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| Titel |
Mineral textures in Serpentine-hosted Alkaline Springs from the Oman
ophiolite |
| VerfasserIn |
Manolis Giampouras, Juan Manuel Garcia-Ruiz, Wolfgang Bach, Carlos J. Garrido, Karin Los, Dario Fussmann, Monien Monien |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250149128
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| Publikation (Nr.) |
 EGU/EGU2017-13451.pdf |
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| Zusammenfassung |
| Meteoric water infiltration in ultramafic rocks leads to serpentinization and the formation of
subaerial, low temperature, hydrothermal alkaline springs. Here, we present a detailed
investigation of the mineral precipitation mechanisms and textural features of mineral
precipitates, along as the geochemical and hydrological characterization, of two
alkaline spring systems in the Semail ophiolite (Nasif and Khafifah sites, Wadi Tayin
massif). The main aim of the study is to provide new insights into mineral and textural
variations in active, on-land, alkaline vents of the Oman ophiolite. Discharge of
circulating fluids forms small-scale, localized hydrological catchments consisting in
unevenly interconnected ponds. Three different types of waters can be distinguished
within the pond systems: i) Mg-type; alkaline (7.9 < pH < 9.5), Mg-HCO3-rich
waters; ii) Ca-type; hyper-alkaline (pH > 11.6), Ca-OH-rich waters; and iii) Mix-type
waters arising from the mixing of Mg-type and Ca-type waters (9.6 < pH < 11.5).
Phreeqc geochemical speciation software was used to determine the saturation
state and the relationship between the theoretical supersaturation (S) and rate of
supersaturation (S˚ ) of solid phases. Simple mixing models using Phreeqc MIX_code
revealed good mixing correlation (R2 ≥0.93) between measured and predicted
values for K, Na, Cl, Mg and sulphate. Al, Ca, Si, Ba, Sr and TIC showed poorer
correlations.
Mineral and textural characterization from different types of water and individual ponds
were carried out by X-ray diffraction (XRD), Raman spectroscopy and field-emission
scanning electron microscopy coupled to dispersive energy spectroscopy (FE-SEM-EDS).
Aragonite and calcite are the dominant minerals (95 vol.%) of the total mineralogical index
in all sites. Mg-type waters host hydrated magnesium carbonates (nesquehonite)
and magnesium hydroxycarbonate hydrates (artinite) due to evaporation. Brucite,
hydromagnesite and dypingite presence in Mix-type waters is spatially controlled by the
hydrology of the system and is localized around mixing zones between Ca-type with
Mg-type waters. Residence time of discharging waters in the ponds before mixing
has an impact on fluid chemistry as it influences the equilibration time with the
atmosphere. Acicular aragonite is the main textural type in hyper-alkaline Ca-type
waters, acting as a substratum for the growth of calcite and brucite crystals. Low
crystallinity, dumbbell shaped and double pyramid aragonite dominates in Mix-type water
precipitates.
Rate of supersaturation is essential for precipitation intensity and textural variation among
the mineral assemblages in the different water types. Low S/ S˚ ratios in Mg-type and
Ca-type waters (<1) reveal limited precipitation. Chemical reactions that lead to rapid
mineral formation are enhanced in cases of Mix-type waters characterized by higher
S/ S˚ ratios (>1.2). Detailed investigation of individual spring sites allowed the
determination of geochemical and hydrological factors controlling the phases and
textures of mineralogical assemblages in active, serpentinization-related, alkaline
environments.
Funding: We acknowledge funding from the People programme (Marie Curie
Actions - ITN) of the European Union FP7 under REA Grant Agreement n˚ 608001. |
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