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

Upper mantle xenoliths from the classical location, Kapfenstein, Styria, as well as from Fehring, Styria, and Tobaj, Burgenland, have been analyzed by laser ablation microprobe inductively-coupled plasma mass spectrometry (LA M-ICP-MS). At all locations spinel Iherzolite is the predominant xenolith t ype and thus our sample contains nine spinel Iherzolites and only one harzb urgite (from the richest location, Kapfenstein, sample Ka 167). All the roc ks have protogranular to protogranular porphyroclastic transitional texture s. Mineral compositions are typical for fertile upper mantle rocks with 0.8 9 [mg] 0.92 for silicates and 0.10 [cr] 0.12 for spinel (0.18 for the harzb urgite). The minerals are equilibrated with respect to major, minor and tra ce elements, except for clinopyroxenes in the Sample from Tobaj (To 100) an d one sample from Fehring (Feh 002) which have variable incompatible trace element contents. Trace element abundances are highest in clinopyroxene (3-4 x primitive mant le rare earth element - REEs - abundances) followed by orthopyroxene (about 0.5 x mantle REEs) and olivine (0.0005-0.05 x mantle REEs). Abundances of trace elements in all phases are usually correlated with their compatibilit y. The most incompatible elements (e.g., U, Th, No, Ta, La, Ce) are deplete d with respect to the more compatible elements in three samples from Fehrin g and two from Kapfenstein. The remaining samples are enriched in either U, or U and Th, or the most incompatible REEs as compared to Nb and Ta which are usually depleted with respect to less incompatible elements. Clinopyrox enes of the sample from Tobaj (To 100) and one sample from Fehring (Feh 002 ) have variable U, Th, Nb and Ta contents. In sample Feh 002 this variation is accompanied by a correlated variation of the light REE contents and the ir abundances are also correlated with the closeness to the surface. The en richments in U and Th are, however, not accompanied by any significant enri chment in Nb and Ta, the concentrations of which stay at low levels. The non-equilibrium trace element distribution in clinopyroxenes suggests t hat the metasomatic events took place shortly before the rocks were deliver ed to the Earth's surface. Thus, metasomatism and volcanic activity seem to be related and a consequence of the rising diapir underneath the Pannonian Basin. Several metasomatic events probably related to fluids dominated by CO2, water, or both were taking place. However, the intensity of that activ ity was generally low, as was the tectonic activity in the border zone of t he Pannonian Basin. Only harzburgite Ka 167 provides evidence for some elev ated activity of depletion and enrichment processes comparable to what has been found in the central region of the basin.