Cryptic metasomatism in the upper mantle beneath Southeastern Austria: a laser ablation microprobe-ICP-MS study

摘要

Upper mantle xenoliths from the classical location,Kapfenstein, Styria, as well as from Fehring, Styria, and Tobaj,Burgenland, have been analyzed by laser ablation microprobeinductively-coupled plasma mass spectrometry (LAM-ICP-MS). Atall locations spinel lherzolite is the predominant xenolith type andthus our sample contains nine spinel lherzolites and only oneharzburgite (from the richest location, Kapfenstein, sample Ka167). All the rocks have protogranular to protogranular-porphyroclastic transitional textures. Mineral compositions thetypical for fertile upper mantle rocks with 0.89 0.92 forsilicates and 0.10 0.12 for spinel (0.18 for the harzburgite).The minerals are equilibrated with respect to major, minor andtrace elements, except for clinopyroxenes in the sample from Tobaj(To 100) and one sample from Fehring (Feh 002) which havevariable incompatible trace element contents.Trace element aboundances are highest in clinopyroxene (3-4×primitive mantle rare earth element- REEs- abundances) followedby orthopyroxene (about 0.5×mantle REEs) and olivine (0.0005-0.05×mantle REEs). Abundances of trace elements in all phasesare usually correlated with their compatibility. The mostincompatible elements (e.g., U, Th, Nb, Ta, La, Ce) are depletedwith respect to the more compatible elements in three samplesfrom Fehring and two from Kapfenstein. The remaining samplesare enriched in either U, or U and Th, or the most incompatibleREEs as compared to Nb and Ta which are usually depleted withrespect to less incompatible elements. Clinopyroxenes of thesample from Tobaj (To 100) And one sample from Fehring (Feh002) have variable U, Th, Nb and Ta contents. In sample Feh 002this variations is accompanied by a correlated variation of the lightREE contents and their abundances are also correlated with thecloseness to the surface. The enrichments in U and Th are, however,not accompanied by any significant enrichment in Nb and Ta, theconcentrations of which stay at low levels.The non-equilibrium trace element distribution inclinopyroxenes suggests that the metasomatic events took placeshortly before the rocks were delivered to the Earth's surface. Thus,metasomatism and volcanaic activity seem to be related and aconsequence of the rising diapir underneath the Pannonian Basin.Several metasomatic events probably related to fluids dominatedby CO2, water, or both were taking place. However, the intensity ofthat activity was generally low, as was the tectonic activity in theborder zone of the Pannonian Basin. Only harzburgite Ka 167provides evidence for some elevated activity of depletion andenrichment processes comparable to what has been found in thecentral region of the basin.