Timing of Magnetite Growth Associated With Peridotite‐Hosted Carbonate Veins in the SE Samail Ophiolite, Wadi Fins, Oman

Abstract

Carbonate‐altered peridotite are common in continental and oceanic settings and it has been suggested that peridotite‐hosted carbonate represent a significant component of the carbon‐cycle and provide an important link in the CO2 dynamics between the atmosphere, hydrosphere, and lithosphere. The ability to constrain the timing of carbonate and accessory phase growth is key to interpreting the mechanisms that contribute to carbonate alteration, veining, and mineralization in ultramafic rocks. Here we examine a mantle section of the Samail ophiolite exposed in Wadi Fins in southeastern Oman where the peridotite is unconformably overlain by Late Cretaceous‐Paleogene limestone and crosscut by an extensive network of carbonate veins and fracture‐controlled alteration. Three previous 87Sr/86Sr measurements on carbonate vein material in the peridotite produce results consistent with vein formation involving Cretaceous to Eocene seawater (de Obeso & Kelemen, 2018, https://doi.org/10.1098/rsta.2018.0433). We employ (U‐Th)/He chronometry to constrain the timing of hydrothermal magnetite in the calcite veins in the peridotite. Magnetite (U‐Th)/He ages of crystal sizes ranging from 1 cm to 200 μm record Miocene growth at 15 ± 4 Ma, which may indicate (1) fluid–rock interaction and carbonate precipitation in the Miocene, or (2) magnetite (re)crystallization within pre‐existing veins. Taken together with published Sr‐isotope values, these results suggest that carbonate veining at Wadi Fins started as early as the Cretaceous, and continued in the Miocene associated with magnetite growth. The timing of hydrothermal magnetite growth is coeval with Neogene shortening and faulting in southern Oman, which points to a tectonic driver for vein (re)opening and fluid‐rock alteration.

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

“…Neal & Stanger, 1984;Chavagnac et al 2013;Giampouras et al 2020), where serpentinization-derived fluids mix with meteoric waters. Earlier evidence of serpentinization of the Semail Ophiolite comes from Wadi Fins in SE Oman where there are veins of calcite and subordinate dolomite within a host rock of serpentinized peridotite, associated with deep clastic dykes filled with sedimentary carbonates (de Obeso & Kelemen, 2018Cooperdock et al 2020). These calcite veins have 87 Sr/ 86 Sr ratios indicative of an age around the Cretaceous-Paleocene boundary (de Obeso & Kelemen, 2018) and (U-Th)/He values in hydrothermal magnetite in the veins, which give an age estimate of 15 ± 4 Ma, equating to the Miocene (Cooperdock et al 2020).…”

“…Earlier evidence of serpentinization of the Semail Ophiolite comes from Wadi Fins in SE Oman where there are veins of calcite and subordinate dolomite within a host rock of serpentinized peridotite, associated with deep clastic dykes filled with sedimentary carbonates (de Obeso & Kelemen, 2018Cooperdock et al 2020). These calcite veins have 87 Sr/ 86 Sr ratios indicative of an age around the Cretaceous-Paleocene boundary (de Obeso & Kelemen, 2018) and (U-Th)/He values in hydrothermal magnetite in the veins, which give an age estimate of 15 ± 4 Ma, equating to the Miocene (Cooperdock et al 2020). Cooperdock et al (2020) interpreted these calcite veins as having formed over a considerable period of time through the interaction of the Semail Ophiolite mantle peridotites with pore fluids derived from overlying Cretaceous and Paleocene limestone formations.…”

“…These calcite veins have 87 Sr/ 86 Sr ratios indicative of an age around the Cretaceous-Paleocene boundary (de Obeso & Kelemen, 2018) and (U-Th)/He values in hydrothermal magnetite in the veins, which give an age estimate of 15 ± 4 Ma, equating to the Miocene (Cooperdock et al 2020). Cooperdock et al (2020) interpreted these calcite veins as having formed over a considerable period of time through the interaction of the Semail Ophiolite mantle peridotites with pore fluids derived from overlying Cretaceous and Paleocene limestone formations. The JHM shows that similar processes started earlier, possibly in late Campanian to early Maastrichtian time, soon after the end of ophiolite obduction onto the Arabian continental margin (Searle & Cox, 1999).…”

“…Whilst serpentinization-derived fluid seepage and associated carbonate mineral formation is now well known in modern settings, direct evidence for these processes in the geological record is sparse and comes largely from carbonate veins in ophiolites, often called ophicalcites (e.g. Lavoie & Chi, 2010;Klein et al 2015;Lafay et al 2017;de Obeso & Kelemen, 2018Cooperdock et al 2020). Nevertheless, the process is of great interest because of the astrobiological implications of serpentinization through the abiogenic formation of organic molecules (Proskurowski et al 2008;Lang et al 2010).…”

Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite

“…Grinding samples prior to thermal degassing to separate the contributions from fluid inclusions and bulk helium is a standard procedure for 3 He measurements for phases such as olivine (e.g., Kurz, 1986). In order to prevent the potential adsorption of helium during crushing (Protin et al, 2016), samples were ground in a mortar and pestle under ethanol (see Cox et al, 2017).…”