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1.
We discuss the concept of components in the Earth's mantle startingfrom a petrological and geochemical approach, but adopting anew method of projection of geochemical and isotopic data. Thisallows the compositional variability of magmatic associationsto be evaluated in multi-dimensional space, thus simultaneouslyaccounting for a large number of compositional variables. Wedemonstrate that ocean island basalts (OIB) and mid-ocean ridgebasalts (MORB) are derived from a marble-cake mantle, in whichdifferent degrees of partial melting of recycled lithosphere,which are heterogeneous in age and composition, contribute tothe magma genesis. This view is supported by the variabilityin the geochemical and isotopic signatures of OIB that are observedon the scale of a single ocean island as well as on that ofan ocean, mostly varying between two extreme compositions, thatare not strictly related to the commonly accepted mantle components(DMM, EMI, EMII, HIMU). Rather they are a distinctive featureof the mantle source sampled at each ocean island and are stronglydependent on the Pb isotope system. We recommend a change inperspective in studies of MORB–OIB geochemistry from onebased on physically distinct mantle components to a model basedon the existence of a marble-cake-like upper mantle. Althoughresembling the statistical upper mantle, this model impliesthat geochemical homogenization can be attained only withinthe limits of local mantle composition, so that a world-wideuniform depleted reservoir cannot be sampled by simply extendingthe volume of the region undergoing partial melting. KEY WORDS: geochemistry; isotope; mantle; OIB 相似文献
2.
Cristina?PerinelliEmail author Pietro?Armienti Luigi?Dallai 《Contributions to Mineralogy and Petrology》2006,151(3):245-266
Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite
to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite
and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite.
Peridotite xenoliths are characterized by negative Al2O3–Mg# and TiO2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend
in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment
in clinopyroxene and sporadic increase of Ti–Fetot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fetot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents
increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium
conditions among clinopyroxene, olivine and orthopyroxene. The highest δ18O values are observed in minerals of the amphibole-bearing xenolith. The δ18Ocpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence
of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory
lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close
to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the
host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent
with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region. 相似文献
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Late Miocene (c. 13–5 Ma) volcanic sequences of the Hallett Volcanic Province (HVP) crop out along >250 km of western Ross
Sea coast in northern Victoria Land. Eight primary volcanic and six sedimentary lithofacies have been identified, and they
are organised into at least five different sequence architectures as a consequence of different combinations of eruptive and/or
depositional conditions. The volcanoes were erupted in association with a Miocene glacial cover and the sequences are overwhelmingly
glaciovolcanic. The commonest and most representative are products of mafic aa lava-fed deltas, a type of glaciovolcanic sequence
that has not been described before. It is distinguished by (1) a subaerially emplaced relatively thin caprock of aa lavas
lying on and passing down-dip into (2) a thicker association of chaotic to crudely bedded hyaloclastite breccias, water-chilled
lava sheets and irregular lava masses, collectively called lobe-hyaloclastite. A second distinctive sequence type present
is characterised by water-cooled lavas and associated sedimentary lithofacies (diamictite (probably glacigenic) and fluvial
sands and gravels) similar to some mafic glaciovolcanic sheet-like sequences (see Smellie, Earth-Science Reviews, 74, 241–268,
2008), but including (for the first time) examples of likely sheet-like sequences with felsic compositions. Other sequence types
in the HVP are minor and include tuff cones, cinder cones and a single ice-marginal lacustrine sequence. The glacial thermal
regime varied from polar, characterised by sequences lacking glacial erosion, glacigenic sediments or evidence for free water,
to temperate or sub-polar for sequences in which all of these features are conspicuously developed. 相似文献
5.
On 28 December 2002, new vents opened on the flanks of Stromboli, just below the summit craters, interrupting the persistent
activity of the volcano with a 7-month-long effusive eruption. We here report on the plagioclase size distribution (PlgSD)
in lava samples collected following the chronology of the 2002–2003 eruption. Data reveal a linear PlgSD similar to that found
in samples of normal Stromboli activity, indicating that the switch from Strombolian explosive to effusive activity is not
associated with changes in texture. Nevertheless, the crystal size distribution slopes and intercepts exhibit slight sinusoidal
temporal variations that are here ascribed to a magma supply mechanism able to induce “resonance” in the crystal size distribution,
with an amplitude that depends on the supply rate. 相似文献
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Olivine crystal size distributions (CSDs) have been measured in three suites of spinel- and garnet-bearing harzburgites and lherzolites found as xenoliths in alkaline basalts from Canary Islands, Africa; Victoria Land, Antarctica; and Pali Aike, South America. The xenoliths derive from lithospheric mantle, from depths ranging from 80 to 20 km. Their textures vary from coarse to porphyroclastic and mosaic–porphyroclastic up to cataclastic. Data have been collected by processing digital images acquired optically from standard petrographic thin sections. The acquisition method is based on a high-resolution colour scanner that allows image capturing of a whole thin section. Image processing was performed using the VISILOG 5.2 package, resolving crystals larger than about 150 μm and applying stereological corrections based on the Schwartz–Saltykov algorithm. Taking account of truncation effects due to resolution limits and thin section size, all samples show scale invariance of crystal size distributions over almost three orders of magnitude (0.2–25 mm). Power law relations show fractal dimensions varying between 2.4 and 3.8, a range of values observed for distributions of fragment sizes in a variety of other geological contexts.
A fragmentation model can reproduce the fractal dimensions around 2.6, which correspond to well-equilibrated granoblastic textures. Fractal dimensions >3 are typical of porphyroclastic and cataclastic samples. Slight bends in some linear arrays suggest selective tectonic crushing of crystals with size larger than 1 mm. The scale invariance shown by lithospheric mantle xenoliths in a variety of tectonic settings forms distant geographic regions, which indicate that this is a common characteristic of the upper mantle and should be taken into account in rheological models and evaluation of metasomatic models. 相似文献
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Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively,
at temperatures close to feldspar liquidus (750–700 °C), were carried out to determine the modality of water exsolution and
vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform
space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (≈0.5 vol.%)
of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size
distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius,
area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions
was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5–1000 μm
were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs
of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano
Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the
general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic
coordinates. The slope of this regression is approximately 2.8±0.1. This dependence is in striking contrast with distributions
theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution
requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous
nucleation mechanisms.
Received: 1 October 1998 / Accepted: 25 June 1999 相似文献
10.
The energy cone concept has been adopted to describe some kinds of surge deposits. The energy cone parameters (height and slope) are evaluated through a regression technique which utilizes deposit thicknesses and the correspondent quotes and heights of the energy cone. The regression also allows to evaluate a coefficient of proportionality linking the deposit thickness to the distance between topographic surface and energy line for a given eruption. Moreover, if an accurate topography is available (in this case a reconstruction of a digitalized topography of the Phlegrean Fields and of the Vesuvius), the energy cone parameters, obtained by the backfitted technique, can be used to evaluate the order of magnitude of the deposit volumes.The hazard map for a surge localized at the Solfatara (Phlegraean Fields, Naples) has been computed. The values of the energy cone parameters and the volume have been assumed to be equal to those estimated with the regression technique applied to a past surge eruption in the same area. 相似文献