Newin situ(U–Th–Sm)/He and U–Pb zircon double dating method replicates results obtained using conventional methods at the Ellendale diamond occurrence.

Cosmogenic 21Ne was utilised to determine exposure ages of young subaerial basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia. The ages (36–53ka) determined from co-existing cosmogenic 21Ne and 3He in olivines separated from basalts are consistent within analytical uncertainties with ages previously determined by cosmogenic 36Cl exposure dating. This paper illustrates the potential of cosmogenic neon exposure ages in studying the eruption, surface morphology, and erosion history of young volcanic rocks, which are difficult to date using other conventional methods, such as K-Ar or 40Ar/39Ar dating. The present study demonstrates that combined cosmogenic 3He and 21Ne dating, specifically measured cosmogenic 3He/21Ne ratios, on the same samples, is powerful for evaluating the validity of calculated cosmogenic 3He and 21Ne surface exposure ages.

The authors present an in-fiber frequency shifter for optical fiber which uses flexural acoustic waves to induce mode coupling between the two optical modes of a dual-mode, elliptical-core fiber. A sharply tapered surface-acoustic-wave horn is used to excite these waves in the fiber, providing a robust, efficient acoustic source. Optical mode filters at the input and output ensure spectral purity of the frequency-shifted light. A 6.3-MHz frequency upshift in 1.3- mu m light with 400-mW input power, with a 35-dB sideband suppression, was demonstrated. Recently studied methods for improving the sideband suppression through mode-filtering techniques are also discussed.<>

The mass accumulation rates (MARs) of aeolian dust in the ocean basins provide an important record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns responsible for dust transport in the geologic past. The incorporation of other terrigenous components such as volcanic ashes in seafloor sediments, however, often obscures the aeolian dust record. We describe a new approach which uses the delivery rate of crustal 4He to seafloor sediments as a proxy for the mass accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components.

Based on the proposition that the Earth contains solar-like, nucleogenic and atmosphere-derived neon components, we calculate the amounts of solar-derived Ne-22(s) and nucleogenic Ne-21* present in a sample. The amounts of Ne-22(s) and Ne-21* can then be compared with the amounts of primordial He-3 and radiogenic He-4. The He-3/Ne-22(s) and He-4/Ne-21* ratios observed in basaltic glasses from mid-oceanic ridges (MORBs) vary by almost 2 orders of magnitude and define a linear correlation with a slope of unity which passes through the point defined by the mean primordial He-3/Ne-22 ratio in the Earth (= 7.7) and the radiogenic He-4 to nucleogenic Ne-21 production ratio (= 2.2 x 10(7)). This indicates that there has been significant recent elemental fractionation which has enriched MORE glasses in both primordial He-3 and radiogenic He-4 with respect to mantle neon. A similar enrichment of helium in MORE glasses is observed relative to mantle-derived Ar-40*.Importantly, there is a positive correlation between absolute helium abundance and the degree of helium enrichment. Specifically, the data show positive linear correlations in plots of [He-3] vs. He-3/Ne-22(s), [He-4] vs. He-4/Ne-21*, and [He-4] vs. He-4/Ar-40*. These positive correlations between helium abundance and elevated He/Ne and He/Ar ratios in MORE glasses are inconsistent with elemental fractionation associated with any form of solubility controlled gas loss process, as these would lead to negative correlations. Additionally there is relatively little fractionation of neon from argon. This observation excludes any type of simple mass dependent process. Rather, it requires some form of "threshold" fractionation process which effects only helium. Although the origin of the correlation between helium abundance and helium fractionation is unclear, we speculate that the systematic helium enrichment observed in MORE glasses may reflect preferentially concentrating helium liberated from the crystallising oceanic crustal section into the relatively small volume of residual magma that is erupted to form glasses. (C) 1999 Elsevier Science Ltd.

We have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the Ontong Java Plateau, western equatorial Pacific Ocean (ODP Site 806). These results provide a two million year record of the burial flux of extraterrestrial He-3, which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles. The He-3 burial flux prior to similar to 700 ka was relatively low, similar to 0.5 pcc cm(-2) kyr(-1), but from 700 ka to the present, the burial flux gradually increased to a value of similar to 1.0 pcc cm(-2) kyr(-1). 100 kyr periodicity in the He-3 burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate, with high He-3 burial fluxes associated with interglacial periods. This periodicity and phase are consistent with previous He-3 measurements in North Atlantic sediments. Although 100 kyr periodicity in He-3 burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris, the measurements and predictions differ by one half cycle in phase. Nevertheless, our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and/or inclination relative to the solar-system invariable plane. Such control is a necessary but not sufficient condition for the hypothesis of Muller and MacDonald (1995) that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period.We also identify several brief (<25 kyr) intervals of strongly enhanced He-3 burial, possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles. Copyright (C) 1998 Elsevier Science Ltd.

We have measured the helium abundance and isotopic composition of a suite of Lower Ordovician marine limestones and associated fossil meteorites from Kinnekulle, Sweden. Limestone 3He/4He ratios as high as 11.5 times the atmospheric value in fused samples and up to 23 times atmospheric in a single step-heat fraction indicate the presence of extraterrestrial helium, and demonstrate that at least a fraction of the extraterrestrial 3He carried by interplanetary dust particles must be retained against diffusive and diagenetic losses for up to 480 Ma. The carrier phase has not been identified but is not magnetic. Extrapolation of high-temperature 3He diffusivities in these sediments is consistent with strong retention of extraterrestrial 3He under ambient Earth-surface conditions. Combination of the observed helium concentrations with sedimentation rates estimated from conodont biostratigraphy suggest that the flux of extraterrestrial 3He in the Early Ordovician was about 0.5 x 10(-12) cm3 STP cm-2 ka-1, ignoring potential post-deposition helium loss. This value is indistinguishable from the average 3He flux estimated for the Cenozoic Era. In contrast, previous studies of fossil meteorites, Ir abundances, and Os isotopic ratios in the limestone suggest that the total accretion rate of extraterrestrial material during the studied interval was at least an order of magnitude higher than the Cenozoic average. This disparity may reflect significant post-depositional loss of 3He from IDPs within these old limestones; if so, the match between the Ordovician flux and the Cenozoic average would be fortuitous. Alternatively, the size distribution of infalling objects during the Early Ordovician may have been enriched only in extraterrestrial material too large to retain 3He during atmospheric entry heating (> approximately 30 micrometers). The fossil meteorites themselves also preserve extraterrestrial helium. Meteorite 3He concentrations of 2 to 9 x 10(-12) cm3 STP g-1 are several orders of magnitude lower than found in most modern meteorites, suggesting very substantial helium loss (probably >99.9%) from these chemically altered objects. The Meteorites carry 3He concentrations only a factor of a few higher than the host limestones. The meteorites themselves cannot be the source of the extraterrestrial 3He observed in the limestones.

We have modeled atmospheric entry heating of interplanetary dust to characterize the population of particles carrying extraterrestrial He to the seafloor. We find that ∼0.5% of the mass and ∼4% of the surface area of the infalling dust transits the atmosphere at temperatures lower than that required for He release (∼600°C). Size-dependent heating causes the particles which retain He to be far smaller than those in the parental interplanetary dust population. The particle-size distribution of He-bearing dust is such that most of the mass is delivered by particles of ∼20 μm diameter, while most of the surface area (relevant for surface-correlated constituents, e.g., implanted solar wind He) is carried by particles of ∼7 μm diameter. Knowledge of these size distributions allows us to evaluate the possibility of sedimentary redistribution of extraterrestrial dust in the atmosphere and ocean. The size distributions also have important consequences for interpretation of He abundances in seafloor sediment samples that integrate over fairly small areas and times. Sediment samples generally will not record a representative distribution of interplanetary dust but will have a strong tendency to undersample rare large particles. We predict a high degree of variability in replicate He analyses of a single sediment sample, with a mass-correlated He component yielding greater variability than a surface-correlated component. Comparison with sediment measurements confirms such variability and demonstrates excellent agreement with the statistical distribution expected for a surface correlated component, consistent with suggestions that seafloor extraterrestrial He is surface-correlated implanted solar wind or solar flare He. A second important statistical effect is that sediment measurements systematically underestimate the true extraterrestrial He flux, typically by 50%.

Helium abundances and isotopic ratios have been measured in samples of geothermal gases, submarine ultramafic xenoliths, and subaerial mafic phenocrysts and xenoliths from the Tabar-Lihir-Tanga-Feni (TLTF) are in the Bismarck Archipelago of Papua New Guinea. These unusual volcanoes are produced by adiabatic decompression melting of subduction-modified upper mantle and apparently carry an exceptionally large slab-derived component. They, therefore, provide an ideal setting for studying the role of subducted helium in are volcanoes. Helium isotopic ratios in the geothermal fluids, submarine xenoliths, and most helium-rich subaerial samples indicate the mantle source regions beneath the Tabar Island Group and Lihir Island have He-3/He-4 ratios of about 7.2 times the atmospheric ratio (R-A), whereas the mantle source for Ambitle Island (Feni Group) in the more southern TLTF are has a lower ratio of about 6.6 R-A These ratios are only slightly lower than typical depleted upper mantle values of about 8.5 R-A, indicating that even severely slab-modified mantle wedge carries a relatively minor slab-derived helium component. The systematically lower He-3/He-4 ratios of the Ambitle samples are interpreted to reflect a slightly greater slab-derived helium component.Helium isotopic ratios as low as 2 R-A, which are correlated with low total helium abundances, were observed in subaerial augitic clinopyroxene phenocrysts. Coexisting olivine and sodium-rich acmitic clinopyroxene have higher helium abundances and isotopic ratios. This isotopic disequilibrium between coexisting mineral phases is interpreted as crustal-level addition of a low He-3/He-4 component to an actively outgassing magma from which different mineral phases trap helium sequentially. Copyright (C) 1997 Elsevier Science Ltd.

MOST of the helium-3 in oceanic sediments comes from interplanetary dust particles (IDPs), and can therefore be used to infer the accretion rate of dust to the Earth through time(1-3). He-3 records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of gears, probably owing to cometary and asteroidal break-up events(3). Muller and MacDonald have proposed(4) that periodic changes in this accretion rate due to a previously unrecognized 100-kyr periodicity in the Earth's orbital inclination might account for the prominence of this frequency in climate records of the past million years(5). Here we report variations in the He-3 flux to the sea floor that support this idea, We find that the flux recorded in rapidly accumulating Quaternary sediments from the Mid-Atlantic Ridge oscillates with a period of about 100 kyr. We cannot yet say, however, whether the 100-kyr climate cycle is a consequence of, a cause of, or an effect independent of these periodic changes in the rate of delivery of interplanetary dust to the sea floor.

Neon isotopic compositions in mantle-derived samples commonly are enriched in (20)Ne and (21)Ne relative to (22)Ne compared with atmospheric neon ((20)Ne/(22)Ne and (21)Ne/(22)Ne ratios in atmospheric neon are 9.8 and 0.029, respectively), together with significant primordial (3)He. Such results have been obtained on MORB's, intraplate plume-related oceanic island basalts, backarc basin basalts, mantle xenoliths, ancient diamonds and CO2 well gases (e.g., 1 - 8). The highest (20)Ne/(22)Ne ratio observed in MORB glasses (= 13.6 plus or minus 1.3 is close to the solar value (= 13.6, as observed in solar wind). In order to explain the enrichment of (20)Ne and (21)Ne relative to atmospheric neon for samples derived from the mantle, it is necessary to postulate the presence of at least two distinct non-atmospheric components. The two most likely candidates are solar and nucleogenic ((20)Ne/(22)Ne solar = 13.6 (21)Ne/(22)Ne solar = 0.032, (20)Ne/(22)Ne nucleogenic = 2.5 and (21)Ne/(22)Ne nucleogenic = 32). This is because solar neon is the only known component with a (20)Ne/(22)Ne ratio greater than both the atmospheric value and that observed in samples derived from the mantle. Nucleogenic neon is well known to elevate (21)Ne/(22)Ne ratios. Neon isotopic signatures observed in mantle-derived samples can be accounted for by mixing of the three neon end members: solar, nucleogenic and atmospheric.

We have determined the elemental and isotopic compositions of noble gases in young subduction-related phenocrystic olivine and clinopyroxene samples from the Taupo Volcanic Zone, central North Island, New Zealand, and in behind-arc intraplate phenocrystic and xenolithic olivine samples from the Northland and Auckland Volcanic Provinces, northern North Island, New Zealand. Helium isotopic ratios range from MORB-like He-3/He-4 values of about 11 x 10(-6) bto lower values of about 6 x 10(-6), consistent with previous measurements of helium isotopic ratios in subduction-related samples. Ar-40/Ar-36 and Ne-21/Ne-22 ratios range from atmosphere-like compositions to maximum values of about 700 and 0.033, respectively. In contrast, most of the Ne-20/Ne-22 ratios are generally indistinguishable from atmospheric values.The variations in helium, neon, and argon isotopic ratios are interpreted as resulting from mixing of: (a) a primordial He-3-rich component derived from the upper mantle and characterised by MORB-like He-3/He-4 ratios of about 12 x 10(-6); (b) a radiogenic He-4-rich component derived from crustal materials and characterised by a He-3/He-4 ratio of less than 2 x 10(-7), and (c) a helium-poor atmosphere-derived component which dominates the heavier noble gases. By combining the helium and argon isotopic results, it is possible to estimate the relative contribution of each of these three components to the total Ar-40 observed in each sample.Based on the present understanding of the origin and evolution of arc magmas, a simple qualitative model of the mechanisms which introduce noble gases to the parent magmas of the samples is developed. However, it remains uncertain whether the atmospheric and crustal-derived noble gas components are introduced to the mantle source regions of the magmas by subduction, or alternatively, are introduced by interactions between the ascending parent magmas and the overlying crust.

Noble gas elemental and isotopic abundances have been analysed in twenty-two samples of basaltic glass dredged from the submarine flanks of two currently active Hawaiian volcanoes, Loihi Seamount and Kilauea. Neon isotopic ratios are enriched in 20Ne and 21Ne by as much as 16% with respect to atmospheric ratios. All the Hawaiian basalt glass samples show relatively high 3He4He ratios. The high 20Ne22Ne values in some of the Hawaiian samples, together with correlations between neon and helium systematics, suggest the presence of a solar component in the source regions of the Hawaiian mantle plume. The solar hypothesis for the Earth's primordial noble gas composition can account for helium and neon isotopic ratios observed in basaltic glasses from both plume and spreading systems, in fluids in continental hydrothermal systems, in CO2 well gases, and in ancient diamonds. These results provide new insights into the origin and evolution of the Earth's atmosphere.

Noble gas elemental and isotopic abundances have been analysed in eight samples of youthful basaltic glass dredged from three different locations within the Lau Backarc Basin: (1) the King's Triple Junction, (2) the Central Lau Spreading Centre at 18°S and (3) the Eastern Lau Spreading Centre at 19°S. Samples from the Lau central and eastern spreading centres have MORB-like helium isotopic ratios of approximately 1.2 × 10−5 (8.5 R/RA). In contrast, the samples from the King's Triple Junction yield helium isotopic ratios averaging 9.4 (±0.8) × 10−6 (6.7 ± 0.6 R/RA), systematically lower than the MORB-like value, which may be reflecting the addition of radiogenic 4He released from the descending slab. Neon isotopic ratios are enriched in 20Ne and 21Ne with respect to atmospheric ratios by as much as 23% and 62% respectively. These observations further confirm that non-atmospheric neon is a common characteristic of samples derived from the mantle. The helium and neon isotopic signatures in the samples can be explained by mixing of a primordial solar component, radiogenic and nucleogenic components produced by radioactive processes inside the Earth, and an atmospheric component. This reconnaissance survey of noble gases in a backarc basin indicates that current volcanism is dominated by magmas from the mantle wedge, a source similar to that from which MORBs are derived. The heavier noble gases (argon, krypton and xenon), however, show more atmosphere-like compositions, either indicating strong interaction of the magmas with the atmosphere or the presence of a recycled component derived from the underlying subducting slab.

The noble-gas elemental and isotopic composition in the Earth is significantly different from that of the present atmosphere, and provides an important clue to the origin and history of the Earth and its atmosphere. Possible candidates for the noble-gas composition of the primordial Earth include a solar-like component, a planetary-like component (as observed in primitive meteorites) and a component similar in composition to the present atmosphere. In an attempt to identify the contributions of such components, we have measured isotope ratios of heliium and neon in fresh basaltic glasses dredged from Loihi seamount and the East Rift Zone of Kilauea 1-3. We find a systematic enrichment in Ne-20, and Ne-21 relative to Ne-22, compared with atmospheric neon. The helium and neon isotope signatures observed in our samples can be explained by mixing of solar, present atmospheric, radiogenic and nucleogenic components. These data suggest that the noble-gas isotopic composition of the mantle source of the Hawaiian plume is different from that of the present atmosphere, and that it includes a significant solar-like component. We infer that this component was acquired during the formation of the Earth.

A diode-laser-pumped Nd3+-doped fibre laser has been actively mode-locked using an integrated fibre phase modulator. Pulses with a sub-80 ps duration are observed at a repetition rate of 417 MHz. Near bandwidth-limited operation is achieved by restricting the laser bandwidth by feedback off a diffraction grating in an external coupled cavity. Low intracavity losses support a submilliwatt laser threshold and a slope efficiency in excess of 48%.

Re-evaluation of available noble gas data obtained from the glassy rims of basalts from Loihi Seamount, Hawaii, shows that contamination of magmas prior to eruption, by addition of a significant component of atmosphere-derived heavy noble gases, is a plausible explanation for the observed atmosphere-like isotopic compositions of Ne, Ar, Kr, and Xe. The most likely source for the atmospheric component is interaction of the magma with seawater carrying dissolved atmosphere-derived noble gases.The possibility of a significant atmospheric component in Loihi samples suggests that the observed heavy noble gas compositions may not be representative of the mantle source of Loihi magmas. While leaving open the question of the noble gas composition in the source region, atmospheric contamination provides a valid alternative to the interpretation that the mantle source region of Loihi magmas has an atmosphere-like noble gas composition.

We describe the construction and operation of an FM mode-locked fiber laser using a large-diameter Nd3+ doped silica fiber with dielectric reflectors deposited directly onto the fiber ends and an integrated ZnO acousto-optic phase modulator clamped to the fiber. Optical pulses of less than 60 ps duration have been generated at 432.77 MHz with 400 mW of power to the transducer, and 300-400 ps pulses have been observed with only 4011,W of modulator power, correspond-ing to less than 130 wad of phase modulation.