The role and conditions of second-stage mantle melting in the generation of low-Ti tholeiites and boninites: the case of the Manihiki Plateau and the Troodos ophiolite

High-Mg, low-Ti volcanic rocks from the Manihiki Plateau in the Western Pacific share many geochemical characteristics with subduction-related boninites such as high-Ca boninites from the Troodos ophiolite on Cyprus, which are believed to originate by hydrous re-melting of previously depleted mantle. In this paper we compare the Manihiki rocks and Troodos boninites using a new dataset on the major and trace element composition of whole rocks and glasses from these locations, and new high-precision, electron microprobe analyses of olivine and Cr-spinel in these rocks. Our results show that both low-Ti Manihiki rocks and Troodos boninites could originate by re-melting of a previously depleted lherzolite mantle source (20–25% of total melting with 8–10% melting during the first stage), as indicated by strong depletion of magmas in more to less incompatible elements (Sm/Yb < 0.8, Zr/Y < 2, Ti/V < 12) and high-Cr-spinel compositions (Cr# > 0.5). In comparison with Troodos boninites, the low-Ti Manihiki magmas had distinctively lower H₂O contents (< 0.2 vs. > 2 wt% in boninites), ~ 100 °C higher liquidus temperatures at a given olivine Fo-number, lower fO₂ (ΔQFM < + 0.2 vs. ΔQFM > + 0.2) and originated from deeper and hotter mantle (1.4–1.7 GPa, ~ 1440 °C vs. 0.8–1.0 GPa, ~ 1300 °C for Troodos boninites). The data provide new evidence that re-melting of residual upper mantle is not only restricted to subduction zones, where it occurs under hydrous conditions, but can also take place due to interaction of previously depleted upper mantle with mantle plumes from the deep and hotter Earth interior.     

Statistical microthermometry of synthetic fluid inclusions in quartz during decompression reequilibration

Fluid inclusion microthermometric data are often reported as homogenization temperature frequency histograms. Interpretation of such histograms for a single fluid inclusion assemblage (FIA) of non-reequilibrated fluid inclusions is usually straightforward and provides an accurate determination of the original density (Th) of that FIA. However, interpretation of such histograms for reequilibrated inclusions is more problematic. Decompression experiments using synthetic inclusions in natural quartz and conducted at 2–5 kbar and 600–700 °C with a maximum internal overpressure of 2 kbar indicate that histogram shape reflects the sample's P-T history. Our results further indicate that the mean, mode, range, standard deviation, extreme values, etc., all have a significance with respect to the P-T history of the sample. Thus, a mound-shaped, unimodal histogram with low range is indicative of a nearly isochoric cooling P-T path. A unimodal histogram that is slightly skewed to the right, and with a low standard deviation but high range, results from inclusion deformation in the plastic regime (high temperature/low strain rates). Fluid inclusions deformed plastically show no correlation between size and density. Histogram outliers should not be ignored and may be used to determine an isochore that passes close to the conditions of entrapment (minimum Th) or close to the final reequilibration conditions (maximum Th). The histogram mean Th value corresponds to an isochore that represents the internal overpressure (about 1 kbar) that can be maintained over geologic time by a majority of reequilibrated fluid inclusions. A multimodal histogram with high range and high standard deviation indicates inclusion brittle deformation (low temperature/high strain environments). Fluid inclusions deformed in a brittle manner show strong positive correlation between size and density. Histograms produced in the laboratory show many similarities to histograms for natural samples, offering the hope that laboratory results may be used to interpret P-T histories of natural samples. Received: 20 May 1997 / Accepted: 3 April 1998     

Coherent Emission Mechanisms in Radio Pulsars

A theory of pulsar radio emission generation, in which the observed waves are produced directly by the maser-type plasma instabilities on the anomalous cyclotron-Cherenkov resonance and the Cherenkov-drift resonance , is capable of explaining the main observational characteristics of pulsar radio emission. The instabilities are due to the interaction of the fast particles of the primary beam and from the tail of the distribution with the normal modes of a strongly magnetized one-dimensional electron-positron plasma. The waves emitted at these resonances are vacuum-like electromagnetic waves that may leave the magnetosphere directly. The cyclotron-Cherenkov instability is responsible for core emission pattern and the Cherenkov-drift instability produces conal emission. The conditions for the development of the cyclotron-Cherenkov instability are satisfied for the both typical and millisecond pulsars provided that the streaming energy of the bulk plasma is not very high γ _p = 5 ÷ 10. In a typical pulsar the cyclotron-Cherenkov and Cherenkov-drift resonances occur in the outer parts of magnetosphere at r _res ≈ 10⁹cm. This theory can account for various aspects of pulsar phenomenology including the morphology of the pulses, their polarization properties and spectral behavior. This revised version was published online in July 2006 with corrections to the Cover Date.     

Waves in a one-dimensional magnetized relativistic pair plasma

Normal modes of a one-dimensional relativistically streaming electron–positron plasma in a superstrong magnetic field are considered, taking into account possible different bulk velocities and thermal effects. This physical picture corresponds to the plasma present on the open field lines of rotating neutron stars where the observed radio emission is generated. Various cases are considered: relativistic and non-relativistic relative streaming of cold components, and relativistically hot distributions. A distinction between superluminous and subluminous waves (which can be excited by the Cherenkov effect) is clearly stated. In the low-frequency regime the Cherenkov and cyclotron two-stream instabilities occur. Polarization of the quasi-transverse modes changes from circular for the propagation along magnetic field lines to linear for angles of propagation larger than some critical angle that depends on the relative velocity of the plasma components.     

Induced Raman scattering in pulsar magnetospheres

It is shown that induced Raman scattering of electromagnetic waves in the strongly magnetized electron–positron plasma of pulsar magnetospheres may be important for wave propagation and as an effective saturation mechanism for electromagnetic instabilities. The frequencies at which strong Raman scattering occurs in the outer parts of a magnetosphere fall into the observed radio band. The typical threshold intensities for the strong Raman scattering are of the order of the observed intensities, implying that pulsar magnetospheres may be optically thick to Raman scattering of electromagnetic waves.     

Maser Action in Methanol Transitions

We report the detection with the ATCA of 6.7 GHz methanol emission towards OMC-1. The source has a size between 40 and 90, is located to the south-east of Ori-KL and may coincide in position with the 25 GHz masers. The source may be an example of an interesting case recently predicted in theory where the transitions of traditionally different methanol maser classes show maser activity simultaneously. In addition, results of recent search for methanol masers from the 25 and 104.3 GHz transitions are reported.     

A Practical Algorithm for the External Annotation of Area Features

One of the subtasks of automated map labelling that has received little attention so far is the labelling of areas. Geographic areas are often are represented by concave polygons which pose severe limitations on straightforward solutions due to their great variety of shape, a fact worsened by the lack of measures for quantifying feature-label relationships. We introduce a novel and efficient algorithm for labelling area features externally, i.e. outside their polygonal boundary. Two main contributions are presented in the following. First, it is a highly optimized algorithm of generating candidate placements utilizing algorithms from the field of computational geometry. Second, we describe a measure for scoring label positions. Both solutions based on a series of well-established cartographic precepts about name positioning in the case of semantic enclaves such as islands or lakes. The results of our experiments show that our algorithm can efficiently place labels with a quality that is close to the quality of traditional cartographic products made by human cartographers.     

Subduction cycling of volatiles and trace elements through the Central American volcanic arc: evidence from melt inclusions

Compositions of melt inclusions in olivine (Fo_90-64) from 11 localities in Guatemala, Nicaragua and Cost Rica along the Central American Volcanic Arc are used to constrain combined systematics of major and trace elements and volatile components (H₂O, S, Cl, F) in parental melts and to estimate volcanic fluxes of volatile elements. The melt inclusions cover the entire range of compositions reported for whole rocks from Central America. They point to large heterogeneity of magma sources on local and regional scales, related to variable contributions of diverse crustal (from the subducting and overriding plates) and mantle (from the wedge and incoming plate) components involved in magma genesis. Water in parental melts correlates inversely with Ti, Y and Na and positively with Ba/La and B/La (with the exception of Irazú Volcano), which indicates mantle melting fluxed by Ba-, B- and H₂O-rich, possibly, serpentinite-derived fluid beneath most parts of the arc. Different components with melt-like characteristics (high LREE, La/Nb and probably also Cl, S and F and low Ba/La) control the geochemical peculiarities of Guatemalan and Costa Rican magmas. The composition of parental magmas together with published data on volcanic volumes and total SO₂ flux from satellite measurements are used to constrain fluxes of volatile components and to estimate total magmatic flux in Central America. We found that volcanic flux accounts for only 13% of total magmatic and volatile fluxes. The remaining 87% of magmas remained in the lithosphere to form cumulates (∼39%) and intrusives (∼48%). The intrusive fraction of magmatic flux may be significantly larger beneath Nicaragua compared to Costa Rica. Interestingly, total fluxes of magmas and volatiles in Central America are quite similar to the global average estimates. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Common temperature signal in four well‐replicated tree growth series from northern Fennoscandia

Four Nordic temperature proxies based on tree growth at the northern timberline – ring‐width from Sweden and Finland, maximum latewood density from Sweden, and height increment from Finland – were compared. Three indexing methods were used to enhance the low (centennial and above), medium (decadal‐to‐multidecadal) and high (decadal‐to‐interannual) frequencies. The proxies are shown to have a strong temperature signal (common variance) at the interannual‐to‐multidecadal scale, while the multidecadal‐to‐centennial trends are less coherent, perhaps reflecting intra‐regional differences in growing conditions but more likely due to the more noisy regional curve standardization method used to retain the longest trends. Various methods of combining the four proxy series were explored and tested by comparison with four long temperature records from northern Fennoscandia. Only relatively high‐frequency, spline‐indexed series produced consistently positive verification statistics as a reconstruction model for summer temperature using all four proxies. Copyright © 2012 John Wiley & Sons, Ltd.     

Resonant cyclotron scattering and Comptonization in neutron star magnetospheres

Resonant cyclotron scattering of the surface radiation in the magnetospheres of neutron stars may considerably modify the emergent spectra and impede efforts to constrain neutron star properties. Resonant cyclotron scattering by a non-relativistic warm plasma in an inhomogeneous magnetic field has a number of unusual characteristics. (i) In the limit of high resonant optical depth, the cyclotron resonant layer is half opaque , in sharp contrast to the case of non-resonant scattering. (ii) The transmitted flux is on average Compton up-scattered by  ∼1 + 2β_T  , where  β_T  is the typical thermal velocity in units of the velocity of light; the reflected flux has on average the initial frequency. (iii) For both the transmitted and reflected fluxes, the dispersion of intensity decreases with increasing optical depth. (iv) The emergent spectrum is appreciably non-Planckian while narrow spectral features produced at the surface may be erased.
We derive semi-analytically modification of the surface Planckian emission due to multiple scattering between the resonant layers and apply the model to the anomalous X-ray pulsar 1E  1048.1 − 5937  . Our simple model fits just as well as the 'canonical' magnetar spectra model of a blackbody plus power law.