Experimental Alteration of Microfractured-core and Powdered Mantle Rocks: Comparison of Reaction Rims, Product Minerals and

Hydrothermal Fluid Chemistry

M. Rough¹*, K. Ding¹ & W.E. Seyfried, Jr.¹

Corresponding author: Mikaella.rough@gmail.com
¹University of Minnesota, Geology and Geophysics Department, Minneapolis, MN, 55455

Abstract:
Fluid-rock interaction experiments simulating deep-ocean hydrothermal environments have typically been conducted in a reaction cells using powdered rock samples and aqueous fluids at high-temperatures and pressures. This experimental technique has provided substantial information on phase relationships and the hydrothermal fluids resulting from seawater-rock interactions, though there are important limitations. For example, damage to reactant minerals during sample processing can enhance formation of metastable minerals not encountered in drill cores from the ocean crust. Moreover, the closed system nature of the experiments precludes information bearing subsurface permeability changes that occur as fluid-rock interaction progresses.

To obtain information on alteration and permeability changes within undamaged rock, we have constructed a fluid flow through system that will function within the basic framework of the Paterson apparatus (Paterson, 1970; Paterson, 1989). Titanium separators are connected to the pore fluid system outside the central sample assembly. The flow of argon gas controls the confining pressure and provides a driving force to the separator, pushing a Ca-bearing evolved seawater solution through the sample in both the upstream and downstream directions, yielding information on time series changes in permeability and chemistry.

The first series of experiments are being conducted on troctolite samples composed of 60% plagioclase, 10% olivine, and 30% pyroxene using both the flexible reaction cell and Paterson apparatus at 500 bars and 420°C. This will provide a direct comparison between the two experimental techniques, while presenting a comprehensive experimental overview of the reactions occurring in mixed gabbro-peridotite rock reaction zones. Results can be applied to the root zones of MOR hydrothermal systems which play a role in the compositional evolution of seafloor vent fluid chemistry.

Contributions to Integration and Synthesis:
Mechanisms for seawater flow through the ocean crust have been predicted using OBS data indicating the effects of thermal cracking near the axial magma chamber, chronicling alteration patterns and temperatures of alteration in exposed sections of the sheeted dike complex at Pito Deep and Hess Deep, and observing the porosity structure and alteration patterns of Ophiolite complexes. All of these analyses provide information bearing on the mechanism of fluid flow through the ocean crust, although these data do not provide information on how the permeability changes as fluid-rock interaction progresses and the timescales necessary to produce the alteration observed in field samples.

Our experiments with the flexible reaction cell and flow-through Paterson apparatus will provide the framework for determining how seawater flows beneath the mid-ocean ridge vent fields. This approach is applicable to all vent environments, including the Juan de Fuca ridge, 9°N, and Lau Basin systems.