Characterizing Hydration of the Ocean Crust Using Shortwave Infrared Microimaging Spectroscopy of ICDP Oman Drilling Project Cores

Michelle Harris,George R Rossman,Peter B Kelemen,Bethany L Ehlmann,Molly A Crotteau,Rebecca N Greenberger,Damon A H Teagle

doi:10.1029/2021jb022676

Abstract

AbstractAlthough ocean crust covers over 60% of Earth's surface, the processes that form, cool, and alter the ocean crust are not completely understood. We utilize shortwave infrared micro‐imaging spectroscopy of ∼1.2 km of rock cored by the International Continental Scientific Drilling Program's Oman Drilling Project to quantify hydration of basaltic dikes and gabbros from the Samail ophiolite as a function of depth, mineralogy and deformation. We develop a regression (R2 = 0.66) between area of the ∼1,350–1,650 nm OH/H2O absorption and measurements of loss on ignition of samples and apply this relationship to generate quantitative ∼250 μm/pixel hydration maps for all cores. The lowest mean hydration is observed in the most pervasively altered dike‐gabbro boundary (GT3A, H2Omean = 2.1 ± 1.6 wt%), consistent with the low H2O content of the dominant alteration minerals, amphibole and epidote. The highest H2O content occurs in deeper foliated and layered gabbros (GT2A, H2Omean = 3.2 ± 3.0 wt%) and layered gabbros (GT1A, H2Omean = 2.8 ± 3.1 wt%). The greater prevalence with depth of zeolite alteration as opposed to lower wt% H2O amphibole at shallow stratigraphic depths, as well as the occurrence of zones of intensive hydration associated with fault zones (H2Omean = 5.7 ± 4.0 wt%) lead to greater hydration of the lower ocean crust. This new approach provides an objective quantification of hydration in these cores, enabling an improved understanding of quantities and characteristics of ocean crust hydration. It highlights the importance of specific phases and faulting in controlling hydration, which has implications for ocean crust cooling, rheological properties, and the role of alteration in global biogeochemical cycling.

Highlights

The ocean crust covers over 60% of the Earth, it is not completely understood how the basaltic and gabbroic ocean crust forms, cools, and is chemically altered by seawater
In order to determine the relationship between hydration and integrated band area of the four different calculated OH/H2O band areas, we considered the loss on ignition (LOI) measurements obtained from whole rock samples in Holes GT1A, GT2A, and GT3A
Our results indicate that the majority of the ocean crust contains 0.5 – 3 wt% H2O, and that overall ocean crust hydration ranges from 0 to ~18 wt% H2O

Summary

Introduction

The ocean crust covers over 60% of the Earth, it is not completely understood how the basaltic and gabbroic ocean crust forms, cools, and is chemically altered by seawater. Study of the magmatic accretion and hydrothermal exchange processes in the ocean crust has required the investigation of ophiolites: ancient sections of ocean crust and upper mantle formed at some style of ocean spreading center, preserved on-land. The International Continental Scientific Drilling Program (ICDP) Oman Drilling Project (OmanDP) has drilled cores from ocean crust exposed in the mountains of Oman (Kelemen et al, 2020; Kelemen et al, 2013). These cores provide selected continuous sections of oceanic crust representative of specific intervals of the mid to lower ocean crust preserved in the Samail ophiolite. On-land diamond coring returned cores with near-100% recovery avoiding the drill sampling bias that commonly affects igneous rock sampling by scientific ocean drilling (e.g. Tominaga et al, 2009)

Objectives

Methods

Results

Discussion

Conclusion