Origins of high pH mineral waters from ultramafic rocks, Central Portugal

This paper reviews the geochemical, isotopic (²H, ¹⁸O, ¹³C, ³H and ¹⁴C) and numerical modelling approaches to evaluate possible geological sources of the high pH (11.5)/Na–Cl/Ca–OH mineral waters from the Cabeço de Vide region (Central-Portugal). Water–rock interaction studies have greatly contributed to a conceptual hydrogeological circulation model of the Cabeço de Vide mineral waters, which was corroborated by numerical modelling approaches. The local shallow groundwaters belong to the Mg–HCO₃ type, and are derived by interaction with the local serpentinized rocks. At depth, these type waters evolve into the high pH/Na–Cl/Ca–OH mineral waters of Cabeço de Vide spas, issuing from the intrusive contact between mafic/ultramafic rocks and an older carbonate sequence. The Cabeço de Vide mineral waters are supersaturated with respect to serpentine indicating that they may cause serpentinization. Magnesium silicate phases (brucite and serpentine) seem to control Mg concentrations in Cabeço de Vide mineral waters. Similar δ²H and δ¹⁸O suggest a common meteoric origin and that the Mg–HCO₃ type waters have evolved towards Cabeço de Vide mineral waters. The reaction path simulations show that the progressive evolution of the Ca–HCO₃ to Mg–HCO₃ waters can be attributed to the interaction of meteoric waters with serpentinites. The sequential dissolution at CO₂ (g) closed system conditions leads to the precipitation of calcite, magnesite, amorphous silica, chrysotile and brucite, indicating that the waters would be responsible for the serpentinization of fresh ultramafic rocks (dunites) present at depth. The apparent age of Cabeço de Vide mineral waters was determined as 2790 ± 40 a BP, on the basis of ¹⁴C and ¹³C values, which is in agreement with the ³H concentrations being below the detection limit.     

Phase transition in K2Cd2(SO4)3: Order parameter and microscopic distortions

The transition from P2₁3(T ⁴) to P2₁2₁2₁(D ₂ ⁴ ) in the langbeinite K₂Cd₂(SO₄)₃ has been analyzed using group theoretical methods and previously published structural data above and below the transition. We find that because the transition is strongly first-order, the primary-order parameter has relatively large values at the transition temperature, and higher order terms which involve the order parameter, the strain, and the coupling of the two must be included in the Landau expansion for the free energy. Complex displacements occur at the transition for all atoms of the unit cell, but these displacements can be resolved into contributions which can be shown from symmetry considerations to transform as the _{2 3} irrep of P2 ₁ 3(T ⁴) as well as contributions from symmetry-preserving displacements which transform under the irrep ₁. Therefore, the transition is not a simple one and involves sulfate rotations and cadmium and potassium ion displacements.     

Holocene record of productivity in the NW Iberian continental shelf

A multiproxy approach (textural characteristics, heavy metal concentrations and benthic foraminifera data) was adopted to study the OMEX core KSGX 40, collected at the Galicia Mud Deposit (NW Iberian outer continental shelf) and recording the last ca. 4.8 ka cal BP. Geochemical profiles of Fe, Mn, Zn, Co, Cr, Cu, Ni, Pb, Al and Ca show significant temporal variations. Benthic foraminifera productivity proxy suggests that the recorded muddy events were contemporaneous of a higher C_org flux to the marine benthic environment. The higher flux of organic matter, following a long-term intensification of the upwelling and coinciding with a finer sedimentation, led to depressed levels of oxygen beneath the water-sediments interface and to early diagenetic processes of several redox-sensitive elements.     

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.     

Characterization of dissolved organic matter across the Subtropical Convergence off the South Island,New Zealand

Dissolved organic matter (DOM) was investigated along a gradient across the Subtropical Convergence (STC) off the South Island, New Zealand. Ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), excitation emission matrix fluorescence (EEM) spectroscopy, and molecular lignin analysis techniques were used to study this DOM. The analysis revealed a group of compounds found only in the coastal DOM samples, which were also characterized by an elevated terrestrial DOM fluorescence pattern and elevated lignin content if compared to the offshore samples. This group exhibited a high degree of carbon unsaturation, as evident from high double bond equivalence minus oxygen values (DBE-O > 9) and maximum fluorescence intensity. Sulfur-containing molecular formulae for summer DOM samples were much more abundant across the entire transect of the STC compared to winter and exhibited distinctly different hydrogen:carbon and oxygen:carbon elemental ratios, suggesting a possible correlation between biological activity and sulfur compounds. The molecular formulae common to all STC samples were compared with those calculated for DOM extracted from freshwater collected from a stream discharging into Doubtful Sound (West Coast, New Zealand). ESI-FT-ICR-MS measurements undertaken in negative electrospray ionization (ESI?) mode indicated that 30% of the molecular formulae were present in both types of DOM, while in positive ESI mode (ESI+) over 90% of the formulae were present in all samples. Hence, a significant portion of the molecular formulae assigned to the solid-phase extractable DOM pool (SPE-DOM) appear to be identical in oceanic and freshwater samples.