Mineralogical assemblages forming at hyperalkaline warm springs hosted on ultramafic rocks: A case study of Oman and Ligurian ophiolites

Abstract

[1] We report on the mineralogical assemblages found in the hyperalkaline springs hosted on Liguria and Oman ophiolites based on exhaustive X-ray diffraction and scanning electron microprobe analyses. In Liguria, hyperalkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric CO 2 uptake and neutralization of the hyperalkaline waters. No brucite and portlandite minerals are observed. The discharge of alkaline waters in Oman ophiolite forms white-orange precipitates. Calcium carbonate minerals (calcite and/or aragonite) are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in Liguria. This process is observed as a thin surface crust made of rhombohedral calcite. Morphological features of aragonite vary from needle-, bouquet-, dumbbell-, spheroidal-like habitus according to the origin of carbon, temperature, and ionic composition of the hyperalkaline springs, and the biochemical and organic compounds. Brucite is observed both at hyperalkaline springs located at the thrust plane and at the paleo-Moho. The varying mixing proportions between the surface runoff waters and the hyperalkaline ones control brucite precipitation. The layered double hydroxide minerals occur solely in the vicinity of hyperalkaline springs emerging within the bedded gabbros. Finally, the dominant mineralogical associations we found in Oman (Ca-bearing carbonates and brucite) in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the Lost City hydrothermal site in a totally marine environment.

Fluid chemistry of the low temperature hyperalkaline hydrothermal system of Prony Bay (New Caledonia)

“…Brucite forms at the vents when high-pH waters mix with Mg-rich seawater. Brucite formation due to mixing of two water types has already been evidenced in the alkaline springs of Oman where magnesium is brought by the runoff waters and hydroxide by the high-pH fluids (Chavagnac et al, 2013a, b). This mode of brucite formation is different Figure 11.…”

“…It is beyond the scope of the present paper to check the thermodynamic data for all the minerals relevant to the serpentinizing environment. We focused on Ca and Mg carbonates and brucite which have been observed to form in the concretions (Pisapia and the HYDROPRONY scientific team, 2012) and also on various forms of silica (quartz, chalcedony, amorphous silica) that have been shown to play a role in the control of aqueous silica in the alkaline waters of Oman (Chavagnac et al, 2013a, b) .…”

Controls on the stable isotope compositions of travertine from hyperalkaline springs in Oman: Insights from clumped isotope measurements

“…yielding pH values between 9 and 11 (Paukert et al, 2012;Chavagnac et al, 2013a), but 441 only one of the fresh carbonate precipitates we analyzed (OM11_07Y) was formed at 442 such a location ("Misbit mixing") with a pH of 10.6 (Table 2). 443 The calibrated 14 C ages of fresh carbonate precipitates and of travertine deposits 480 considered in this study, range from modern to 350 years and from modern to 45,000 481 years, respectively, where "modern" is defined by >95% of the 14 C activity for AD 1950 482 same aliquots of sample powders used in the clumped isotope analyses, we refer to these 490 14 C ages in Table 1 and in the following discussion.…”

Carbonate formation on ophiolitic rocks at different pH, salinity and particle size conditions in CO2-sparged suspensions

“…Geochemical data on spring waters involving serpentinitic rocks and ultramafic rocks have shown this potential [7]. Alkaline springs are associated with ophiolitic rocks and assumed to equilibrate with atmospheric CO 2 and eventually form calcite, brucite and aragonite [6,30,34]. Salinity and pH are important features of these alkaline springs and would prove beneficial in the dissolution step.…”