A noble record

Alexander B Verchovsky and Mark A Sephton review the origins and significance of noble gases in meteorites, focusing on what this unique record reveals about the early solar system.     

The influence of particle composition on thorium scavenging in the NE Atlantic ocean (POMME experiment)

²³⁰Th, ²³²Th and ²³⁴Th were analyzed in sinking particles collected by moored and drifting sediment traps in the NE Atlantic Ocean (POMME experiment) in order to constrain the phase(s) carrying Th isotopes in the water column. It reveals a contrasted behaviour between ²³⁴Th and ²³⁰Th. ²³⁴Th is correlated to the particulate organic carbon suggesting that it is primarily scavenged by organic compounds in the surface waters. ²³⁰Th_xs is correlated with Mn, Ba and the lithogenic fraction that are enriched in small suspended particles and incorporated in the sinking particulate flux throughout the water column. The lack of correlation between ²³⁰Th_xs and CaCO₃ or biogenic silica (bSi) indicates that CaCO₃ and bSi are not responsible for ²³⁰Th scavenging in the deep waters of this oceanic region. ²³⁰Th is generally correlated with the lithogenic content of the trapped material but this correlation disappears in winter during strong atmospheric dust inputs suggesting that lithogenic matter is not directly responsible for ²³⁰Th scavenging in the deep waters or that sufficient time is required to achieve particle–solution equilibration. MnO₂ could be the prevalent ²³⁰Th_xs-bearing phase. The narrow range of K_{d_MnO2}^Th obtained for very contrasted oceanic environments supports a global control of ²³⁰Th_xs scavenging by MnO₂ and raises the possibility that the ²³⁰Th–²³¹Pa fractionation is controlled by the amount of colloidal MnO₂ in seawater.     

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

V_p and V_s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust.V_p values for the metapelitic granulites range between 6.26 and 7.99 km s^{− 1} with a mean value of 7.09 ± 0.4 km s^{− 1}. Psammite and granitic orthogneiss samples have calculated V_p values of 6.51 and 6.23 km s^{− 1}, respectively. V_s values for the metapelites are between 3.86 and 4.34 km s^{− 1}, with a mean value of 4.1 ± 0.15 km s^{− 1}. The psammite and orthogneiss have calculated V_s values of 3.95 and 3.97 km s^{− 1}, respectively.The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V_p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V_s anisotropy ranges from 1% to 11%. Poisson's ratio (σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V_p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation.When corrected for present-day lower crustal temperature, the experimentally determined V_p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V_p values of 6.85–6.9 and 6.9–8.0 km s^{− 1}, respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.     

Karst database development in Minnesota: design and data assembly

The Karst Feature Database (KFD) of Minnesota is a relational GIS-based Database Management System (DBMS). Previous karst feature datasets used inconsistent attributes to describe karst features in different areas of Minnesota. Existing metadata were modified and standardized to represent a comprehensive metadata for all the karst features in Minnesota. Microsoft Access 2000 and ArcView 3.2 were used to develop this working database. Existing county and sub-county karst feature datasets have been assembled into the KFD, which is capable of visualizing and analyzing the entire data set. By November 17 2002, 11,682 karst features were stored in the KFD of Minnesota. Data tables are stored in a Microsoft Access 2000 DBMS and linked to corresponding ArcView applications. The current KFD of Minnesota has been moved from a Windows NT server to a Windows 2000 Citrix server accessible to researchers and planners through networked interfaces.     

Unique Marine Olenekian-Anisian Boundary Section from South Primorye, Russian Far East

INTRODUCTION LateOlenekianandAnisianmarinedepositsin SouthPrimoryewerefirststudiedbyD.L.Ivanov,thechiefofageologicalteammakingreconnaissance workfortheconstructionofthetrans Siberianrail road.HecollectedEarlyandMiddleTriassicam monoidsonRussianIsland.Arep…     

6: Classification of VMS deposits: Lessons from the South Uralides

VMS deposits of the South Urals developed within the evolving Urals palaeo-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada.Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early arc tholeiites host Cu–Zn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, arc tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cu–(Zn) and Cu–Co VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of arc-continent collision in Late Devonian times. The arc tholeiites host Cu–Zn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to Zn–Pb–Cu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the Cu–Zn VMS deposits.The exceptional degree of preservation in the Urals has permitted the identification of early seafloor clastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification.It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis.     

Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions

Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa_<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk_<15). ∼10% of AOAs contain low-Ca pyroxene (Fs_1-3Wo_1-5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3-5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally ¹⁶O-rich isotopic compositions (Δ¹⁷O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are ¹⁶O-enriched (Δ¹⁷O < −20‰), whereas those of (iii) are ¹⁶O-depleted (Δ¹⁷O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is ¹⁶O-rich (Δ¹⁷O ∼ −22‰), whereas the neighboring plagioclase mesostasis is ¹⁶O-depleted (Δ¹⁷O ∼ −11‰).We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an ¹⁶O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted ¹⁶O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into chondrules. The original ¹⁶O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains.     

Experimental and theoretical study of hydration of halide ions

We have studied thermochemistry of the first hydration steps for Cl⁻, Br⁻, and I⁻ in the gas phase both experimentally using high-pressure mass spectrometry (HPMS) and theoretically using density functional theory (DFT) calculations. The highest hydration steps measured experimentally were n = 8 for Cl⁻, n = 7 for Br⁻, and n = 5 for I⁻, all of them being higher than previously reported. Both experimental and theoretical stepwise enthalpies and entropies of hydration for these halides exhibited non-monotonic behavior for successive hydration steps that was not reported in previous HPMS investigations of these reactions. This behavior can be successfully interpreted using halide water cluster geometries obtained from DFT calculations by considering the number of additional hydrogen bonds formed at each hydration step and simultaneous weakening of ion-solvent interaction with increasing cluster size. Results of DFT calculations for surface cluster geometries agree better with experimental results than do the results for interior cluster geometries. We conclude that predominantly surface clusters were observed in our experiments and that small surface clusters have larger number of possible isomers than the interior clusters of the same size. The results for enthalpies of hydration for the studied halide ions lead to the conclusion that ion-solvent interaction is stronger than solvent-solvent interaction for chloride-water clusters. The difference between the two types of interaction diminishes with increasing anion size. The ion-solvent and solvent-solvent interactions are of nearly equal magnitude for iodide.     

Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered mackinawite

Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The K_d (L g⁻¹) values obtained from linear fits to the isotherm data are ∼9 for As(V) and ∼2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered mackinawite controls arsenic mobility.