Does Anisotropy of Thermal Contraction Control Hydrothermal Circulation at the Moho Level below Fast Spreading Oceanic Ridges?

Abstract

It has been proposed that deep, high-T (up to 1000°C) hydrothermal alteration occurred within the gabbro section, near the paleo-ridge axis in the Oman ophiolite. In the deep, hot gabbros, the main water channels are submillimetric microcracks with a dominantly vertical orientation (Nicolas et al., 2003). Sr and O isotopic investigations point to seawater as the most likely hydrothermal contaminant (Bosch et al., 2004). It was proposed that the mechanism for seawater ingression at temperatures above 700°C was anisotropy of thermal contraction in gabbros, opening microcracks that are controlled by crystallographic fabrics. The exceptionally large anisotropy of thermal contraction of single-crystal calcic plagioclase, when combined with the lattice fabrics of the gabbros, results in the largest thermal contraction being parallel to the mineral lineation, which is itself normal to the ridge, thus inducing vertical fractures parallel to the ridge plane. High-T hydrothermal alteration in gabbros extends to the Moho. Interestingly, in the underlying peridotites, with a dominantly horizontal foliation, the direction of largest thermal contraction, calculated as above from fabrics and thermal expansion coefficients of olivine, is vertical, potentially responsible for subhorizontal cracking. Thus, during the off-axis drift of newly accreted lithosphere, at the limits of the axial magma chamber, thermal contraction opens vertical cracks in the crustal gabbros, favoring seawater ingression down to the Moho. Just below the Moho, the horizontal cracking preferred orientation could be closing the high-temperature hydrothermal circuit at this level. At temperatures below 700°C, oceanic ridge spreading-related tensile stress is dominant, inducing ridge-parallel cracking in both the crust and lithospheric mantle. Circulation of seawater-derived fluids in the Moho transition zone at gabbro solidus conditions should have important consequences for magmatic processes.

Mechanical anisotropy control on strain localization in upper mantle shear zones

“…Note the inherited CPO of the porphyroclastic fabrics with the E‐W alignment of the b axis. (b) Diagram displays changes in thermal contraction of olivine and enstatite along the different crystallographic axes during progressive cooling using thermal expansion coefficients given in Boudier et al []. Contraction is given in meters per intervals of 500 m across the shear zone.…”

“…The latter cannot be excluded during ductile fracturing, particularly in cases where accelerated crack propagation may occur under the presence of interconnected melt [ Weinberg and Regenauer‐Lieb , ]. In any case, vertical fracturing additionally might be facilitated by the anisotropy of thermal expansion/contraction of olivine ( b > c > a axes, see Boudier et al []). Given the inherited orientation of olivine in porphyroclastic harzburgite, with the a axis parallel to the principal stretching direction σ 1 , ridge parallel dilatancy would support the evolution of the vertical dike systems (Figure ).…”

Fluid Infiltration Through Oceanic Lower Crust in Response to Reaction‐Induced Fracturing: Insights From Serpentinized Troctolite and Numerical Models

“…Various types of alteration have been recognized in the crust and mantle sections of the Oman ophiolite, including (1) seawater penetration and hydrothermal circulation on the seafloor (e.g., Bosch et al, 2004;Boudier et al, 2005;Gregory & Taylor, 1981;Manning et al, 2000;Nicolas et al, 2003); (2) alteration by fluids released from the metamorphic rocks during subduction and obduction (e.g., Alabaster et al, 1982);…”

“…Various types of alteration have been recognized in the crust and mantle sections of the Oman ophiolite, including (1) seawater penetration and hydrothermal circulation on the seafloor (e.g., Bosch et al, 2004; Boudier et al, 2005; Gregory & Taylor, 1981; Manning et al, 2000; Nicolas et al, 2003); (2) alteration by fluids released from the metamorphic rocks during subduction and obduction (e.g., Alabaster et al, 1982); (3) surface and subsurface alteration by meteoric waters (e.g., Kelemen & Matter, 2008). For example, Sr‐O isotopic data for rocks in crustal sections are indicative of HT hydrothermal circulation on the seafloor (Bosch et al, 2004; Gregory & Taylor, 1981), whereas the Sr isotopic compositions of clinopyroxene suggest that the basal peridotites of the Fizh massif were influenced by the fluid that originally derived from the subducted slab (Yoshikawa et al, 2015).…”

Modes of crustal accretion and their implications for hydrothermal circulation

“…Permeability decreases from 600°C to the final cutoff temperature of 800°C above which no fluid circulation is assumed [ Hasenclever et al, ]. This higher temperature is in agreement with observations from the Oman ophiolite that describe evidence for fluid circulation at very high temperatures of up to 800–900°C [ Boudier et al, ; Koepke et al, ; Nicolas and Mainprice , ].…”