Gerald Hartmann

Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton : evidence for cratonic evolution and redistribution of REE during crustal anatexis

Abstract Twenty-three clastic metasediments from the Kongling high-grade terrain of the Yangtze craton, South China were analyzed for major, trace and rare earth elements and Sm-Nd isotopic ratios. Associated dioritic-tonalitic-trondhjemitic (DTT) and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance. The results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical, geochemical and Sm-Nd isotopic compositions. Group A is characterized by having no to slight negative Eu anomalies (Eu/Eu∗ = 0.82–1.07), being high in Cr (191–396 ppm) and Ni (68–137 ppm), and low in Th (3.3–7.8 ppm) and REE (ΣREE = 99–156 ppm). These characteristics are similar to those of metasediments from Archean greenstone belts. In addition, the Group A metasediments have the value of the Chemical Index of Alteration (CIW) close to felsic gneisses. Their Sm-Nd isotopic, REE and trace element compositions can be interpreted by mixtures of the DTT gneisses and amphibolites. Dating of detrital zircons from 2 Group A samples by SHRIMP reveals a major concordant age group of 2.87–3.0 Ga, which is identical to the age of the trondhjemitic gneiss. These results strongly suggest that Group A was principally the first-cycle erosion product of the local Kongling DTT gneiss and amphibolite. Moreover, the higher than amphibolite Cr content and slight Eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3.0–3.3 Ga in age also played a role. Group B is characterized by the presence of graphite and shows a more evolved composition similar to post-Archean shales with a prominent negative Eu anomaly (Eu/Eu∗ = 0.48–0.77) and high CIW. On paired Cr/Th vs La/Co and Co/Th plots, Group B samples conform to a two-end member mixing line of the Kongling granitic gneiss and amphibolite. However, data on Nd model age and CIW suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source. Both Groups A and B exhibit a clear positive correlation between CIW and TDM and a negative one between CIW and Eu/Eu∗. These correlations point to the crustal evolution of the Yangtze craton towards coupled increasing CIW and Eu depletion with decreasing age. This in turn reflects the change of granitoid magmatism from local Na-rich dioritic-tonalitic-trondhjemitic rocks to widespread K-feldspar granite. The change led to the intracrustal differentiation, stabilization and growth of the craton. Group C is restite and contains abundant sillimanite and garnet and unusually high ilmenite (7–11vol%), which can be seen to be dehydration melting products of biotite under the microscope. This group shows extremely varied REE distributions from LREE enriched to depleted and from negative to strong positive (Eu/Eu∗ = 1.63) Eu anomalies. Compared to Groups A and B, Group C is severely depleted in Na2O, K2O, LREE, Rb and Ba, whereas TiO2, Co, V, Sc and HREE and Y are considerably enriched. This is accompanied by anomalous high Sm/Nd (0.21–0.28), 147Sm/144Nd (0.1361–0.1738) and 143Nd/144Nd (0.511589–0.511958) ratios. TDM correlates clearly with Sm/Nd ratio and 2 out of 3 samples give significantly older to unrealistic TDM (3.9–4.9 Ga). The results document redistribution of REE and an open behavior of the Sm-Nd isotope system during the biotite dehydration melting of metasediments.

The composition of peridotite tectonites from the Ivrea Complex, northern Italy: Residues from melt extraction

A total of forty-three samples from the Balmuccia, Baldissero, and Finero peridotite tectonites have been analyzed for major elements, forty minor elements, mineral composition, and a subset of samples have been investigated for δD, δ18O, δ34S, and 87Sr/86Sr ratios. The spinel Iherzolites from Balmuccia and Baldissero contain on average 56% olivine, 28% orthopyroxene, 14% clinopyroxene, and 1.5% spinel. They have a neodymium and strontium isotopic signature similar to MORB. These peridotites were moderately depleted in Al and Ca and highly depleted in incompatible elements by the separation of 4.5% P-MORB. The MORB in excess to the formation and subduction of the oceanic crust fractionated into a tonalitic continental crust and a pyroxenitic residue which was recycled into the mantle. The crustal mass fraction of the upper mantle-crust system is 2.8%. A primitive mantle composition can be calculated from 97.2% Balmuccia model peridotite plus 2.8% bulk crust. The new set of primitive mantle concentrations is in accordance, within about 10%, with the data from fertile peridotite xenoliths and primitive meteorites for the elements Li, Mg, Si, Ca, Sc, V, Mn, Fe, Co, Ni, Zn, Ga, Y, Zr, La, Ce, Nd, Eu, Tb, Dy, Yb, Lu, and Hf. Larger differences occur for the elements F, Na, P, Ti, Cr, and Cu. The bulk crust concentrations of the highly incompatible and mobile elements K, Rb, Ba, Th, and U exceed those being supplied from a primitive upper mantle reservoir of 660 km depth. An additional lower mantle source is required for about half of the crustal accumulation of these elements. Moderately depleted peridotite bodies with a cross section of 3 km can be sufficiently well homogenized in compatible and moderately incompatible elements to represent MORB producing upper mantle. Concentrations of Na, Sc, Ti, V, Ga, Y, Zr, and Yb are well correlated with the Al abundance so that primitive mantle values can also be estimated using the cosmically derived primordial Al concentration. The spinel harzburgites from Finero contain on average 74% olivine, 16% orthopyroxene, 3% clinopyroxene, 5% amphibole, 1% phlogopite, and 1% spinel. These peridotites were depleted by a separation of about 18% MORB. Subsequently they have undergone metasomatic alteration probably from dehydration of a subducted lithospheric slab. They received two pulses of water-rich fluids at temperatures of about 800°C to form amphibole low in incompatible elements or amphibole plus phlogopite high in incompatible elements without complete reequilibration of the preexisting rocks. The accumulation of elements is correlated to their incompatibility with the exception of a depletion of U, Th, Nb, and Ti. Isotopic ratios of H, O, S, Sr, Nd, and Pb, chlorine in phlogopite and a relative deficit of Nb and Ti are consistent with a crustal imprint in the metasomatic fluids.

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr−Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of δD with 87Sr/86Sr in amphibole and phlogopite and a slightly larger δ18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.

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

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.

Geophagy in moustached tamarins,Saguinus mystax (platyrrhini: Callitrichidae), at the Río Blanco, Peruvian Amazonia

During a field study at the Río Blanco in northeastern Peru between June and September 1990, moustached tamarins (Saguinus mystax) were observed to feed on soil material on three different occasions. Soil was taken directly from bare spots of surface soil (two observations) and from the broken mound of leaf-cutting ants (Atta sp.; one observation). Exposed surface soil is rare throughout the study area. Geochemical analyses of the soils that were consumed showed elevated concentrations of several elements in the ant mound sample compared to the other two soil samples. The most likely hypothesis for the function of soil feeding in moustached tamarins is that it serves in mineral supplementation.

Comparison of methods for measuring heavy metals and total phosphorus in soils contaminated by different sources

The relationships between the concentrations of zinc (Zn), lead (Pb), copper (Cu), nickel (Ni) and chromium (Cr) as measured by X-ray fluorescence analysis (XRF), aqua regia, and HNO3 pressure digestion were studied in soil samples covering a wide range of heavy metal concentrations. The soils were contaminated by sewage sludge, exhaust depositions, river sediments of mining residues, and dump material. The question was addressed whether the source of heavy metals or other soil properties affect the relationship between these three methods. The aqua regia-digestible fraction of the five heavy metals reached on average 64% of the XRF-detectable content. The pressure accelerated HNO3-digestible fraction of the five heavy metals was on average 71% of the XRF-detectable content; the respective phosphorus (P) fraction reached a median of 75%. This suggests that HNO3 pressure digestion can also be used for characterizing the total P content of soils. Aqua regia extraction and HNO3 pressure digestion gave similar values for Zn, Pb, and Cu, which dominate the heavy metal load of most contamination sources. Significantly higher Cr values were obtained by HNO3 pressure digestion than by aqua regia extraction. Additionally, the Cr contents were affected by the source, e.g. sewage sludge had relatively high contents of aqua regia and HNO3 pressure extractable contents in comparison to the XRF values. The element-specific relationships between the three methods were all highly significant. However, the respective multiple linear regression models were in most cases affected by soil organic carbon (C), in some cases by clay or soil pH.

Igneous Geology and Geochemistry of the Upper Río Chagres Basin

The geological basement of the upper Rio Chagres basin (RCB) is primarily a mixture of Cretaceous to Upper Tertiary age volcanic and intrusive rocks. Exposed rocks consist of highly deformed mafic basalts, basaltic andesites, gabbros, diorites as well as chemically more evolved granodiorites, tonalities, and granites. Ultramafic rocks, that would provide evidence for an oceanic basement/lithospheric mantle basement to the RCB, are absent. Primary stratigraphic relations and contacts are generally obscured, either tectonically or by deep weathering. Most rocks, in particular the volcanics, volcaniclastic sediments, and granites are all strongly deformed and chemically altered. Mafic and granitic rocks have distinct weathering characteristics that influence stream channel morphologies throughout the RCB. The mafic complexes are most resistant to weathering and mechanical erosion, producing narrow river channels, rapids, and deeply cut gorges. Granitic lithologies are most easily weathered and generate straight and wide river courses. Massive altered basalts are intermediate in their style of weathering. Dike swarms crosscut all lithologies and strongly influence river channel form and orientation. The geochemical composition of the rocks suggest that the majority are derived from extensive volumes