TEACHING & RESEARCH
- Ph.D., l973, Geology and Geophysics, University of California at Berkeley
- Ingeniero de Minas, l967, School of Mining Engineering, Madrid
After an early interest in water–rock equilibrium relations in sedimentary basins and in calculating the distribution of species dissolved in natural waters, I became interested in the dynamics of geochemical phenomena. It is through its reaction–transport dynamics, not through equilibrium, that a geochemical system may develop a characteristic spatial repetitive pattern. With collaborators, I studied stylolitization and metamorphic banding, intracrystalline oscillatory zoning of trace elements in calcite, orbicular zoning, the genesis of agate banding, the genesis of zebra veins in dolomites and in serpentinized ultrabasics, and the genesis of Banded Iron Formations – all cases of what I called in 1984 geochemical self–organization, and all produced by disequilibrium and feedback. Understanding and modeling each of those self–organizational patterns requires finding out which feedbacks take place in its genesis and incorporating them into the continuity equation. Petrography is essential, both to help construct the reaction–transport models and to check their spatial predictions.
Starting in 1990 I became interested in the geochemical dynamics, or chemical geodynamics, of larger scale phenomena, weathering and dolomitization, ore deposit genesis and metamorphism. Since these involve mineral replacement, understanding their dynamics required first understanding how replacement happens. Replacement of B by A is identified by its characteristic spatial properties – to preserve both volume and morphological details (as ghosts). These properties in turn require that A growth and B dissolution be strictly simultaneous and take place at mutually equal rates. The coupling factor that equalizes the rates turns out to be the growth–driven stress. The guest mineral, via the local stress it generates as it grows, pressure-dissolves the adjacent host mineral. The induced stress self–adjusts so as to always equalize the volumetric rates of guest mineral growth and host pressure–solution: this is why replacement preserves solid volume – the feature that petrographers have long reported. This discovery has led to insights into the dynamics of weathering (Merino et al., 1993), of bauxite (or terra rossa) formation (Merino and Banerjee, 2008), of metamorphic reactions such as the replacement of periclase by brucite in marbles, and of burial dolomitization (Merino and Canals 2011)
For example, we have discovered petrographically that the red clays known as terra rossa, or bauxite, form not residually or as a sediment as long held, but by replacement of the underlying limestone – a surprise to geochemists and soils scientists – and that the replacement, by releasing acid which generates additional dissolution porosity, may trigger the reactive–infiltration instability that "carves" the dissolution funnels and sinkholes of the karst limestone that contains the terra rossa itself – a surprise to geomorphologists. The chemical dynamics imposed by the replacement ends up explaining why bauxite and karst are associated.
Understanding the physics of replacement helps unravel the old problem of burial dolomitization, a paradigm of metasomatism: the dolomite–for–calcite replacement, because it is self–accelerating via the Ca2+ pore fluid concentration, because it happens by pressure–solution, and because crystalline carbonates are strain–rate–softening, spontaneously passes – continuously – from replacive to displacive dolomite growth, and this is why characteristic sets of displacive, self–organized dolomitic zebra veins occur in burial dolostones the world over.