Trace element compositions of amphibole and clinopyroxenes from composite mantle xenoliths crosscut by micaceous hornblendite veinlets that served as conduits for melts moving through vein systems in the upper mantle have been investigated by ion-microprobe analysis to evaluate the process of vein metasomatism in the lithosphere beneath the West Eifel (Germany). The geochemical and Sr–Nd isotopic characteristics of the veinlets suggest that these are products of megacrysts precipitation of melts of the same composition as West Eifel alkali basalts. Their geochemical imprint on the host peridotite is constrained by the evolution of compositional gradients in trace element contents within a small-scale, 0.5–1.0 cm wide transition zone at the veinlet–host contact. Highly LREE-enriched, HSFE-poor clinopyroxenes identified in some host rocks have 87Sr/86Sr (0.7041) and 143ND/144Nd ratios (0.5125) different from those of veinlets and West Eifel basalts, and provide evidence for a metasomatizing agent before melt injection. Trace element modelling of zoning profiles developed in a single amphibole grain of the transition zone shows that the veining event manifested in the composite xenoliths was rapid and represents the brief final stage of multiple enrichment processes, probably as a consequence of the Quaternary Eifel volcanism.
INTRODUCTION
Polymict peridotites are a complex suite of mantle xenoliths recovered from kimberlites in Kimberley in South Africa. They are unlike other mantle xenoliths having breccia- like characteristics and containing rock and mineral fragments of lherzolites, harzburgites, eclogites and the megacryst association. These fragments are embedded in a matrix rich in ilmenite and phlogopite together with rutile and sulphides. Polymicts are not only distinct from other mantle xenoliths but can also be texturally distinct from each other.
PETROGRAPHY
Typically, polymict peridotites include the presence of all or some of the phases garnet, olivine, orthopyroxene, clinopyroxene, phlogopite, ilmenite, rutile and sulphides. Texturally large single crystals (up to 7 mm) are seen embedded in a finer grained matrix. The large crystals are often extensively deformed and recrystallised. This is particularly true of JJG 1414 which contains olivine, garnet, orthopyroxene and opaque minerals within a sea of olivine neoblasts. Commonly the clasts of orthopyroxene and phlogopite within polymicts have distinct petrographic rims. The areas of matrix consist of intergrown fine grained orthopyroxene, phlogopite, opaques and occasionally olivine. These areas are found in nodules JJG 2115, DB3 and UNCAT. No such matrix area is seen in JJG 1414. Clinopyroxene- orthopyroxene intergrowths are found in JJG 1414, JJG 2115, and UNCAT. The only large single clinopyroxene is seen in DB3.
MAJOR ELEMENT DATA
Electron probe analysis has revealed a wide compositional variety in each of the polymicts. Fig. 1 plots Ca, Mg and total Fe compositions for all the phases in seven different polymicts.
More specifically, electron probe analyses were performed on the rims and cores of individual phases within the polymicts. Fig. 2 show the results of orthopyroxene and phlogopite analyses in two nodules. These minerals indicate clear core-rim variations. In each nodule orthopyroxene clasts have variable core compositions whereas the rims show a common Fe/(Fe+Mg) value of approximately 0.1 - 0.11. In Fig. 2a, orthopyroxene compositions from the clinopyroxene-orthopyroxene intergrowth in UNCAT are also plotted. These show compositions comparable to the rims of the single clasts. In JJG 1414 individual orthopyroxenes show two rim compositions - an inner rim with higher Fe/(Fe+Mg) values and an outermost rim of slightly lower Fe/(Fe+Mg) values. The phlogopite clasts also show variable core compositions and similar rim compositions of between 0.9 - 0.11 Fe/(Fe+Mg). Phlogopites in JJG 2115 show two distinct rim compositions- both of higher Fe/(Fe+Mg) values than the cores (Fig. 2b). With respect to other elements, the phlogopite rims are consistently higher in Na, Cr, Al and Ti than the cores. This is not true of the orthopyroxene clasts. Analysis of olivine clasts in these four nodules reveal similar core-rim variations although a wider spread of Fe/(Fe+Mg) values in the rims is seen between nodules. All garnets which have been analysed to date have been homogeneous.
Matrix minerals have also been analysed for major element compositions. Generally these data plot in close conjunction to the rim compositions of the clasts - especially with respect to Fe/(Fe+Mg) value. This is shown in Fig. 2.
DISCUSSION
These features are suggestive of metasomatic processes operating on clasts derived from a variety of sources. Due to the close association of the rim compositions and the matrix mineral data, it is thought that the fluid which was responsible for this metasomatism was also crystallising the matrix minerals. Preliminary interpretations involve the possibility of more than one pulse of a metasomatising fluid which has resulted in the heterogeneous rim compositions in some of the nodules.
Consideration of theoretical, experimental and natural rock data show that basic-ultrabasic melt will disperse along mineral grain edges in olivine-rich mantle rock and thereby form a connected three-dimensional network throughout the rock even when present in only small (less than 1%) volumes. The viscosity of such melts will also allow small (less than 1-5%) volumes to move on appropriate geological timescales as a result of gravity-driven compaction. These features mean that small volume basic-ultrabasic melts are capable of infiltrating and metasomatizing mantle peridotites. Modally metasomatized mantle xenoliths are commonly closely associated with an array of dyke-like and vein injection phenomena. Textural, structural and modal characteristics of a wide array of mantle dykes, veins and metasomatic rocks suggest that such rocks have certain features in common with cumulates, and might usefully be distinguished as dyke cumulates and metasomatic infill cumulates . They represent partial crystal precipitates from melt flowing along channelways or pervasively through peridotite, and their bulk rock compositions provide poor guides to actual mantle melt compositions. The crystallization of the minerals in dykes/veins/ metasomites causes differentiation of the melt by crystal fractionation processes, but at the same time the melt may maintain equilibrium with host rock phases (e.g. olivine) and chromatographic column or percolation effects will control the range of transport of different chemical components by the melt. These combined processes are referred to as percolative fractional crystallization . Data on the actual trace element compositions of melt in equilibrium with the minerals of mantle dykes/veins/metasomites are calculated from trace element analyses of the minerals by using partition coefficients. For a wide variety of metasomatic suites, the calculated melt compositions show a progression of trace element abundances from ones similar to primitive asthenospheric OIB-like compositions towards more incompatible element enriched compositions. Thus they support the hypothesis that fractional crystallization and percolative fractional crystallization processes operating upon initial primitive asthenospheric melts may yield melt compositions matching those necessary for wide varieties of mantle metasomatism. The differentiation of the melts and evolution of the metasomatic rocks proceed together. No evidence for the involvement of volatile-rich fluids distinct from melts has been found. The trace element compositions of many kimberlitic and lamproitic melts may also arise by processes of percolative fractional crystallization of initially primitive melts with oIB-like trace element compositions, as a result of flow through mantle peridotite.
Summary Limestones and quartzites within the Tarfside culmination of Glen Esk (Angus) are correlated with the upper part of the Dalradian Argyll Group, and form part of a right way up succession extending upwards into the Southern Highland Group. This correlation makes the Tarfside limestones stratigraphically equivalent to the Loch Tay and Deeside limestones in the inverted flat belt of the Tay nappe. The zone of inversion between the Tarfside succession and that of Deeside and Loch Tay lies in the headwaters of Glen Esk and further south; the Tarfside succession occupies the structural position of the right way up limb of the Pitlochry-Kirkmichael recumbent syncline beneath the Tay nappe. The actual inversion is thought to take place across a slide which also causes the sudden disappearance of the Green Beds. Evidence for sedimentary facies variation in the Tarfside rocks and their stratigraphical equivalents supports the interpretation of some of Read’s Boyne Line phenomena (in Aberdeenshire and Banffshire) as being sedimentary facies variations.
The nature of magma bodies associated with the formation of Cr-poor megacryst suites included in kimberlites is considered with particular regard to the possibility of a virtually isobaric body containing crystals at widely varying temperatures. A model is proposed in which a complex of magma sheets and other apophyses exists in a temperature gradient, thereby allowing the simultaneous existence of different states of magma differentiation and crystallization.
Abstract High-pressure-temperature (P-T) experiments were conducted in an attempt to determine the diffusion rates of C atoms in diamond, and the possibility of changes in the isotope compositions of diamond at high P-T in the Earth’s mantle. The starting material consisted of a polished plate of natural diamond (very largely 12 C), which had been coated with 13 C diamond by chemical-vapourdeposition to form a sharp interface between 12 C and 13 C diamond. Three experiments were performed at 1800, 2000 and 2300ºC, all at 7.7 GPa, for0.5 –20 h. Isotopic profiles obtained by ion microprobe before and after each experiment showed no evidence of relaxation of the sharp interface between 12 C and 13 C, and so diffusion must have been on a scale less than the ~32 nm depth resolution for this technique. Using 32 nm as the maximum length scale of diffusion across the interface, the maximum ln D (diffusion coefficient) values for the experiments were calculated to be in the range –38 to –42. Unlike previous experimental data, these results show that changes in the isotopic compositions of diamond on long time scales in the Earth’s upper mantle are unlikely. Furthermore, the results support empirical evidence from mapping of C isotope distributions in natural diamonds that C isotope compositions reflect diamond growth compositions.
The phase relations of muscovite-quartz-bearing pelitic schists containing combinations of garnet (Grt), staurolite (St), chloritoid (Cld), biotitt (Bt) and chlorite (Chl) are examined (1) to assess the influence of manganese on natural assemblages, and (2) to constrain the topologies of petrogenetic grids, particularly with respect to the controversial assemblage Cld +Bt. Two field areas were studied: Stonehaven, NE Scotland (p ∼4⋅5 kbar) and the SE Tauern Window, Austria (P ∼7 kbar), both characterized by the up-grade progression from typical 'garnet-zone' Grt+Chl assemblages to 'staurolite-zone' St+Bt±Grt assemblages via a narrow, complex zone containing Cld+Bt assemblages. In both areas, the following commonly observed chemographic relations hold: Mg/(Mg+Fe): Grt
Abstract Sulphur isotope ratio measurements (δ 34 S) of diagenetic pyrite are commonly used to identify S sources and mechanisms of sulphide formation in basinal sediments. This study reports such data for a diagenetic pyrite nodule from the Brent Group sandstones of the northern North Sea at three sampling scales: 50 cm (core subsample), 500 μm (laser microprobe) and 50 μm (ion microprobe). Similar δ 34 S variations are found by the laser and ion microprobe techniques. There is a very wide range in δ 34 S (<−10% to >+50%) within the nodule and isotopically heavy S (δ 34 S >+20%) is common at all scales. The nodule δ 34 S distribution does not fit a Rayleigh fractionation pattern. The laser microprobe sampling at 100–500 μm scales seems to be adequate to characterize S isotope variations in this material.
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