Carbon isotope composition of graptolite periderm and whole-rock from the Aeronian (Silurian, Llandovery) in Wales and Scotland and its use in chemostratigraphy     

Andrea M. Snelling  Melanie J. Leng  Christopher C. Kendrick 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(1):82-91

Here we show the use of graptolite periderm for chemostratigraphic study. Using material from the Aeronian (Silurian) interval from Wales and Scotland as examples, we show that the carbon isotope composition of the periderm (δ¹³C_grap.) provides a signal that is locally different but not consistently so from surrounding whole-rock samples (δ¹³C_whole-rock). Graptolite periderm δ¹³C seems not influenced by astogenetic stage of development or gross rhabdosome type and differences between δ¹³C_grap. from different metamorphic grades are minimal. Taken as a whole, the Aeronian interval examined shows little overall change, but large variations are seen on the small scale, possibly reflecting very local carbon cycling. For carbon isotope stratigraphy in such rocks, therefore, large-scale bulk sampling will likely reduce inhomogeneities and give more reproducible results. Furthermore, in situations (for instance associated with sea level fluctuations) where terrestrial organic matter has been incorporated into the sediment, then graptolite carbon may more faithfully reflect bulk marine organic matter.  相似文献   

Sb–Hg ore deposit distribution controlled by brittle structures: The case of the Selvena mining district (Monte Amiata, Tuscany, Italy)     

Andrea Brogi  Lorenzo Fabbrini  Domenico Liotta 《Ore Geology Reviews》2011,41(1):35-48

This paper deals with the tectonic control on the hydrothermal system that gave rise to Sb–Hg ore deposits in the Monte Amiata area that was one of the most relevant mining district for the exploitation of mercury in Italy. The study area (Selvena mining district) is located in southern Tuscany (inner Northern Apennines) one of the most important mineralized area in the western Mediterranean region. Southern Tuscany was severely affected by Middle–Late Miocene low-angle normal faults, later dissected by Pliocene–Pleistocene faults, coeval magmatism (Late Miocene–Pleistocene) and hydrothermal activity (Pliocene–Present). The Selvena mining district is located south of Middle Pleistocene Monte Amiata volcanic complex. Our structural and kinematic study is based on the integration among fieldwork, borehole and mine data. The results highlight two Pleistocene–Holocene left-lateral transtensional shear zones linked by normal faults, defining a coeval pull-apart structure. Here, the Sb–Hg mineralization, transported by meteoric hydrothermal fluids mainly, is particularly diffuse and concentrated in the cataclasites and in damage zones of the normal faults. Furthermore, a widespread mineralization also occurs in the cataclasites of Miocene low-angle normal faults. Mine evidence suggests that ore-bearing fluids percolated through structural conduits located along the fault planes and resulting parallel to the intermediate stress axis. Geological structures and ore deposit distribution are related to a single hydrothermal circuit, with meteoric water channelled to depth through conduits parallel to the intermediate stress axis of the transcurrent shear zones; then, hydrothermal fluids mainly ascended through the almost vertical deformation zones located at the intersection between normal and strike-slip faults. Thus, hydrothermal fluids permeated also the Middle–Late Miocene cataclasites. This study shed light on the relationships between geological structures and mineralization in southern Tuscany and underlines the importance to investigate mine areas to understand hydrothermal fluids path.  相似文献   

Dissolution and carbonation of a serpentinite: Inferences from acid attack and high P-T experiments performed in aqueous solutions at variable salinity     

Andrea Orlando  Daniele BorriniLuigi Marini 《Applied Geochemistry》2011,26(8):1569-1583

Dissolution experiments on a serpentinite were performed at 70 °C, 0.1 MPa, in H₂SO₄ solution, in open and closed systems, in order to evaluate the overall dissolution rate of mineral components over different times (4, 9 and 24 h). In addition, the serpentinite powder was reacted with a NaCl-bearing aqueous solution and supercritical CO₂ for 24 h at higher pressures (9-30 MPa) and temperatures (250-300 °C) either in a stirred reactor or in an externally-heated pressure vessel to assess both the dissolution rate of serpentinite minerals and the progress of the carbonation reaction. Results show that, at 0.1 MPa, MgO extraction from serpentinite ranges from 82% to 98% and dissolution rate varies from 8.5 × 10⁻¹⁰ mole m⁻² s⁻¹ to 4.2 × 10⁻⁹ mole m⁻² s⁻¹. Attempts to obtain carbonates from the Mg-rich solutions by increasing their pH failed since Mg- and NH₄- bearing sulfates promptly precipitated. On the other hand, at higher pressures, significant crystallization (5.0-10.4 wt%) of Ca- and Fe-bearing magnesite was accomplished at 30 MPa and 300 °C using 100 g L⁻¹ NaCl aqueous solutions. The corresponding amount of CO₂ sequestered by crystallization of carbonates is 9.4-15.9 mole%. Dissolution rate (from 6.3 × 10⁻¹¹ mole m⁻² s⁻¹ to 1.3 × 10⁻¹⁰ mole m⁻² s⁻¹) is lower than that obtained at 0.1 MPa and 70 °C but it is related to pH values much higher (3.3-4.4) than that (−0.65) calculated for the H₂SO₄ solution.Through a thorough review of previous experimental investigations on the dissolution kinetics of serpentine minerals the authors propose adopting: (i) the log rate [mole m⁻² s⁻¹] value of −12.08 ± 0.16 (1σ), as representative of the neutral dissolution mechanism at 25 °C and (ii) the following relationship for the acidic dissolution mechanism at 25 °C: