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

Geophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VSdue to the occurrence of cracks. However, the anomalously low VP/VSobserved at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VSdue to crack development. In this study, we present the first evidence that shows the low VP/VSin the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model. The low velocity and high VP/VSstructure in the upper oceanic crust (“seismic layer 2”) is generally associated with the development of fluid‐filled thin cracks. However, the anomalously low VP/VSratios that have been observed in the layer 2/3 transition cannot be explained by the classical thin crack model. We provide the first direct evidence of low VP/VSratios in epidotized rocks that are associated with the occurrence of spheroidal pores and quartz precipitation. Our theoretical calculations show that both the crack geometry and alteration minerals are required to explain the anomalously low VP/VSratios in the transition zone. Knowledge of these unique physical properties of epidotized rocks will facilitate the detection of fluid upwelling zones in the oceanic crust, which are possibly linked to the hydrothermal ore resources. Epidosites are characterized by low VP/VSdue to the presence of spheroidal poresLow VP/VSanomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotizationThe unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust Epidosites are characterized by low VP/VSdue to the presence of spheroidal pores Low VP/VSanomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotization The unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust