H.-G. Stosch

Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher, West Germany

Abstract Among the spinel peridotite nodules from Dreiser Weiher. West Germany which represent fragments of the earths upper mantle two series may be distinguished. One group (Ib) is anhydrous while the second one (Ia) is characterized by the presence of amphibole and/or breakdown-products of amphibole. Both suites display a wide range in modal composition. Pyroxene geothermometry yields equilibration temperatures of ~ 1150°C for group Ib and ~950°C for group Ia. Rare earth element (REE) patterns reveal marked differences between both groups: nearly unfractionated, light REE depleted or slightly light REE enriched chondrite normalized patterns in nodules from group Ib, but a high relative light REE enrichment in the amphibole containing suite Ia. An inverse correlation between the clinopyroxene content (or CaO) and relative light REE enrichment is observed in group Ia only. Two sources of light REE enrichment of the nodules can be distinguished: The first is a contaminant on mineral surfaces and can be removed by acid leaching. The second is an integral part of the constituent minerals of the nodules. Trace element abundances and modelling show that nodules of group Ib cannot be derived from upper mantle pyrolite by a one-stage partial melting process. Nodules of type Ia are interpreted as being the result of a reaction between Ib-type mantle and a fluid or liquid which provides H2O and incompatible elements. At Dreiser Weiher this ‘open system’ upper mantle metasomatism may be related to the young uplift of the Rhenish Shield in a similar way as previously proposed by Lloyd and Bailey (1975). A model for the lithospheric mantle below the Westeifel is different from the San Carlos model proposed by Frey and Prinz (1978) and more complex.

PARTITIONING OF HIGH FIELD-STRENGTH AND RARE-EARTH ELEMENTS BETWEEN AMPHIBOLE AND QUARTZ-DIORITIC TO TONALITIC MELTS : AN EXPERIMENTAL STUDY

The knowledge of rare-earth element (REE) and high field-strength element (HFSE) partitioning between minerals such as amphibole, pyroxenes or garnet and tonalitic liquids is essential to understand where and how tonalitic melts are generated. In this paper we present the results of trace-element partitioning studies between amphibole and quartz-dioritic to tonalitic liquids which have been conducted at 1 GPa and temperatures of 800°C, 850°C, and 900°C. Amphiboles crystallized from tonalitic liquids are nearly homogeneous, indicating that equilibrium has been closely attained. Henrys Law behaviour was confirmed for trace-element concentrations ranging from a few ppm as in natural systems up to doping levels of 0.5 wt.%. Partition coefficients (D) for the REE on the M4-site and of Ti, Zr and Hf on the M2-site of the amphibole structure can be fitted to parabolas, using the equation of Blundy and Wood (1994). Parabolas for all isovalent series of cations have the same shape in the temperature range investigated. The maxima of the parabolas for the REE defining the size of the unstrained site r0 lie at Dy. At 1 GPa partition coefficients DAmph/L for Dy range from 1.77 at 900°C to 2.68 at 800°C. DAmph/L partition coefficients of Nb and Ta are identical within error limits. From this observation, together with the coincidence of their ionic radii with fitted radii of r0 for the M2-site, it is concluded that partition coefficients of Nb and Ta plot close to the peak of the parabola for the pentavalent cations. From a comparison of the fitted parabola parameters of the REE for amphibole and clinopyroxene it is inferred that at fixed conditions the amphibole/clinopyroxene partition coefficients DCpx/Amph for all REE have to be constant. The partition coefficients of the cations investigated in this study correlate negatively with the degree of depolymerisation in the melt expressed as the ratio of nonbridging oxygens to the number of tetrahedrons (NBO/T). Logarithms of partition coefficients normalized to the degree of depolymerisation define a linear function when plotted versus reciprocal temperature. By means of the fitted parabola parameters, straight-line relationships between DREE and DCa as well as DREE and DTi can be calculated. The predicted correlations agree very well with the experimentally determined data.

Geochemistry and evolution of MORB-type eclogites from the Münchberg Massif, southern Germany

In the Munchberg Massif in the Variscan foldbelt of southern Germany two varieties of eclogite are known which are intercalated with amphibolite-facies meta-igneous and meta-sedimentary rocks: a dark kyanite-free and a lighter colored kyanite-bearing type. Kyanite-free eclogites, which are discussed here, have a major and trace element composition which suggests derivation from ocean-floor basalts with melt to cumulate compositions. Internal SmNd isochrons (clinopyroxene-amphibole-garnet) and one RbSr isochron (clinopyroxene-amphibole-mica) yield eclogitization ages in the range of 380 to 395 Ma. Thus, the age of eclogitization is only marginally higher ( < 15 Ma) than the age of amphibolite-facies metamorphism in the Munchberg Massif as derived from KAr ages of amphiboles and micas from metasediments and meta-igneous rocks. A seven point whole-rock SmNd isochron for one eclogite body results in an age of480 ± 23Ma with an initialeNd of8.7 ± 0.6 and is likely to record the age of igneous formation of the eclogite protoliths. Sr isotopic compositions back-calculated to that time are anomalously high and variable if compared to Nd isotopes. This can be explained by alteration with an aqueous or fluid phase with high87Sr86Sr, most likely seawater, either during igneous formation in an oceanic rift environment or subduction-related eclogitization. In addition, some eclogites show a marked enrichment of incompatible, immobile elements and plot far below the whole-rock SmNd isochron. These features are ascribed to the presence of an evolved crustal component, probably acquired during extrusion of the basaltic protoliths by mixing with country-rock gneisses.

Distribution of titanium and the rare earth elements between peridotitic minerals

The concentrations of titanium and rare earth elements (REE) in olivines, orthopyroxenes, clinopyroxenes and spinels from four anhydrous, spinel-bearing peridotite xenoliths have been determined. The distribution of titanium (used as an analogue for the high field strength elements: HFSE) relative to the REE between clinopyroxenes and orthopyroxenes varies as a function of the whole rock composition and modal mineralogy. The distribution coefficients for titanium and the REE in these peridotites do not reflect mineral-melt equilibria. It is believed that subsolidus distribution coefficients for HFSE relative to REE vary with temperature. Ratios of various incompatible elements (e.g., Ti/Eu, Zr/Sm, Hf/Sm and P/Nd) in peridotite minerals differ from those in most primary basalts. However, the abundance ratios of incompatible elements in the bulk peridotite are comparable to those found in modern basalts. Given this and the differing contribution of melt from each phase during melting, near constant ratios of such incompatible elements in primary and primitive basalts and komatiites reflect the “buffering” of the melt by its residue. These ratios are fixed in the magma during the initial stages of melting because of similar and low distribution coefficients between melt and bulk residue for these element pairs. Differences in the relative abundances of titanium and REE in clinopyroxenes and orthopyroxenes demonstrate that mantle normalized abundance patterns for clinopyroxene are not equivalent to those of the whole rock. Therefore, claims of a widespread HFSE-depleted reservoir in the upper mantle base solely on the relative abundances of incompatible elements in peridotitic clinopyroxenes are invalid.

Experimental partitioning of high field strength and rare earth elements between clinopyroxene and garnet in andesitic to tonalitic systems

Abstract Partition coefficients of the rare earth elements (REE) and the high field strength elements (HFSE: Ti, Zr, Hf, Nb, Ta) among clinopyroxenes, garnets, and andesitic to granodioritic melts were experimentally determined at pressures ranging from 1 to 3 GPa and temperatures between 900 and 1150°C. Natural rocks of quartz–dioritic and basaltic composition were used as starting materials. Melt compositions covered a range from dioritic to quartz–dioritic and granodioritic. Partition coefficients obtained from experiments with different doping levels at the same run condition indicate that Henry’s Law is fulfilled. Partition coefficients were investigated as a function of temperature and phase compositions. Apparent correlations between the clinopyroxene partition coefficients of the REE and degree of depolymerization expressed as nonbridging oxygens per tetrahedron (NBO/T) are due to variations in the Na contents of the clinopyroxenes which are controlled by pressure or NBO/T. Based on the model of Blundy and Wood (1994 ), which accounts for the strain associated with placing a cation on a particular crystallographic site when the radius of the cation differs from the optimal radius for that site, significant differences between Zr− and Hf partition coefficients as well as correlations between Ti and REE partition coefficients can be explained in terms of differences of their ionic radii. For garnets D La /D Yb ratios change as a function of temperature much more than for clinopyroxenes. This is also consistent with the Blundy and Wood (1994 ) model.

Open-system O-isotope behaviour and trace element enrichment in the sub-Eifel mantle

Abstract Spinel peridotites from the West Eifel, western Germany exhibit a wide range of δ 18 O SMOW values for olivine ( +5.1 to +6.3‰), smaller ranges for pyroxenes ( +5.7 to +6.3‰ for orthopyroxene; +5.6 to +6.4‰ for clinopyroxene) and variable 18 O/ 16 O fractionations between olivine and pyroxenes ( Δ clinopyroxene-olivine = +0.6 to −0.3‰ and Δ orthopyroxene-olivine = +0.7 to −0.1‰). These features are characteristic of a disequilibrium distribution of O-isotopes between coexising olivine and pyroxenes. Enrichment in 18 O is observed primarily in olivine from amphibole and/or phlogopite-bearing peridotites and correlates with degree of LREE/HREE enrichment, indicating that the processes responsible for trace element enrichment (i.e. mantle metasomatism) and O-isotope disequilibrium are linked. The 18 O- and incompatible trace element-enriched fluid(s) cannot be generated by intra-mantle fractionation processes as presently understood, and are attributed to recycling of altered oceanic lithosphere and/or pelagic sediments into the mantle via subduction. This is supported by δD SMOW values for associated Eifel amphibole and mica ( −50 to −45‰ respectively) which fall midway between δD values for upper mantle ( −80‰) and seawater (0‰). Coexisting amphibole and mica also have distinctly different δ 18 O values ( +7.0 and +5.9‰, respectively) which suggests that they did not originate from the same fluids/melts. This implies that the sub-Eifel mantle has experienced at least two distinct metasomatic episodes.

Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects

Petrographic, petrological and geochemical data for 16 mafic meta-igneous, granulite facies lower crustal xenoliths from the East Eifel were collected in order to develop a model for the lower crustal history for this region. The xenoliths consist of plagioclase±amphibole±clinopyroxene±garnet±orthopyroxene±scapolite + opaque minerals±apatite±rutile±zircon. Garnet has reacted to a variable extent with plagioclase and clinopyroxene to form a corona of plagioclaseII+ amphibole + orthopyroxeneII. Pyroxenes and plagioclases show complex zoning patterns with regard to Al and Ca which can be interpreted in terms of P, T history. Decreasing temperature and pressure conditions are recorded by decreasing Al in clinopyroxene rims coexisting with increasing anorthite contents in plagioclase rims and the breakdown of garnet. In addition, a young heating event that affected the granulites to different degrees is inferred from the complementary Ca-zoning patterns in clino- and orthopyroxenes. Rare earth element (REE) patterns of whole rocks together with the trends displayed and fractionated liquids. REE analyses of the mineral separates display equilibrium partitioning patterns for amphibole and clinopyroxene, although isotopic data show that amphibole contains externally-derived Sr and Nd components not recognized in other minerals. At least a 4-stage history for the granulites is recorded: (1) intrusion and crystal fractionation of basaltic magmas in the lower crust, probably accompanied by crustal assimilation, (2) granulite facies metamorphism, (3) a decrease in temperature and pressure, and (4) a later heating event. The complicated thermal history is reflected in Sm−Nd mineral isochron ages which range from about 170 Ma down to about 100 Ma and cannot be assigned to distinct geological events. These ages correlate with inferred temperatures; the low ages are measured for xenoliths with the highest temperatures. In some cases the young heating event is likely to be responsible for partial resetting of the mineral isochrons.

Crustal xenoliths from Cenozoic volcanic fields of West Germany: Implications for structure and composition of the continental crust

Crustal xenoliths in three Cenozoic volcanic fields of West Germany, the Northern Hessian Depression (NHD), the Eifel and the Urach/Hegau, include medium to high-grade meta sedimentary and felsic to mafic meta-igneous rocks. Also present in all three suites are pyroxenites and hornblendites. For each volcanic field, a model crustal profile is proposed based on calculated or measured P-wave velocities of xenoliths and depth-Vp relationships (EGT Central Segment and Rhenish Massif traverses). The xenolith lithologies from the NHD and the Eifel show some similarities. The middle crust between the depths of about 10 and 25 km consists mainly of meta-sediments, felsic gneisses and granulites. Meta-sedimentary rock types are particularly abundant in the Eifel at depths of between about 5 and 15 km but are less common within the NHD xenolith collection. The felsic gneisses range from meta-granites to meta-tonalites (I-and S-type). Eifel meta-sediments range from meta-pelites to meta-greywackes and meta-quartzites. The NHD xenolith suite contains a few highly depleted granulite-facies meta-sedimentary fragments. At depths between 24 and 26 km, the increase in Vp from about 6.8 to > 8 km s−1 (28–34 km) is correlated with the presence of mafic granulites intercalated with eclogites, pyroxenites and hornblendites. Beneath North Hessia, the granulite layer problably grades into a composite eclogite-peridotite layer at the lower part of this transition zone. The crust beneath the Urach/Hegau consists largely of meta-sediments with subordinate felsic meta-igneous rocks. Most of the meta-sedimentary samples seem to be depleted in felsic components, suggesting intra-crustal differentiation by partial melting. The Urach crust contains lithologies which are similar to the outcropping Moldanubian-type para-gneisses of the Black Forest. Mafic and ultramafic xenoliths from the Urach/Hegau differ in their mineralogy and chemical composition from the Eifel and NHD mafic granulites. They represent meta-cumulates derived from alkaline magmas which intruded the base of the crust and underwent deformation and recrystallization. Petrographic and chemical differences between Urach/Hegau and Eifel/NHD are believed to reflect the contrasting styles of crustal evolution in the Rhenohercynian and the Moldanubian belts of the Variscan orogen.

Evolution of the upper mantle beneath the southern Baikal rift zone: an Sr-Nd isotope study of xenoliths from the Bartoy volcanoes

Anhydrous and amphibole-bearing peridotite xenoliths occur in roughly equal quantitites in the Bartoy volcanic field about 100 km south of the southern tip of Lake Baikal in Siberia (Russia). Whole-rock samples and pure mineral separates from nine xenoliths have been analyzed for Sr and Nd isotopes in order to characterize the upper mantle beneath the southern Baikal rift zone. In an Sr-Nd isotope diagram both dry and hydrous xenoliths from Bartoy plot at the junction between the fields of MORB and ocean island basalts. This contrasts with data available on two other localities around Lake Baikal (Tariat and Vitim) where peridotites typically have Sr−Nd isotope compositions indicative of strong long-term depletion in incompatible elements. Our data indicate significant chemical and isotopic heterogeneity in the mantle beneath Bartoy that may be attributed to its position close to an ancient suture zone separating the Siberian Platform from the Mongol-Okhotsk mobile belt and occupied now by the Baikal rift. Two peridotites have clinopyroxenes depleted in light rare earth elements (LREE) with Sr and Nd model ages of about 2 Ga and seem to retain the trace element and isotopic signatures of old depleted lithospheric mantle, while all other xenoliths show different degrees of LREE-enrichment. Amphiboles and clinopyroxenes in the hydrous peridotites are in Sr−Nd isotopic disequilibrium. If this reflects in situ decay of 147Sm and 87Rb rather than heterogeneities produced by recent metasomatic formation of amphiboles then 300–400 Ma have passed since the minerals were last in equilibrium. This age range then indicates an old enrichment episode or repeated events during the Paleozoic in the lithospheric mantle initially depleted maybe ∼2 Ga ago. The Bartoy hydrous and enriched dry peridotites, therefore, are unlikely to represent fragments of a young asthenospheric bulge which, according to seismic reflection studies, reached the Moho at the axis of the Baikal rift zone a few Ma ago. By contrast, hydrous veins in peridotites may be associated with rift formation processes.

18O/16O ratios in anhydrous spinel lherzolite xenoliths from the Shavaryn-Tsaram volcano, Mongolia

Abstract Anhydrous spinel lherzolite xenoliths from the Shavaryn-Tsaram volcano, which represent unusually clinopyroxenerich samples of upper mantle beneath the Tariat Depression in north-central Mongolia, have particularly low δ18O values of +4.9 to +5.7‰ SMOW. Constituent minerals exhibit small (ca. 0.5–0.7‰) variations in18O content (olivine = +4.6 to +5.3‰, clinopyroxene = +5.6 to +6.1‰, orthopyroxene = +5.8 to +6.5‰), that are unrelated to xenolith modal mineralogy, chemical composition, radiogenic isotope character, or pyroxene equilibration temperature. This O-isotope character of the Tariat xenoliths is interpreted to reflect the closed-system distribution of oxygen isotopes in a slowly cooling mantle diapir emplaced into the lithosphere from a relatively primitive region of the asthenosphere.