Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges

Abstract

40To understand how the mantle cools beneath mid-ocean ridge spreading centers, we 41 applied a REE-in-two-pyroxene thermometer and major element thermometers to peridotites 42 from the Wadi Tayin massif in the southern part of the Samail ophiolite in the Sultanate of 43Oman, which represent more than 10 km of structural depth beneath the paleo-Moho. Closure 44 temperatures for REEs in pyroxenes deduced from the REE-in-two-pyroxene thermometer (T REE ) 45 decrease smoothly and systematically with depth in the section, from >1300°C near the crust to 46 <1100°C near the metamorphic sole, consistent with previously observed, similar variations in 47 mineral thermometers with lower cooling temperatures. Estimated cooling rates decrease from 48 ~0.3 °C/y just below the crust-mantle transition zone (MTZ) to ~10 -3 °C/y at a depth of six km 49 below the MTZ. Cooling rates derived from Ca-in-olivine thermometry also decrease moving 50 deeper into the section. These variations in cooling rate are most consistent with conductive 51 cooling of the mantle beneath a cold overlying crust. In turn, this suggests that hydrothermal 52 circulation extended to the MTZ near the axis of the fast-spreading ridge where the igneous crust 53 of the Samail ophiolite formed. These observations are consistent with the Sheeted Sills model 54 for accretion of lower oceanic crust, and with previous work demonstrating very rapid cooling 55 rates in the crust of the Wadi Tayin massif. Our observations, combined with previous results, 56 suggest that efficient hydrothermal circulation beneath fast spreading centers cools the 57 uppermost mantle from magmatic temperatures to <1000°C as quickly as tectonic exhumation at 58 amagmatic spreading centers. In contrast, thermometers sensitive to cooling over lower 59 temperature intervals indicate that the Wadi Tayin peridotites cooled more slowly than 60 tectonically exhumed peridotites sampled near the seafloor along mid-ocean ridges. 61Hydrothermal cooling of the crust may have waned, so that the crust-mantle package cooled 62 3 more slowly, whereas rapid cooling of abyssal peridotites during tectonic exhumation continued 63 to seafloor temperatures. 64 65

Constraints on Cooling of the Lower Ocean Crust From Epidote Veins in the Wadi Gideah Section, Oman Ophiolite

“…Geospeedometry‐based estimates of rapid magmatic cooling rates (e.g., Sun & Lissenberg, ; VanTongeren et al, ) require circulation of seawater to the base of the crust very near the ridge axis. Initial cooling (from 1300 to 1100 °C) of the upper mantle is also fast, consistent with the idea that the overlying crust was already cooled (Dygert et al, ). Other geospeedometry studies (Faak et al, ; Faak & Gillis, ) indicate much slower cooling rates in the lower oceanic crust and hence less requirement for deep near‐axis seawater circulation.…”

“…Other geospeedometry studies (Faak et al, 2015;Faak & Gillis, 2016) indicate much slower cooling rates in the lower oceanic crust and hence less requirement for deep near-axis seawater circulation. Dygert et al (2017) suggested that perhaps the geospeedometers more sensitive to cooling at lower temperatures (e.g., Ca-in-olivine) provide lower cooling rates than rare earth element (REE)-based geospeedometers, which record cooling at T > 1000°C (Sun & Lissenberg, 2018). This may indicate that cooling rates slow down as temperatures drop below around 1000°C (Dygert et al, 2017).…”

“…Dygert et al (2017) suggested that perhaps the geospeedometers more sensitive to cooling at lower temperatures (e.g., Ca-in-olivine) provide lower cooling rates than rare earth element (REE)-based geospeedometers, which record cooling at T > 1000°C (Sun & Lissenberg, 2018). This may indicate that cooling rates slow down as temperatures drop below around 1000°C (Dygert et al, 2017).…”

Great Basin Mantle Xenoliths Record Active Lithospheric Downwelling Beneath Central Nevada

“…The three thermometers can be used together to estimate the thermal stability of a geologic environment and the rate at which it heated or cooled. In environments where cooling is driven by tectonic processes (which are slow compared to volcanic processes), T REE > T BKN ≳ T Ca‐ol (e.g., Dygert et al, ; Dygert & Liang, ). By contrast, some mantle xenolith suites originate from thermally equilibrated environments and were entrained and quenched rapidly by volcanic eruption; these exhibit similar temperatures for all thermometers (e.g., Witt‐Eickschen & O'Neill, ; Figure S5).…”

Plate tectonic cycling modulates Earth's 3He/22Ne ratio

“…Shallow regions of subducting lithosphere presumably lose noble gases and other volatiles from pore fluids (Sumino et al, 2010) and hydrous phases (Jackson et al, 2013b;Kendrick et al, 2011; owing to cracking, heating and prograde metamorphism (e.g., Chavrit et al, 2016;Holland and Ballentine, 2006;Staudacher and Allègre, 1988;Smye et al, 2017). Isotopic, lithologic, and geospeedometric evidence suggests hydration beneath oceanic spreading centers is limited to approximately the upper km of the mantle section (Dygert and Liang, 2015;Dygert et al, 2017;Gregory and Taylor, 1981;Rospabe et al, 2017). Seismic reflection-refraction data suggest hydration of old lithospheric slabs is restricted to the upper 10km of mantle lithosphere (e.g., Han et al, 2016;Van Avendonk et al, 2011).…”