Characterization of hydrothermal fluids that alter the upper oceanic crust to spilite and epidosite: Fluid inclusion evidence from the Semail (Oman) and Troodos (Cyprus) ophiolites

Rock‐Matrix Porosity and Permeability of the Hydrothermally Altered, Upper Oceanic Crust, Oman Ophiolite

“…As our porosity measurements were conducted at room P–T , the question arises as to whether the values are valid for the original in situ P–T conditions of active spilite and epidosite alteration at depth in the Semail crust. Stratigraphic reconstructions to the base of the SDC (Belgrano et al., 2019, 2021) and fluid inclusion data (Richter & Diamond, 2022) suggest that the studied alteration occurred at temperatures of 200–440°C and that maximum and minimum pressures through to the base of the SDC varied as follows: P confining = P ocean + P lithostatic = 44–190 MPa and P effective = P confining − P hydrostatic = 8–90 MPa (mean ≈ 50 MPa). As epidosite alteration mostly occurred during post‐axial Tholeiitic Alley time (Section 2.2), the maxima of these pressure ranges include the Tholeiitic Alley overburden above the axial SDC and Geotimes lavas.…”

“…The above alteration is estimated to have occurred at temperatures of ∼200–440°C at near‐hydrostatic fluid pressures of >31–54 MPa (taking into account a seafloor depth of ∼3,100–3,500 m b.s.l. ; Belgrano et al., 2021), at a range of depths up to 2,050 m below seafloor (Richter & Diamond, 2022). The post‐axial Tholeiitic Alley and Boninitic Alley lavas are regionally altered mostly to smectite + albite with pore‐fillings of zeolite and celadonite.…”

“…, well below the ∼3.2 wt.% NaCl equiv. seawater-like salinity of the hydrothermal fluids that caused the spilite and epidosite alteration (Richter & Diamond, 2022). Second, open fractures induced during obduction and subaerial weathering are ubiquitous and abundant.…”

Simultaneous Measurements of Elastic Wave Velocity and Porosity of Epidosites Collected From the Oman Ophiolite: Implication for Low VP/VS Anomaly in the Oceanic Crust

“…Seismic velocity at shallow depths is controlled mainly by the large‐scale cracks such as fissures, therefore the contribution of epidosites to the V P /V S structure could be limited at the deeper portions where most cracks are closed. Cyprus‐type volcanogenic massive sulfide deposits are considered to be associated with epidosites due to leaching of base and precious metals during hydrothermal alteration (Patten et al., 2016), although the Fe‐poor concentration of the epidosite fluids has led to debate for its connection to the seafloor resources (Richter & Diamond, 2022). In any cases, the unique geophysical characteristics of epidosites is effective for detecting fluid upwelling zones in the oceanic crust.…”

“…Epidosites are metasomatic products consisting mainly of epidote and quartz, which are commonly observed in ophiolite complexes, including the Oman (Gilgen et al., 2016; Juteau et al., 2000; Nehlig et al., 1994) and Troodos ophiolites (Cann et al., 2015; Jowitt et al., 2012; Schiffman et al., 1987). They are formed at major zones of upward fluid flow in the extensive hydrothermal convection systems that occur close to mid‐oceanic spreading axes (Juteau et al., 2000), and have been linked to the formation of the volcanogenic massive sulfide deposits (Gilgen et al., 2016; Schiffman et al., 1987), while their origin is still under debate (Richter & Diamond, 2022; Weber et al., 2021). The bottom of layer 2 nearly coincides with the occurrence of epidosites in the Cyprus‐type model and, consequently, extensive fluid‐rock interactions may explain the anomalously low V P /V S ratio in this part of the oceanic crust (Figure 1).…”