Klaus Simon

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.

Stable isotope fractionation between liquid and vapour in water–salt systems up to 600°C

Abstract Hydrogen and oxygen isotope fractionation factors between liquid and vapour were measured between 350°C and 600°C in the systems H2O–NaCl and H2O–KCl. Fractionation factors are similar for the NaCl- and KCl-bearing systems. At all conditions, D preferentially fractionates into the vapour phase relative to H, 18 O into the liquid relative to 16 O . Fractionation factors are given in terms of 1000 lnαD/H(L–V) and 1000 lnα 18 O / 16 O (L–V), respectively. At constant temperature, fractionations are approximated by linear correlations of the fractionation factor with the salt concentration in the liquid. Isotherms all have similar slopes and start at the respective critical composition where the fractionation is zero. D/H fractionation increases by 0.55‰ per wt.% NaCl(L), 18 O / 16 O fractionation by about 0.05‰ per wt.% NaCl(L). If extrapolated up to the limit of salt-saturation in the system, maximum fractionations of 28‰ in D/H(V–L) and about 2‰ in 18 O / 16 O (L–V) result along the liquid+vapour+halite curve over the temperature range from 350°C to 600°C. Maximum fractionations occur in boiling hydrothermal systems when the water–salt solvus opens up to salt-saturated conditions. This happens most likely in hydrothermal solutions at the roof of shallow intrusions. In this environment, salt-bearing solutions of magmatic, meteoric, or mixed origin intersect the solvus during isothermal or adiabatic decompression. Open-system behaviour may rapidly result in salt saturation of the liquid residue. Vapour leaving the hydrothermal system may have an isotopic signature similar to seawater whereas brine residues may become increasingly depleted in D and slightly enriched in 18 O . If the strong fractionation effect inherent in a boiling fluid system is disregarded, one may easily misinterpret the stable isotope ratios of hydrothermal minerals from such systems since boiling may strongly mask the source of the parent fluids.

Does δD from fluid inclusion in quartz reflect the original hydrothermal fluid

Abstract Hydrogen isotope investigations on hydrothermal quartz reveal two H-reservoirs: (i) trapped fluid inclusions, and (ii) structurally bound water in homogenously distributed small clusters or bubbles. Varying mixing ratios of the two reservoirs are sampled by means of mechanical and thermal decrepitation applied to different grain size fractions. A two-component mixing calculation results in an isotopic characterisation of the two H-reservoirs, which fractionate hydrogen isotopes close to the known MOH–H 2 O system, with water being enriched in deuterium. Temperature controls both the internal fractionation as well as the abundance ratio of inclusion water to bonded water. At high temperatures, fractionation is small but bound water becomes more abundant, comprising a significant amount of water from both thermal and mechanical extraction techniques. Hence, the isotope composition of the extracted water does not reflect the original hydrogen isotope composition of the hydrothermal fluid especially at temperatures higher than 200°C. Previously reported δ D data of fluid inclusion, which were used to elucidate the origin of the hydrothermal fluid, tend to be too low.

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

Hydrothermal alteration of Variscan granites, southern Schwarzwald, Federal Republic of Germany

Hercynian S-type granites from the southeastern Schwarzwald granite series represent cogenetic biotite-and two-mica granites. Oxygen- and hydrogen-isotope data show that hydrothermal alteration invoking isotopically light surface waters resulted in a drastic reduction in δ18O and δD and pronounced disequilibrium between the minerals. Effective water-rock ratios are calculated to be high, about 0.8 vol units. A shift in the18O/16O and the chemical composition of the fluid due to water-rock interaction is continuously traced from pure H2O with meteoric isotopic character in the deep-seated biotite granites to slightly saline water with rock-equilibrated isotopic composition in the two-mica granites at a shallower level. Substantial retrograde hydrometamorphism in the temperature range 500° to 200° C resulted mineralogically in high-temperature chloritization of biotite, and low-temperature muscovitization as well as feldspar alteration, respectively. Another result of the re-equilibration of cations is strong disturbance of the Rb−Sr system which affects measured ages and initial87Sr/86Sr values. Hydrothermal differentiation and alteration probably overlap to a very large extent magmatic differentiation processes.

Ancient microbial activity recorded in fracture fillings from granitic rocks (Äspö Hard Rock Laboratory, Sweden)

Fracture minerals within the 1.8-Ga-old Äspö Diorite (Sweden) were investigated for fossil traces of subterranean microbial activity. To track the potential organic and inorganic biosignatures, an approach combining complementary analytical techniques of high lateral resolution was applied to drill core material obtained at -450 m depth in the Äspö Hard Rock Laboratory. This approach included polarization microscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), confocal Raman microscopy, electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The fracture mineral succession, consisting of fluorite and low-temperature calcite, showed a thin (20-100 μm), dark amorphous layer lining the boundary between the two phases. Microscopic investigations of the amorphous layer revealed corrosion marks and, in places, branched tubular structures within the fluorite. Geochemical analysis showed significant accumulations of Si, Al, Mg, Fe and the light rare earth elements (REE) in the amorphous layer. In the same area, ToF-SIMS imaging revealed abundant, partly functionalized organic moieties, for example, C(x)H(y)⁺, C(x)H(y)N⁺, C(x)H(y)O⁺. The presence of such functionalized organic compounds was corroborated by Raman imaging showing bands characteristic of C-C, C-N and C-O bonds. According to its organic nature and the abundance of relatively unstable N- and O- heterocompounds, the organic-rich amorphous layer is interpreted to represent the remains of a microbial biofilm that established much later than the initial cooling of the Precambrian host rock. Indeed, δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr isotope data of the fracture minerals and the host rock point to an association with a fracture reactivation event in the most recent geological past.

O, H, C isotope study of rocks from the KTB pilot hole: crustal profile and constraints on fluid evolution

The pilot hole of the Continental Deep Borehole (KTB) drilling project is located in the Bavarian Oberpfalz at the western margin of the Bohemian Massif. The 4-km deep borehole penetrated various paragneisses and minor orthogneisses with intercalations of amphibolites and metagabbros. The different lithologies have systematically different whole-rock oxygen isotope values and give little evidence for large scale water-rock interaction. Minor fluid interaction is well documented during retrograde metamorphism by non-equilibrium fractionations between refractory minerals (quartz, garnet and hornblende) and altered minerals (chlorite/biolite and feldspar). Ubiquitous vein mineralisation indicates fluid-induced retrogression at temperatures between 150°C and 400°C. The δD values of hydroxylbearing minerals are very uniform in all lithologic units. The calculated hydrogen isotope composition of the fluid in equilibrium with matrix and vein minerals increases from -45‰ for metabasic rocks, to -20‰ for gneisses, to about -5‰ for vein minerals. The oxygen isotope composition of the fluid has been buffered by the rock and decreases with decreasing temperature because of increasing fractionations at low temperatures and low water-rock ratios. Modern fluids sampled from open cavities within the borehole have isotopic compositions that suggest a continuous fluid evolution during retrogression in a closed system. The δ13C values of calcite and graphite also indicate closed system mixing processes.

Assessing the utility of trace and rare earth elements as biosignatures in microbial iron oxyhydroxides

Microbial iron oxyhydroxides are common deposits in natural waters, recent sediments and mine drainage systems and often contain significant accumulations of trace and rare earth elements (TREE). TREE patterns are widely used to characterize minerals and rocks, and to elucidate their evolution and origin. Whether and which characteristic TREE signatures distinguish between a biological and an abiological origin of iron minerals is still not well understood. Long-term flow reactor studies were performed in the Aspo Hard Rock Laboratory to investigate the development of microbial mats dominated by iron-oxidizing bacteria, namely Mariprofundus sp. and Gallionella sp. The experiments investigated the accumulation and fractionation of TREE under controlled conditions and enabled us to assess potential biosignatures evolving within the microbial iron oxyhydroxides. Concentrations of Be, Y, Zn, Zr, Hf, W, Th, Pb, and U in the microbial mats were 1e3- to 1e5-fold higher than in the feeder fluids whereas the rare earth elements and Y (REE+Y) contents were 1e4 and 1e6 fold enriched. Except for a hydrothermally induced Eu anomaly, the normalized REE+Y patterns of the microbial iron oxyhydroxides were very similar to published REE+Y distributions of Archaean Banded Iron Formations. The microbial iron oxyhydroxides from the flow reactors were compared to iron oxyhydroxides that were artificially precipitated from the same feeder fluid. These abiotic and inorganic iron oxyhydroxides show the same REE+Y distribution patterns. Our results indicate that the REE+Y mirror quite exactly the water chemistry, but they do not allow to distinguish microbially mediated from inorganic iron precipitates. All TREE studied showed an overall similar fractionation behavior in biogenic, abiotic and inorganic iron oxyhydroxides. Exceptions are Ni and Tl, which were only accumulated in the microbial iron oxyhydroxides and may point to a potential usage of these elements as microbial biosignatures.

Effects of meteoric water interaction on Hercynian granites from the Südschwarzwald, southwest Germany

Abstract Hercynian granites from the Sudschwarzwald (plutons of Klemmbach, St. Blasien, Schluchsee, Barhalde) show a wide range of whole-rock δ 18 O-values from +2.5‰ to + 10.9‰ (SMOW). Detailed 18 O analysis of separated minerals indicates strong isotopic disequilibrium, a feature which can be explained by large-scale post-magmatic hydrothermal interaction of the granites with isotopically light meteoric water at temperatures below 500°C. Microthermometric investigation of homogenization and melting temperatures from two-phase (vapour, liquid) secondary fluid inclusions in quartz indicates a temperature range between 200° and 400°C for the alteration. The aqueous fluid inclusions typically have low salinities, but reach values up to 9 eq. wt.% NaCl in samples with evidence for secondary mineral development (i.e. chloritization, sericitization, etc.). Trace-element variations in K-feldspar (Rb, Sr, Ba, Tl, Pb, Bi) provide a guide to elemental mobilization with increasing water/rock ratios. A gain of Rb, Tl and Bi and a loss of Sr, Ba and Pb is observed during alteration. Combining all data, a three-stage reaction scheme for the hydrothermal alteration of the granites is proposed: (1) T∼400°C: granite (prim.) +fluid 1→granite (alter.) +fluid 2 (2) T∼200–300°C: fluid 2 +granite (alter.) → secondary mineralization+fluid 3 (3) T⩽200°C: fluid 3→gangue mineralization.

Description of an aerodynamic levitation apparatus with applications in Earth sciences.

BackgroundIn aerodynamic levitation, solids and liquids are floated in a vertical gas stream. In combination with CO2-laser heating, containerless melting at high temperature of oxides and silicates is possible. We apply aerodynamic levitation to bulk rocks in preparation for microchemical analyses, and for evaporation and reduction experiments.ResultsLiquid silicate droplets (~2 mm) were maintained stable in levitation using a nozzle with a 0.8 mm bore and an opening angle of 60°. The gas flow was ~250 ml min-1. Rock powders were melted and homogenized for microchemcial analyses. Laser melting produced chemically homogeneous glass spheres. Only highly (e.g. H2O) and moderately volatile components (Na, K) were partially lost. The composition of evaporated materials was determined by directly combining levitation and inductively coupled plasma mass spectrometry. It is shown that the evaporated material is composed of Na > K >> Si. Levitation of metal oxide-rich material in a mixture of H2 and Ar resulted in the exsolution of liquid metal.ConclusionsLevitation melting is a rapid technique or for the preparation of bulk rock powders for major, minor and trace element analysis. With exception of moderately volatile elements Na and K, bulk rock analyses can be performed with an uncertainty of ± 5% relative. The technique has great potential for the quantitative determination of evaporated materials from silicate melts. Reduction of oxides to metal is a means for the extraction and analysis of siderophile elements from silicates and can be used to better understand the origin of chondritic metal.