Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

Abstract

The Oman Drilling Project “Multi‐Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This article reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites, and replacive. Dunites show evidence for “reactive fractionation,” in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65%–100% hydrated. Ferric to total iron ratios vary from 50% to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30–150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14, and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite, and olivine. fO2 deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H2O‐H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores versus oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth.

Listvenite Formation During Mass Transfer into the Leading Edge of the Mantle Wedge: Initial Results from Oman Drilling Project Hole BT1B

“…These partially serpentinized peridotites commonly show a "mesh texture", with relict olivine and pyroxene "cores" transected by a "mesh" of cross-cutting serpentine veins-typically 10 to 100 microns apart (Francis, 1956;Green, 1961Green, , 1964Raleigh & Paterson, 1965). In some regions within the Samail ophiolite, particularly areas of relatively subdued topography that have undergone extensive, penetrative weathering, relict mantle minerals in the mesh cores are completely replaced by serpentine (e.g., OmanDP Sites BA1, BA2, BA3 and BA4, Kelemen, Matter, et al, 2020;Kelemen et al, 2021). However, along the steep canyons and narrow ridges that are typical of outcrops in the mantle section of the ophiolite, subject to relatively rapid erosion, the pervasive presence of the serpentine vein mesh surrounding relict mantle minerals attests to relatively rapid fluid transport in fractures and veins, compared to slow transport of H 2 O into the mesh cores by diffusion and/or imbibition.…”

Gas Migration Episodes Observed During Peridotite Alteration in the Samail Ophiolite, Oman

“…A variety of petrological, hydrological, geochemical, and microbiological investigations have been carried out for borehole BA1B (e.g., Cocomazzi et al., 2020 ; Ellison et al., 2021 ; Kelemen et al., 2021 ; Ternieten et al., 2021 ), providing important context for interpreting our results. We cannot determine the composition of the bubbles we detected on the basis of our acoustic data, but peridotite alteration (Nothaft et al., 2021 ), and microbial processes supported by alteration products (Schrenk et al., 2013 ), can both generate H 2 and CH 4 gas.…”

“…Theoretical (Kelemen & Hirth, 2012 ), laboratory (Xing et al., 2018 ; Zhu et al., 2016 ), and numerical simulations (Malvoisin et al., 2020 ; O. Evans et al., 2020 ) generally assume that gases generated by peridotite alteration (H 2 and CH 4 ) are dissolved into the reaction product fluids, and that the fluids flowing through the formation are single‐phase liquids. H 2 and CH 4 gases can be observed bubbling up in alkaline springs in some peridotite terrains (Kelemen et al., 2021 ; Leong et al., 2021 ), but it has been hypothesized that this represents the near‐surface exsolution of dissolved gases produced at higher pressures deeper in the subsurface (Sleep et al., 2004 ). The fate of the gas produced by peridotite alteration, and how it is partitioned between dissolved and free gas phases, has important consequences for fluid flow.…”

“…Most peridotite terrains are located in the deep sea, making them difficult to study. However, the Oman Drilling Project (OmanDP) (Kelemen et al., 2020 ) has provided a unique opportunity to advance our understanding of peridotite alteration by establishing a Multi‐Borehole Observatory (MBO) in an actively serpentinizing terrain of the Samail ophiolite (Kelemen et al., 2021 ). Four, 400 m deep boreholes were drilled within a ∼100 × 100 m region (Figure 1 ), providing an opportunity to study the geological and alteration history of the uplifted mantle rocks, and to monitor presently occurring alteration processes.…”

An untargeted metabolomics approach to characterize dissolved organic matter in groundwater of the Samail Ophiolite

“…The Samail Ophiolite consists of a large (~15,000 km 3 ) block of ocean crust and upper mantle (Nicolas et al, 2000) that was rapidly emplaced onto the Arabian continental plate approximately 70 million years ago (Glennie et al, 1973; Coleman, 1981; Tilton et al, 1981; Hacker et al, 1996). Here, mafic and ultramafic rocks that once underwent alteration in seafloor hydrothermal systems continue to be altered by active serpentinization on the continent (Canovas et al, 2017; Keleman et al, 2021). We used an untargeted metabolomics approach (Patti et al, 2012) to characterize compounds in the dissolved organic matter (DOM) pool of groundwaters collected from varied geochemical conditions, using metagenomic data acquired from biomass filtered from the same samples to verify the annotation of metabolomic features.…”