(Pa/U)-(Th/U) of young mafic volcanic rocks from Nicaragua and Costa Rica and the influence of flux melting on U-series systematics of arc lavas     

Rebecca B. Thomas  Marc M. HirschmannHai Cheng  Mark K. ReaganR.Lawrence Edwards 《Geochimica et cosmochimica acta》2002,66(24):4287-4309

We present U, Th, and Pa isotope data for young lavas from Costa Rica and Nicaragua in the Central American arc. Thorium isotopic ratios for Costa Rica and Nicaragua differ dramatically: Costa Rican lavas are characterized by low (²³⁰Th/²³²Th) (1 to 1.2) and, for four out of five lavas, (²³⁰Th/²³⁸U) greater than unity. Nicaraguan lavas have high (²³⁰Th/²³²Th) (2.2 to 2.7) and, for five of six samples, (²³⁰Th/²³⁸U) less than unity. All lavas have (²³¹Pa/²³⁵U) greater than unity, with initial values ranging from 1.27 to 1.77, but those from Costa Rica have larger ²³¹Pa excesses. There is a broad positive correlation between (²³¹Pa/²³⁵U) and (²³⁰Th/²³⁸U) similar to the worldwide trend for arcs outlined by Pickett and Murrell (1997), although many of the Nicaraguan lavas skirt the high end of that trend. In greater detail, the Central American data appear to divide into separate high-(²³¹Pa/²³⁵U) and low-(²³¹Pa/²³⁵U) tiers. These tiers may be different because of either different residence times in the crust or different proportions of sedimentary components from the slab.Substantial (²³¹Pa/²³⁵U) excesses (>1.5) in both Costa Rica and Nicaragua require a melting process that allows for enhanced daughter (²³¹Pa) ingrowth. With increasing U addition, (²³¹Pa/²³⁰Th) increases in a manner that cannot be explained adequately by aging of fluid components before partial melting and eruption. Thus, either some ²³¹Pa is added from the slab, or melting-enhanced ²³¹Pa ingrowth is greater in sources that have experienced a larger amount of slab-derived flux and a higher extent of melting. These observations can be explained if regions that have undergone greater extents of fluxing and melting have experienced these processes over a longer time interval than those that have had little flux added and little melt extracted. We propose a flux-ingrowth melting model in which corner flow in the mantle wedge supplies fresh hot mantle into a zone of slab fluid addition. Partial melting occurs in response to this fluxing. We assume critical melting at low porosity (∼10⁻³), rapid fluid flux to the melting region, and rapid melt transport. Solid mantle traverses the melting region over 10⁵ to 10⁶ yr, thereby allowing ²³¹Pa and ²³⁰Th ingrowth from U retained in the residues of melt extraction. Magmas are aggregated from all parts of the melting regime, mixing melts from incipiently fluxed regions with those from sources that have experienced more extensive fluid addition, partial melting, and daughter nuclide ingrowth. With suitable assumptions about component addition from the slab, this flux-ingrowth model matches a wide range of U-series and trace element data from Costa Rican and Nicaraguan lavas, with required average extents of melting of ∼1 to 3% and 7 to 15%, respectively. Upwelling and/or extensive melt-rock reaction are not required to explain large (²³¹Pa/²³⁵U) excesses in Central America or other arcs. On Th isotope equiline plots, the model produces linear arrays that resemble isochrons but that have no age significance. Instead, these arrays are generated by mixing of melts from sources that have experienced fluid addition and partial melting over a range of time intervals, as seems likely in arc source regions. Finally, the flux-ingrowth model predicts considerable ²²⁶Ra excesses for integrated magmas. If we assume that ²²⁶Ra is added continuously with the slab-derived fluid, the model predicts large and increasing (²²⁶Ra/²³⁰Th) with increasing melting and slab-component addition, without requiring the addition of a distinct late fluid.  相似文献   

From oblique subduction to intra-continental transpression: Structures of the southern Kermadec-Hikurangi margin from multibeam bathymetry,side-scan sonar and seismic reflection     

Jean-Yves Collot  Jean Delteil  Keith B. Lewis  Bryan Davy  Geoffroy Lamarche  Jean-Christophe Audru  Phil Barnes  Franck Chanier  Eric Chaumillon  Serge Lallemand  Bernard Mercier de Lepinay  Alan Orpin  Bernard Pelletier  Marc Sosson  Bertrand Toussaint  Chris Uruski 《Marine Geophysical Researches》1996,18(2-4):357-381

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.  相似文献   

Atypical heat-flow near gas hydrate irregularities and cold seeps in the Baikal Rift Zone     

Maarten Vanneste  Jeffrey Poort  Marc De Batist  Jan Klerkx 《Marine and Petroleum Geology》2002,19(10)

In this paper, we address the irregular behaviour and geometry of the gas hydrate stability zone (HSZ) inferred from reflection seismic data in relation to heat-flow measurements. The study area lies in the hanging wall of the Posolsky fault in the Southern Baikal Basin (SBB). Side-scan sonar imagery already revealed an undulating antithetic active fault structure and several isolated active vent structures. Remarkably, these fluid discharge structures occur only where the base of the hydrate stability zone (BHSZ), as inferred from seismic reflection profiles, is fluctuating and discontinuous, independent of lake floor morphology. The correlation between the interpreted BHSZ and heat-flow data across the Malenki seep is reasonable. On a seismic profile south of the fluid escape features, the BHSZ is expressed as an oscillatory, but continuous reflection, and shows poor correlation with heat-flow measurements. In nearly all cases, measured heat-flow exceeds inferred heat-flow. Additionally, the local inferred minima are anomalously low compared to the expected background values in the SBB. These observations suggest that the present-day hydrate accumulation and its (meta-)stability are more complicated than originally suspected. The limited area of these anomalies, their amplitudes and their occurrence in the immediate vicinity of faults and fluid escape features suggest that fluid convection cells disturb local gas hydrate stability conditions.  相似文献   

A Survey of the Southwest Indian Ridge Axis Between Atlantis II Fracture Zone and the Indian Ocean Triple Junction: Regional Setting and Large Scale Segmentation   总被引：4，自引：0，他引：4  

Patriat  Philippe  Sauter  Daniel  Munschy  Marc  Parson  Lindsay 《Marine Geophysical Researches》1997,19(6):457-480

The study of very low-spreading ridges has become essential to ourunderstanding of the mid-oceanic ridge processes. The Southwest Indian Ridge(SWIR) , a major plate boundary of the world oceans, separating Africa fromAntarctica for more than 100 Ma, has such an ultra slow-spreadingrate. Its other characteristic is the fast lengthening of its axis at bothBouvet and Rodrigues triple junctions. A survey was carried out in thespring of 1993 to complete a multibeam bathymetric coverage of the axisbetween Atlantis II Fracture Zone (57° E) and the Rodrigues triplejunction (70° E). After a review of what is known about the geometry,structure and evolution of the SWIR, we present an analysis of the newalong-axis bathymetric data together with previously acquiredacross-axis profiles. Only three transform faults, represented byAtlantis II FZ, Novara FZ, and Melville FZ, offset this more than 1000 kmlong section of the SWIR, showing that the offsets are more generallyaccommodated by ridge obliquity and non-transform discontinuities. From comparison of the axial geometry, bathymetry, mantle Bouguer anomaly and central magnetic anomaly, three large sections (east of Melville FZ, between Melville FZ and about 65°30 E, and from there to the Rodrigues triple junction) can be distinguished. The central member, east of Melville FZ, does not resemble any other known mid-oceanic ridge section: the classical signs of the accretion (mantle Bouguer anomaly, central magnetic anomaly) are only observed over three very narrow and shallow axis sections. We also apply image processing techniques to the satellite gravity anomaly map of Smith and Sandwell (1995) to determine the off-axis characteristics of the Southwest Indian Ridge domain, more especially the location of the triple junction and discontinuities traces. We conclude that the large-scale segmentation of the axis has been inherited from the evolution of the Rodrigues triple junction.  相似文献   

Quantitative assessment of intertidal blue mussel (Mytilus edulis L.) stocks: combined methods of remote sensing, field investigation and sampling   总被引：3，自引：0，他引：3  

Marc Herlyn 《Journal of Sea Research》2005,53(4):243-253

The aim of this investigation was to develop appropriate methods for quantitative assessment of blue mussel beds. Combined methods of remote sensing, ground truth investigation and sampling in the field were applied to mature and young intertidal mussel beds. Three variables were measured to obtain reliable quantitative estimates of biomass and abundance: the total area covered by a bed; the cover, i.e. the area of mussel-covered patches in relation to the total mussel bed area; and the proportion, i.e. the area covered by mussels within the patches in relation to the total area of mussel patches. Cover and proportion of intertidal blue mussel beds were measured in the field by the transection method. Aerial photographs enabled the total area of the beds to be determined and large-scale surveys of mussel beds to be carried out synchronously. By examining large-scale aerial photographs it was possible also to determine the cover by remote sensing. The proportion could not be derived from aerial photographs, even from pictures at the largest scale tested during this investigation (scale 1:2500). By statistical methods it was estimated that 12 sub-samples (each covering 177 cm²) from mature beds, and 12 sub-samples (each covering 38 cm²) from young beds are needed to obtain reliable data on biomass and abundance.  相似文献   

Non-numeric computation for high eccentricity orbits     

R. Sridharan  Marc L. Renard 《Celestial Mechanics and Dynamical Astronomy》1975,11(2):179-194

Geocentric orbits of large eccentricity (e=0.9 to 0.95) are significantly perturbed in cislunar space by the Sun and Moon. The time-history of the height of perigee, subsequent to launch, is particularly critical. The determination of ‘launch windows’ is mostly concerned with preventing the height of perigee from falling below its low initial value before the mission lifetime has elapsed. Between the extremes of high accuracy digital integration of the equations of motion and of using an approximate, but very fast, stability criteria method, this paper is concerned with the development of a method of intermediate complexity using non-numeric computation. The computer is used as the theory generator to generalize Lidov's theory using six osculating elements. Symbolic integration is completely automatized and the output is a set of condensed formulae well suited for repeated applications in launch window analysis. Examples of applications are given.  相似文献   

Luna 24 ferrobasalt as a low-Mg primary melt     

Marc Norman  Graham Ryder 《Earth, Moon, and Planets》1980,23(3):271-292

Luna 24 very-low titanium (VLT) ferrobasalts, metabasalts, brown glasses and impact melts form a tight compositional cluster with no gradation to other groupings postulated for the Luna 24 core components. This suggests that the Luna 24 VLT ferrobasalt was extruded as a liquid of its own composition and was not derived by fractional crystallization from a more magnesian parent in a surface flow. Furthermore, the characteristics of the core lithologies are not easily visualized as components of such a differentiated flow, e.g. brown glasses. Gravitative settling models purporting to demonstrate the validity of the flow differentiation model are merely permissive.Subsurface fractionation requires that plagioclase, not olivine, be the liquidus phase.The high-Mg component in the Luna 24 core can be constrained, though not identified, chemically, and it has neither the major element, trace element, isotopic, nor mineralogical characteristics required of a possible parent to the Luna 24 VLT ferrobasalt. Thus models of fractionation lack a physical expression of the less differentiated compositions, contrary to the belief that the high-Mg component in the core is the parent material.The Luna 24 VLT ferrobasalt is probably a primary low-Mg melt from a plagioclase-bearing source region, and may have undergone little or no fractionation prior to eruption. Such a model is compatible with, and suggested by, chemical and experimental data. Caution against postulating that all Mg-poor melts are fractionated products, based on terrestrial models, is advised. The terrestrial oceanic situation of primary melts with similar Mg/Fe is probably not valid for the Moon.  相似文献   

Supergranular line profile variation of Mg i λ2852     

Marc S. Allen 《Solar physics》1978,60(2):265-267

The chromospheric network is barely visible in the Mg i 2852 line. Unlike in plages, where it shows well-defined self-reversed emission structure in the core, the line is an unreversed absorption feature in the network boundaries, exhibiting only a small blue asymmetry. In the network cell interiors, the line is unreversed and symmetrical.  相似文献   

Echelle observations of C iii λ1909 and Si iii λ1892     

Allen  Marc S. 《Solar physics》1979,64(1):71-75

Profiles of C iii 1909 and Si iii 1892 obtained on and near the limb during the 1976 flight of the University of Hawaii echelle rocket spectrograph were reduced and analyzed to determine electron densities and mass motions. The electron pressure derived (N _eT_e 4 × 10¹⁵ cm^–3) agrees well with that determined by Cook and Nicolas (1979) from ATM data. Nonthermal velocities in the region of formation of Si iii 1892 on the disk were found to be 10–12 km s^–1, somewhat lower than the values obtained by Doschek et al. (1976), also from ATM spectra. However, velocities derived at and above the limb were in closer agreement, about 17 km s^–1.Geosciences Systems Department, Computer Sciences Corporation, 8728 Colesville Road, Silver Spring, Md. 20910, U.S.A.  相似文献   

Remote sensing for groundwater modelling in large semiarid areas: Lake Chad Basin,Africa     

Marc Leblanc  Guillaume Favreau  Sarah Tweed  Christian Leduc  Moumtaz Razack  Linus Mofor 《Hydrogeology Journal》2007,15(1):97-100

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