Mineral Evolution

8. MINERAL EVOLUTION

At a Christmas party on December 6, 2006, Harold Morowitz asked a simple question: "Were there clay minerals in the Archean?" It's a simple question, with important implications for origin-of-life models that depend on clays. But the answer is not at all obvious. A year later I've developed a manuscript with Dominic Papineau (CIW), Wouter Bleeker (Geological Survey of Canada), Robert Downs (University of Arizona), John Ferry (Johns Hopkins), Tim McCoy (Smithsonian), Dimitri Sverjensky (Johns Hopkins), and Hexiong Yang (University of Arizona) on the subject, which essentially reframes the science of mineralogy. The paper has been submitted to American Mineralogist. Here is an abstract of the idea.

The mineralogy of terrestrial planets and moons evolves as a consequence of selective physical, chemical and biological processes. In a stellar nebula prior to planetary accretion, unaltered chondritic material with approximately 60 different refractory minerals represents the starting point of mineral evolution. Subsequent aqueous and thermal alteration of chondrites, asteroidal accretion and differentiation, and the consequent formation of achondrites results in a mineralogical repertoire limited to the approximately 250 minerals now found in unweathered lunar and meteorite samples.

Following planetary accretion and differentiation, the mineral evolution of a terrestrial planet depends initially on a sequence of geochemical and petrologic processes, which depend principally on the size and volatile content of the body. These processes may include volcanism and degassing, fractional crystallization, crystal settling, assimilation reactions, regional and contact metamorphism, plate tectonics and associated large-scale fluid-rock interactions. In addition to the familiar igneous lithologies of Bowen’s reaction series, these processes may result in the formation of pegmatites, hydrothermal ore deposits, metamorphic terrains, evaporites, and zones of surface weathering. According to some origin-of-life scenarios, a planet must progress through at least some of these stages of chemical processing as a prerequisite for life.

Biological processes began to affect Earth’s surface mineralogy by the Paleoarchean (~3.8 Ga), when large-scale surface mineral deposits, including carbonate and banded iron formations, were precipitated under the influences of changing atmospheric and ocean chemistry. The Paleoproterozoic “great oxidation event” (2.2 to 2.0 Ga), when atmospheric oxygen may have risen to >1% of modern levels, and the Neoproterozoic increase in atmospheric oxygen following several major glaciation events, gave rise to multicellular life and skeletal biomineralization and irreversibly transformed Earth’s surface mineralogy.

Sequential stages of mineral evolution arise from three primary mechanisms: (1) the progressive separation and concentration of the elements from their original relatively uniform distribution in the presolar nebula; (2) the increase in range of intensive variables such as pressure, temperature, and the activities of H₂O, CO₂ and O₂; and (3) the generation of far-from-equilibrium conditions by living systems. The sequential evolution of Earth’s mineralogy from chondritic simplicity to Phanerozoic complexity introduces the dimension of geologic time to mineralogy and thus provides a dynamic alternate approach to framing, and to teaching, the mineral sciences.

The general principles observed for the emergence of mineralogical complexity on Earth apply equally to any differentiated asteroid, moon or terrestrial planet. In every instance mineral evolution will occur in a progression of stages as a result of local, regional and global selective processes. The degree to which a body will advance in mineralogical complexity beyond the relatively simple achondrite stage is dictated by the nature and intensity of subsequent cycling (and hence repeated separation and concentration of elements). Consequently, a planet’s surface mineralogy will directly reflect the extent to which cyclic processes have affected the body’s history. Accordingly, remote observations of the mineralogy of other moons and planets may provide crucial evidence for biological influences beyond Earth.

While an exact division of geological history into stages of mineral evolution is somewhat arbitrary, we propose that Earth's mineral evolution can be divided into at least ten stages, each of which increased the mineral diversity of the planet:

Stage 1. Chondrites (>4.56 Ga): the most primitive meteorites are also the simplest mineralogically, with only about 60 different primary minerals..

Stage 2. Asteroidal differentiation and achondrites (>4.55 Ga): Asteroidal melting produced metallic cores and rocky mantles.

Stage 3. Igneous rock evolution (4.55 to 4.0 Ga): Processes of volcanism and igneous rock differentiation.

Stage 4. Granitoid formation (4.0 to 3.5 Ga): Remelting of basalt and sedimentary deposits produced sequences of granitoid rocks.

Stage 5. Plate tectonics (>>3.0 Ga): Subduction resulted in large-scale reworking of crustal material.

Stage 6. Anoxic biological world (3.9-2.5 Ga): Precipitation of banded iron formations, carbonates and other massive deposits.

Stage 7. Great oxidation event (2.5 to 1.9 Ga): The flowering of photosynthetic microorganisms led to a dramatic rise in atmospheric oxygen, and associated formation of a host of oxidized, weathered minerals.

Stage 8. Intermediate ocean (1.9 to 1.0 Ga): At ~1.85 Ga the production of banded iron formations ceased relatively abruptly, signaling a significant change in ocean chemistry likely driven by microbial activity. This gradual change to an “intermediate ocean” appears to have resulted from increased microbial sulfide reduction and surface oxidation.

Stage 9. Snowball Earth (1.0 to 0.57 Ga): Multiple lines of evidence indicate that Earth experienced dramatic fluctuations in climate and atmospheric composition between about 1.0 and 0.57 Ga, with at least two (and possibly as many as four) snowball Earth events between about 0.75 and 0.57 Ga.

Stage 10. Phanerozoic Era (0.57 to present): By the beginning of the Phanerozoic Eon (0.57 Ga) biology came to dominate the mineralogical diversification of Earth’s surface.