ALBERT

Description: Mantle xenoliths entrained in late Carboniferous to mid-Permian silica-poor basic magmas from northern Scotland were investigated using major and trace element compositions of minerals and isotopic compositions of whole-rocks and clinopyroxenes. The aim of the study was to gain more precise information on the nature of the lithospheric mantle beneath this terrane, including evidence for its metasomatic modification and evolution. This study concerns peridotites from two localities in the ENE and WSW of the Scottish Northern Highlands Terrane: Rinibar (South Ronaldsay, Orkney) and Streap Com'laidh (near Glenfinnan). Two groups of clinopyroxenes can be distinguished both at Streap Com'laidh (Type-S1 and Type-S2) and Rinibar (Type-R1 and Type-R2) based on different trace element contents and isotopic ratios. Type-S1 is characterized by an almost flat profile from middle rare earth elements (MREE) to heavy REE (HREE) accompanied by an overall light REE (LREE) enrichment. It shows the highest Th and U, coupled with low Sr, Zr and TiO2 contents. Type-S2 exhibits humped LREE-enriched patterns and a steep decrease from Nd to Yb. It has the lowest Th and U, coupled with the highest Sr, TiO2 and Zr contents. Both groups of clinopyroxenes present analogous isotopic features. They have measured 87Sr/86Sr values from 0.70652 to 0.70826, 144Nd/143Nd from 0.512093 to 0.512687 and 176Hf/177Hf from 0.282727 to 0.283088. These isotopic features could be explained by the addition in the mantle wedge of a slab component, made up of altered oceanic crust plus a moderate quantity of subducted sediments. The most recent subduction event in the geological history of Scotland is at about 400 Ma. It may have been during this convergent stage that the metasomatism affecting the sub-Streap lithospheric mantle occurred. Type-R1 is characterized by the lowest concentrations of Ba, Rb, Sr, LREE and UTh, associated with remarkably high levels of Ti and Zr. These clinopyroxenes have measured 87Sr/86Sr ranging from 0.70330 to 0.70383, 144Nd/143Nd from 0.512643 to 0.512761 and 176Hf/177Hf from 0.282705 to 0.282899. In contrast, Type-R2 shows the highest concentrations of Ba, Rb, Sr, LREE and UTh, and pronounced Ti and Zr negative anomalies. They have measured 87Sr/86Sr isotopic ratios similar to Type-R1, but lower 144Nd/143Nd (0.5124310.512524) and higher 176Hf/177Hf (0.2829210.283014). Calculated melt in equilibrium with Type-R1 is very similar to inferred primary kimberlitic magmas and the clinopyroxene trace element profiles may have resulted from an efficient chemical exchange between a percolating melt and the peridotite host rock. On the other hand the calculated Type-R2 melt overlaps the field of Proterozoic carbonatites. Significantly, at the age of 550 ({+/-}50) Ma, the two groups have almost identical SrNd compositions, similar to average depleted mid-ocean ridge basalt mantle (DMM) at 550 Ma. This strongly suggests contemporaneous overprinting of DMM by kimberlitic and carbonatitic metasomatic agents at c. 550 Ma, which may be related to the opening of the Iapetus Ocean following the break-up of the Rodinia supercontinent. These data indicate a complex metasomatic history of the Scottish lithospheric mantle, which relate different geological events, most probably prior to the juxtaposition of the various tectonic blocks that nowadays constitute the Northern Highland Terranes.

Description: In the foreword of the volume Mantle Metasomatism by Menzies & Hawkesworth (1987), Boettcher stated that the concept of mantle metasomatism has been of immense heuristic value for Earth scientists. At that time, metasomatism was still strongly related to allochemical metamorphism, where chemical composition of the rock is changed by the additional or removal of material. However, the concept of modal or patent (where a new phase is petrographically evident) and cryptic (where chemical enrichment is not accompanied by the presence of a newly formed phase) metasomatism had already been introduced by the pioneering works of Harte (1983), Menzies (1983) and Dawson (1984). Outstanding progress has characterized the past two decades, and the processes and agents of metasomatism are now much better understood, in part as a result of the significant advancements of in situ microanalytical techniques such as secondary ionization mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The in situ analyses substantially deepened knowledge of intermineral and solid

Description: Mantle xenolith bearing olivine melanephelinites from the Okhotsk sector of the OkhotskChukotka Volcanic Belt (OCVB), northeastern Russia, occur as small isolated volcanoes emplaced within massive late Early to Late Cretaceous subduction-related calc-alkaline rocks. The xenoliths are typical medium- to fine-grained anhydrous mainly spinel lherzolites that are strongly to weakly foliated with intensive to minor recrystallization to equigranular texture. The primitive mantle normalized whole-rock REE have flat patterns or patterns with slightly elevated light REE (LREE) ((La/Yb)N=0.481.38). The REE in clinopyroxenes have systematically decreasing normalized abundances from Sm to La, implying that the LREE enrichments in the whole-rock REE patterns are attributed to circulation of minor intergranular fluids or melts. Equilibration temperatures and pressures calculated for the Viliga samples are in the range of 10501160 {degrees}C and 1521 kbar, respectively. Ca diffusion rates in olivine reveal a rapid transport to the surface (26 days) of these peridotites. Model calculations have shown that the fertile lherzolites can be produced by 29% batch melting, whereas the depleted peridotites require 15% batch melting of a primitive source. The cessation of the interaction between the palaeo-Pacific plate and the NE Russian margin at c. 87 Ma apparently caused a piecemeal' collapse of the former followed by intrusion and ascent of olivine melanephelinitic magma, which entrained xenoliths from the asthenospheric mantle of the subducted plate during the Pliocene through the generated window(s). Moreover, clinopyroxenes that have low 87Sr/86Sr and high 143Nd/144Nd and plot in and above the mid-ocean ridge basalt (MORB) field are consistent with an upwelling asthenospheric mantle through the window(s) created by the piecemeal' collapse of the palaeo-Pacific plate.