Tensile Strength of Serpentinized Harzburgite

Abstract

ABSTRACT: Ultramafic rocks play an active role in geologic processes including the carbon cycle and serpentinization reactions. Many of these processes accompany mineral transformations that result in volume increase of the product phase (e.g. serpentinization, carbonation). Accommodation of changes in volume is often associated with formation of fracture surfaces as new minerals form. Thus, fracture properties such as tensile strength or fracture toughness are of interest in analyses of volumetric deformation during mineral replacement. Cores extracted from Oman ophiolite were used to measure the tensile strength of the ultramafic matrix. A specimen of serpentinized harzburgite was machined with one notch and used for a three-point bending test within a closed-loop, servo-hydraulic system with crack opening displacement (COD) as the feedback signal to achieve controlled fracture. Acoustic emission (AE) was used to provide additional details on the nature of crack propagation. 1. INTRODUCTION Coleman (1977) and Evans (2004) argue that an increase in volume during serpentinization of peridotites can be explained by immobility of Mg or Si (i.e. similar MgO/SiO2 ratios between protolith and serpentine equivalents) and formation of brucite. Field evidence of kernel structures (O'Hanley 1992; Evans 2004) is also in favor of volume increase during hydration processes. Hess (1955) suggested hydration of peridotites below the Mohorovicic (Moho) boundary can cause ocean floor rise, where the volume increase is equal to the volume of water added. The reaction describing the formation of serpentine minerals in closed system (constant composition) can be written to conserve Si and Mg as: (equation) The reaction in Equation (1), however, requires (i) continuous circulation of hydrothermal fluid in the rock mass, and (ii) access to fresh minerals enabling dissolution and precipitation to move forward. Potential clogging of the fracture network through vein formation, therefore, can stop the processes of serpentinization or carbonation if the hydrothermal fluids carry dissolved inorganic carbon resulting in dihedral angles greater than 60 degrees (Watson and Brenan 1987). Accommodation of changes in volume is often associated with formation of new fracture surfaces as new minerals form. Thus, fracture properties such as tensile strength or fracture toughness are of interest in analyzing of consequences of volumetric deformation during mineral replacement processes. We designed a suite of experiments to investigate tensile strength of a harzburgite, where evidence for present day serpentinization and volume change has been documented (Evans 2004). 2. THEORY Formation of fracture surfaces in the matrix surrounding the ultramafic mineral undergoing serpentinization or carbonation and hence volume increase requires local stresses to equal the tensile strength of the matrix. The nominal strength, i.e. nominal stress at peak load, of geometrically similar specimens can be defined as (Bazant and Planas 1998): (equation)