Mineral systems, hydridic fluids, the Earth’s core, mass extinction events and related phenomena

Abstract

We argue that hydridic fluids from the deep-earth are an important fluid type in mineral systems. The Carboniferous through Triassic interval of Earth history is used to illustrate our hypothesis that flux of hydridic fluid is a causative link between many earth processes such as mass extinction, evolution of ocean chemistry , climate change, anoxia, large-scale volcanism and mineral systems. The Earth's core is considered the dominant reservoir of hydrogen. An enhanced flux of hydridic fluids mobilizes the mantle and sustains tectonism and metallogenesis over 100s of millions of years. 1 Why are metal resources rare? Metal accumulations within the Earth are rare occurrences and the accumulations of high grade, large tonnage metal deposits much more so. Most models of formation of these deposits are based on some combination of processes that operate within the middle to upper crust and the hydro-and bio-sphere: metamorphism, basin dewatering, de-volatilization of magmas, sea floor metamorphism, meteoric fluid circulation, fluvial win-nowing and detrital accumulations. But these are relatively common Earth processes. Arguably such common processes ought to give rise to more common occurrences of the giant deposits. So is the "special-ness" of the major metal accumulations a function of rare combinations of common processes or have we yet to understand some key elements of the mineral systems that produced the giant deposits? We argue that anhydrous fluids, composed dominantly of H 2-CH 4-H 2 S (hydridic fluids in the sense of Larin, 1993) are an important fluid type in the mineral systems that produced the Earth's giant mineral deposits and provinces. The hints from the available stable and radiogenic isotope data and from mineral systems such as the Archean gold provinces, that afford an opportunity to study a crustal section, is that these fluids are sourced from deep within the earth. In this contribution we explore the questions of "where" within the earth, what are the properties of such fluids at P and T and what other earth processes could have been affected, possibly effected, by such fluids? 2 Sources and potential reservoirs of hydridic fluids: The Earth's core? Given present knowledge of the Earth, two obvious reservoirs of hydridic fluids are within serpentinized mantle wedges of subduction zones and within the Earth's core, or at least the outer parts of the core. Sleep et al. (2004) discuss the formation of H 2 saturated fluids from serpentinization of ultramafic rocks. Magnetite and awaruite (FeNi 3) catalyze methane and organic matter formation abiotically within serpentinite. Such extremely reduced fluids will not be stable in the presence of water bearing silicate melts, thus limiting potential sites of hydridic fluid reservoirs to relatively cold, serpentinized upper mantle. The "flat slab" setting is one possible loci in modern arc settings and interestingly there is a correlation of some giant porphyry and epithermal deposits with zones of low angle subduction in several of the important mineral provinces of the Pacific Rim. Recently Ranero and Sallarès (2004) have provided geophysical evidence (anomalously low seismic velocities and densities of the crust and upper mantle) for serpentinization of the crust and mantle of the Nazca plate during bending at the north Chile trench. Alternatively the hydridic fluids may be sourced from the core of the Earth. Williams and Hemley (2001) reviewed the development over the last seventy years of the concept of a hydridic core, noting the importance of the experimental work (Antonov et al. 1980) in demonstrating the significant increase in affinity between hydrogen and iron with increasing pressure such that a sto-ichiometric iron hydride forms at high pressure. Okuchi (1997) concluded that hydrogen may be the primary light element in the core accounting for the major part of the 10% density deficiency of the outer core. 3 Hydridic fluids and metallogenic epochs? Some first order observations about the temporal distribution of giant deposits through Earth history provide clues as to the dominant source of H 2. Gold deposits are distributed heterogeneously through Earth history with two Precambrian peaks at about 2800 to