Petrology Of Peridotite Xenoliths Research Articles

Petrology of peridotite xenoliths from Sabga volcano in the Cameroon Volcanic Line (North-West Cameroon)

Petrology of peridotite xenoliths from Sabga volcano in the Cameroon Volcanic Line (North-West Cameroon)

백령도 네오기 진촌 현무암에 포획된 첨정석 페리도타이트의 암석학적 특성

백령도 네오기 진촌 현무암에 포획된 첨정석 페리도타이트의 암석학적 특성

Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in Northern Victoria Land lithospheric mantle (Antarctica)

The petrology of peridotite xenoliths in the Cenozoic volcanics from Greene Point (Northern Victoria Land, Antarctica) provides new constraints on the characterisation of the lithospheric mantle beneath the West Antarctic Rift.Based on mineral major and trace element models, this mantle domain is proposed to represent a residuum after 10% and 20% partial melting. Moreover, melting models and isotopic results for Sr and Nd systematics highlight the substantial contribution of tholeiitic melts percolating through peridotites. Close correlation with trace element contents in clinopyroxene phenocrysts from Ferrar and Karoo tholeiites allows us to ascribe this refertilisation event to the Jurassic. This asthenospheric melt was also able to transfer a garnet signature to the Northern Victoria Land mantle segment. The rare presence of glass and secondary phases indicate that Greene Point xenoliths were heterogeneously affected by alkaline metasomatism, probably related to the West Antarctic Rift System opening; this has also been widely observed in other Northern Victoria Land localities (i.e., Baker Rocks).Temperature and fO2 were calculated (950°C; Δlog fO2 (QFM), −1.70 to −0.39) at a fixed pressure of 15kbar, confirming the tendency of the anhydrous Greene Point xenolith population to have higher equilibration temperatures and comparable redox conditions, compared to the nearby amphibole-bearing peridotites from Baker Rocks.

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al2O3 and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (≤3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na2O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.

Petrology of Peridotite Xenoliths from Iraya Volcano, Philippines, and its Implication for Dynamic Mantle-Wedge Processes

Peridotite xenoliths entrained in calc-alkaline andesites from the Iraya volcano, Philippines, were petrologically examined to constrain the nature of the mantle-wedge materials and processes. They can be classified into two types: C-type (coarse-grained type) and Ftype (fine-grained type) peridotites. C-type peridotites are mostly coarse-grained (olivine, 1 mm across) harzburgites with porphyroclastic to protogranular textures but include subordinate dunites. Ftype peridotites are fine-grained (olivine, 60---70 mm across). Secondary orthopyroxenes that replace olivine and sometimes show radial (spherulitic) aggregation are very common in F-type peridotites and, subordinately, in C-type peridotites, in which the total amount of orthopyroxene increased in volume. Fine-grained olivine in F-type peridotites characteristically has minute glass and chromian spinel inclusions. Mineral chemistry is clearly different between the two types of peridotite: olivine is around Fo91---92 and Fo89---91 in C-type and F-type peridotites, respectively. The Cr/(Cr þ Al) atomic ratio (Cr number) and Fe3þ/(Cr þ Al þ Fe3þ) atomic ratio of chromian spinel are 0 2---0 3 and 50 1, respectively, in C-type peridotites, and 0 4---0 7 and around 0 1, respectively, in F-type peridotites. The secondary orthopyroxenes are appreciably lower in Al2O3, Cr2O3 and CaO than the primary ones. A textural transition from C-type to F-type peridotites can be observed; coarse olivine becomes recrystallized into fine grains through subgrains that preserve the previous coarse texture. The C-type harzburgites are similar in mineral chemistry to arc-type harzburgites, e.g. mantle xenoliths from the Japanese island arcs, and may represent samples of the sub-arc lithospheric mantle. The C-type harzburgites beneath the Iraya volcano may have been strained and deformed during oblique subduction of the South China Basin. A silicate melt rich in SiO2, H2O and Fe, possibly derived by fractional crystallization from a primitive arc magma, assisted the recrystallization of the C-type peridotites to the F-type peridotites with metasomatic chemical modification. Oblique subduction is common in arc---trench systems, suggesting that F-type peridotite formation may be common within the mantle wedge.

Petrology of peridotite xenoliths in lamprophyre from Shingu, Southwestern Japan: Implications for origin of Fe-rich mantle peridotites

Xenoliths of harzburgite, lherzolite, dunite and wehrlite (= Group I rocks) in lamprophyre dikes from Shingu are accompanied by large amounts of ultramafic-mafic xeno liths with Al- and Ti-rich clinopyroxene and/or kaersuite (websterite, clinopyroxenite, kaersutite rock, gabbro and anorthosite) (= Group II rocks). The latter rocks often crosscut the Group I rocks as veinlets, indicating that Group II rocks are younger. Although harzburgites and lherzolite from Shingu have ordinary modal compositions, the constituent minerals have extraordinary chemical characteristics; low Mg and Cr and high Ti, Al and Fe3+. Fo values of olivine range from 91 to 77. Cr/(Cr + Al) atomic ratios of spinel are lower than 0.5 even in harzburgites. Fe3+/(Cr+Al+Fe3+) atomic ratios of spinel are sometimes over 0.1. TiO2 contents of clinopyroxene often exceed 0.5 wt%. These characteristics are revealed when Group I rocks are veined or selvaged by Group 11 rocks; chemical compositions of minerals in peridotites systematically change forwards the latter. This strongly suggests that injections of melts with alkali basaltic affinity which had precipitated Group 11 rocks resulted in diffusion metasomatism on the Group I rocks. It is likely that the metasomatized peridotites are widespread underneath the areas where alkali basalt magmatism had fluorished, such as southwestern Japan. Some of Fe-rich lherzolite and harzburgite xenoliths reported in the literature are possibly metasomatites.

Plate motion and structure of the continental asthenosphere: A realistic model of the upper mantle

A one-dimensional shear flow model of the upper mantle beneath continents is presented. The model includes a two-layer crust with values of thermal conductivity and radiogenic heat production distinct from those of the underlying mantle, in which the conductivity is generally a function of temperature and depth. Temperature, velocity, effective viscosity, and volumetric heat generation profiles are determined, accounting for viscous dissipation and using a temperature and pressure dependent nonlinear mantle rheology in which the rate of strain is directly proportional to a power n of the shear stress (n = 3 is generally assumed, although the Newtonian case is investigated). The solutions provide the values of lower crustal radiogenic heat production and shear stress, which are a priori unknown. The boundary conditions on the model are the values of velocity, temperature, and heat flux at the surface and the vanishing of velocity with a prescribed upward heat flux at great depth. The models produce a shear zone (asthenosphere), the region where the geotherm bends over, the velocity drops rapidly to zero, the viscosity has a minimum value, and the shear stress heating has a maximum value. The thickness and depth of the shear zone, the value of viscosity therein, the associated shear stress, the lower crustal radioactivity, and other characteristics of the model depend on parameters such as the activation energy and volume of the constitutive stress-rate of strain equation, plate velocity, mantle radioactivity, and heat influx from the lower mantle. These dependences are explicitly demonstrated through the presentation of a series of numerical calculations. A realistic model of the mantle beneath stable continental regions is found which satisfies the additional constraints of the temperature at and the depth to the shear zone inferred from the petrology of peridotite xenoliths in kimberlites from South Africa, the seismic shear wave structure beneath shields, and the value of radioactivity in gabbroic rocks likely to represent the lower crust. The activation volume V* is the most important constitutive parameter controlling the depth of the shear zone. The model for stable continental regions predicts that V* is perhaps twice as large as would be appropriate for diffusion of O− − ions. Viscous dissipation contributes about 0.1 HFU (μcal cm−2 s−1) to the continental surface heat flux. Through thermal feedback as a result of the pressure and temperature dependence of the viscosity, shear stress heating leads to the formation of an asthenosphere decoupling the plates from the underlying mantle.