Metasomatically altered peridotite xenoliths from the hessian depression (Northwest Germany)

Abstract

Abstract A set of 36 spinel lherzolite and spinel harzburgite xenoliths from 7 occurrences of Tertiary alkali olivine basalt and olivine nephelinite have been investigated for modal composition, major elements, and 39 minor elements. The peridotites were depleted during former partial melting events. The proportion of clinopyroxene (7.3% cpx), elements indicating depletion (1.9% A12O3, 1.9% CaO), and compatible trace elements (Ni, Mn, Co, Sc, Cr, V, Zn) are on average close to the means of the worldwide subcontinental lithosphere. Accumulation of potassium and of correlated incompatible trace elements has been used as an index of metasomatic alteration. It divides the peridotites into an L-group ( 1000 ppm K). Immediately before the alkali-basaltic volcanism, mantle metasomatism produced phlogopite, amphibole, and secondary clinopyroxene (the latter is enriched in LREE). Some proportions of these minerals have formed an interstitial melt from heat of the transporting magma. The metasomatic alteration of the sampled mantle varies in a complicated three-dimensional pattern without vertical layering. Mobilization of elements in mantle fluids (H2O-CO2-C1) and accumulation in certain peridotites is a complex open-system process. Accumulation in H-group (relative to L-group) peridotites exceeds primitive mantle rocks and decreases in the following sequence: Ba, P, K, F, Nd, Sm, Tl, Eu, Gd, Rb, Sr, Ce, La, Ti, Dy, Ta, Nb, Zr, Y, Hf, Er, Li, Bi, Pb, Yb, Na. This order is almost conformable to increasing compatibilities in mantle minerals (or decreasing accumulation in the continental crust). The exceptional behaviour of high field-strength elements might be controlled by the formation of Ti minerals. The metasomatic transport can be explained by extraction of the incompatible elements either from mantle minerals or from subducted crustal sources into water-rich fluids. Chlorine in phlogopite has equilibrated with fluids containing chloride concentrations close to seawater. Values of δ18O in bulk peridotites and in phlogopite exceeding typical mantle compositions probably reflect a crustal contribution to the metasomatic fluids. Nd and Sr isotopic ratios of both peridotites and alkali basalts received a crustal signature from an environment with time-integrated LREE accumulation and Rb depletion.