak of Grenville
metamorphism
Reanalysis of Copernicus Measurements on Interstellar Carbon Monoxide
We used archival data acquired with the Copernicus satellite to reexamine CO
column densities because self-consistent oscillator strengths are now
available. Our focus is on lines of sight containing modest amounts of
molecular species. Our resulting column densities are small enough that
self-shielding from photodissociation is not occurring in the clouds probed by
the observations. While our sample shows that the column densities of CO and H2
are related, no correspondence with the CH column density is evident. The case
for the CH+ column density is less clear. Recent chemical models for these
sight lines suggest that CH is mainly a by-product of CH+ synthesis in low
density gas. The models are most successful in reproducing the amounts of CO in
the densest sight lines. Thus, much of the CO absorption must arise from denser
clumps along the line of sight to account for the trend with H2.Comment: 19 pages, 6 figures. Accepted for publication in Ap
Temporal behavior of two-wave-mixing in photorefractive InP:Fe versus temperature
The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc
electric field at different temperatures has been studied. In particular, the
temperature dependence of the characteristic time constant has been studied
both theoretically and experimentally, showing a strongly decreasing time
constant with increasing temperature
Stable isotopic disequilibrium in high-T metamorphic systems
A principal use of stable isotopes in metamorphic rocks is as thermometers, or as tests for isotopic equilibrium
where metamorphic temperatures are known. Applications are often complicated when apparent isotopic
temperatures are discordant and disagree with petrologic temperatures, indicating a failure of isotopic systems
to record and/or preserve equilibrium, peak-T fractionations. In low-T, fluid-hosted environments such features
often clearly reflect open system exchange. However, in high-T metamorphic environments a slow cooling
history can be sufficient to produce such features by retrograde, closed system exchange between coexisting
minerals
Oxygen-isotopic composition and high-resolution secondary ion mass spectrometry imaging of Martian carbonate in Lafayette meteorite
Carbonate from SNC meteorites gives insight into a
variety of processes on and/or beneath the surface of Mars. In Lafayette,
carbonate occurs in unusually intimate association with hydrous phases
when compared with other carbonate-bearing SNCs [1]. We have measured
the ^(18)O/^(16)O ratio of carbonate in the alteration veins of Lafayette
using the magnetic sector ion microprobe. In addition, we obtained isotope
images of major- and minor-element cations in veinlets with the
focused ion beam (FIB) quadrupole SIMS. The spot size of the FIB probe
used in this study (~0.03 ”m) provides an opportunity for high spatial
resolution chemical imaging, which is ideal for examining features in the
fine-grained alteration assemblage
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