Shallow-marine serpentinization-derived fluid seepage in the Upper Cretaceous Qahlah Formation, United Arab Emirates

Abstract

Serpentinization of ultramafic rocks in the sea and on land leads to the generation of alkaline fluids rich in molecular hydrogen (H2) and methane (CH4) that favour the formation of carbonate mineralization, such as veins in the sub-seafloor, seafloor carbonate chimneys and terrestrial hyperalkaline spring deposits. Examples of this type of seawater–rock interaction and the formation of serpentinization-derived carbonates in a shallow-marine environment are scarce, and almost entirely lacking in the geological record. Here we present evidence for serpentinization-induced fluid seepage in shallow-marine sedimentary rocks from the Upper Cretaceous (upper Campanian to lower Maastrichtian) Qahlah Formation at Jebel Huwayyah, United Arab Emirates. The research object is a metre-scale structure (the Jebel Huwayyah Mound) formed of calcite-cemented sand grains, which formed a positive seafloor feature. The Jebel Huwayyah Mound contains numerous vertically orientated fluid conduits containing two main phases of calcite cement. We use C and O stable isotopes and elemental composition to reconstruct the fluids from which these cements precipitated and infer that the fluids consisted of variable mixtures of seawater and fluids derived from serpentinization of the underlying Semail Ophiolite. Based on their negative δ13C values, hardgrounds in the same section as the Jebel Huwayyah Mound may also have had a similar origin. The Jebel Huwayyah Mound shows that serpentinization of the Semail Ophiolite by seawater occurred very soon after obduction and marine transgression, a process that continued through to the Miocene, and, with interaction of meteoric water, up to the present day.

Mineralogical, petrographic, and geochemical analyzes which confirm the hydrothermal origin of the sediments that overlie the peridotites of Cerro Matoso, Colombia

“…The element and stable isotope composition of the sedimentary succession have been used to reconstruct the origin and nature of the fluids from which they are formed. In this sense oxygen, and carbon isotope values of the bulk sediments from Cerro Matoso are consistent with those observed in serpentinization settings of ultramafic rocks (e.g., Lavoie and Chi, 2010;Lartaud et al, 2011;Klein et al, 2015;Eickmann et al, 2021).…”

“…In general, carbon dioxide is derived from magmatic degassing, which retains values of δ 13 C, mainly between -4 and -8‰ (Campbell and Larson, 1998), and carbonates in the mantle, between -5 and -7‰ (Alt et al, 1986;Hoefs and Sywall, 1997), will be related only to the most negative δ 13 C isotopic signature of the marine sediments in Cerro Matoso because in general, all samples present a wide range of δ 13 C. The more negative δ 13 C values in Cerro Matoso samples point out the presence of methane oxidation events, favoring the growth of carbonates in the sediments overlying the ultramafic rocks. Similar conditions have been reported around deep marine hydrothermal vents by Kelley et al (2001), Früh-Green et al (2004), and Eickmann et al (2021. (Klein et al, 2015), Saldanha sediments (Dias et al, 2011), carbonates at Ghost City (Lartaud et al, 2011), carbonates and brecciated serpentinites to the south of the Gaspe Peninsula (Lavoie and Chi, 2010); and hydrothermal fluids at Rainbow and Logatchev (Charlou et al, 2002).…”

“…Fissured and brecciated appearances in the sediments accompanied by a network of microcracks, open and common ducts, and high porosity, with vug and mold porosity, have been associated with the exhalation of fluids. The formation of these sedimentary structures is therefore analogous to fluid-induced chimney formation on the modern ocean floor (e.g., Eickmann et al, 2021).…”

“…We suggest the production of abiogenic methane by serpentinization of peridotite as the light δ 13 C source. Methane in modern serpentinitederived fluids in both the deep-sea and ophiolite environments have negative δ 13 C values (e.g., -18‰ at Elba and -7.7‰ in the Zambales ophiolite; -16.7‰ at Rainbow; -10.3‰ at Logatchev; -11.9‰ at Lost City; Abrajano et al, 1988;Lilley et al, 1993;Charlou et al, 2002;Proskurowski et al, 2008a;Meister et al, 2018;Sciarra et al, 2019;Eickmann et al, 2021). Part of this methane will be oxidized close to the seafloor to produce 13 C-depleted DIC, and high pH and Ca concentrations in the serpentinite-derived fluids trigger carbonate and mineral precipitation on mixing with seawater (Palandri and Reed, 2004;Proskurowski et al, 2008b).…”

Deep Sea Biodiversity on the Continents, How It Happens?

“…[4,25]), the source of fluid hosted by them has been related to the serpentinization process that suffers peridotites when they are under intense activity provided by the heat of intrusion of mafic magmas or by the cooling of the ultramafic lithosphere, the later results in a fluid circulation that is driven by convectiondissipated heat from the bottom and exothermic chemical reactions between the circulating fluids and host [73]. Serpentinite-derived fluids in both deep-sea and ophiolite environments, where methane has been measured, present negative δ 13 C values, including −11.9‰ at Lost City, −10.3‰ at Logatchev, −16.7‰ at Rainbow, −18‰ at Elba, and − 7.7‰ in the Zambales ophiolite [86][87][88][89][90][91][92]. Hydrothermal systems hosted in ultramafic rocks are present in Logatchev, Rainbow, and Lost City Hydrothermal Field-LCHF [3].…”