GEO-GIANTS OF THE PAST

The occasional series continues, laying bare the trials and tribulations of our geological forebears.     

U–Pb baddeleyite ages and Hf,Nd isotope chemistry constraining repeated mafic magmatism in the Fennoscandian Shield from 1.6 to 0.9 Ga

The Palaeoproterozoic (1.90–1.60 Ga) crust of central Fennoscandia was intruded repeatedly by dolerite dikes and sills during the Neo- and Mesoproterozoic eons. We report 17 new baddeleyite U–Pb dates comprising six generations of dolerites (in Ma):

Blekinge-Dalarna dolerites	946–978
Protogine Zone dolerites	1,215–1,221
Central Scandinavian Dolerite Group	1,264–1,271
Tuna dikes and age equivalents in Dalarna	1,461–1,462
Värmland dolerites	～1,568
Breven-Hällefors dolerites	～1,595

The favoured tectonic model implies that the majority of these suites were related to active margin processes somewhere west (and possibly south) of the Fennoscandian Shield. Dolerite intrusions are interpreted to reflect discrete events of back-arc extension as the arc retreated oceanward. Initial Hf and Nd isotope compositions of the dolerite swarms fall between CHUR and normal-depleted mantle, and suggest a variably depleted and re-enriched mantle as the source for the here investigated 1.6 to 0.95 Ga old mafic rocks. Repeated recycling of older crustal components, mainly sediments (dominated by material with short residence ages) in earlier subduction systems may have been very efficient at producing geochemically and isotopically variably enriched lithospheric mantle sections beneath the Fennoscandian Shield.

     

INFRARED (JHK) PHOTOMETRY OF METEORITES AND ASTEROIDS

We present JHK colors observed for ten asteroids and synthesized JHK colors for seven meteorite groups, samples of iron and nickel metal, pyroxene, olivine, feldspar, a lunar anorthite and some terrestrial mineral samples. Pronounced differences are apparent between the chondritic and achondritic meteorite classes; the chondritic classes show less subdued trends in J-H color which reflect their metamorphic grade We find small but significant differences between the JHK colors of the predominant C and S classes of asteroids. All JHK colors of asteroids observed here fall within the limited domain defined by the various chondritic and iron-rich meteorites but are strikingly different from those of most achondritic meteorites     

Ecosystem scale evapotranspiration and net CO2 exchange from a restored peatland

An understanding of the symbiotic water and gas exchange processes at the ecosystem scale is essential to the development of appropriate restoration plans of extracted peatlands. This paper presents ecosystem scale measurements of the atmospheric exchange of water and carbon dioxide (CO₂) from a restored vacuum extracted peatland in eastern Québec, utilizing full‐scale micrometeorological measurements of both evaporation and CO₂. The results indicate that the adopted restoration practices reduce the loss of water from the peat, but CO₂ emissions are ～25% greater than an adjacent nonrestored comparison site. The blockage of drainage ditches and the existence of a mulch cover at the site keep the moisture conditions more or less constant. Consequently, the CO₂ flux, which is predominantly soil respiration, is strongly controlled by peat temperature fluctuations. Copyright © 2001 John Wiley & Sons, Ltd.     

Photometric properties of powdered sulfur

The spectrophotometric (0.39 < λ < 0.7 λm) properties of three particle-size fractions (diameters <10 λm, <150 λm, and 420–850 λm) of sulfur have been investigated in the laboratory. Particle size, temperature, thermal history, and scattering geometry are all shown to influence the spectral reflectance of the normal (S₈) sulfur samples and an “orange-colored” S₈ sample produced by quenching molten sulfur. A scattering law consisting of a linear combination of lunar-like and Lambertian terms adequately describes the data for all particle sizes. Where sulfur is darkest (λ < 0.45 λm), the reflectance decreases with increasing particle size, whereas where sulfur is brightest (λ > 0.45 λm) the reflectance increases with decreasing particle size. In reflected light, the long wavelength edge of the strong ultraviolet absorption retreats smoothly to shorter wavelengths with decreasing temperature at ～1.6Å/°K, a value lower than the 2.2Å/°K value previously reported for transmitted light. Near opposition, sulfur powders are found to follow closely a Minnaert limb darkening law except where the reflectance is low, i.e., in the strong ultraviolet absorption band of the larger particle size fractions. It is clear from our data that quantitative comparisons between disk-integrated observations of Io and laboratory measurements of flat samples of sulfur are not adequate unless temperature effects and changes in scattering geometry are included.     

Formation of the galilean satellites in a gaseous nebula

A model for Galilean satellite formation was analyzed in which the satellites accrete in the presence of a dense, gaseous disk-shaped nebula and rapidly form optically thick, gravitationally bound primordial atmospheres. Upper-bound temperatures expected during accretion lead to partially differentiated structures for both Ganymede and Callisto, although with Ganymede much more differentiated than Callisto. When allowance is made for the aerodynamic breaking of infalling planetesimal fragments, lower surface temperatures result, and the amount of partial differentiation of Callisto is small, possibly approaching zero for a narrow size distribution of infalling planetesimals. The model is chosen to be consistent with the observed densities of the Galilean satellites and our current understanding of Jupiter formation. The retention of ices more volatile than H₂O is considered but not modeled in detail. A nominal nebula of ～0.1 Jupiter masses is constructed by consideration of likely surface density profiles and existing Jupiter collapse calculations. This nebula is optically thick (even if grain opacity is ignored) in both radial and vertical directions and has a temperature profile T ～ 3600 (R_J/R), where R_J is Jupiter's radius and R is the radial distance in the disk midplane. Satellites accrete very rapidly (dynamical time scales being 10²–10⁴ years) and their optically thick gaseous envelopes are unable to eliminate the heat of accretion by radiation. Water-saturated, convective, adiabatic envelopes form, through which planetesimals fall, break up, and partially disseminate their mass. The resulting satellite surface temperatures during accretion are calculated. Possible implications of these models for the subsequent evolution of Ganymede and Callisto are explored and it is suggested that the extensive differentiation undergone by Ganymede may provide the right environment for subsequent resurfacing, whereas the relative lack of extensive differentiation for Callisto may explain the inferred absence of endogenic tectonism.     

AMS 14C dating of tundra lake sediments using chironomid head capsules

Radiocarbon dating of late-Quaternary sediments from high-latitude lakes is often complicated by the influx of old carbon, reservoir effects, or both. If terrestrial plant macrofossils are also absent, the dating of bulk sediment often provides the only means to establish chronologies for these problematic sediment sequences. Given that chironomid (non-biting midge) remains are sufficiently abundant in many northern lakes to be ¹⁴C-dated via the accelerator mass spectrometry (AMS) method, we decided to explore their utility in age-model development. Five age determinations based on chironomid material were obtained from a lake sediment core sampled in the shrub tundra of northern Québec. These results were compared to six AMS bulk sediment ages, as well as to a date obtained from Drepanocladus spp. The chironomids yielded consistently younger ages (with increasing age offset upcore), confirming both the presence of a reservoir effect and the value of chironomids in establishing more reliable ¹⁴C chronologies.     

Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, NV, USA

Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from 45–75 wt.% SiO₂. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate.

Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting.     

Experimental constraints on major and trace element partitioning during partial melting of eclogite

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al₂O₃). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO₂ content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile.Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high-field strength elements indicate complementary D_Zr/D_Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.