A few weeks ago I wrote about the revolution in thinking concerning granite formation which has taken place in the last couple of decades. In that post, I alluded to Andrew Snelling’s work on polonium (Po) radiohalos, the formation of which seems to impose severe constraints on the cooling history of granite bodies.
It soon became evident that there’s a lot of confusion among creationists and others about Snelling’s model for the origin of these radiohalos and how it compares with the original model proposed by another creationist, Robert Gentry. Although Gentry’s pioneering work did so much to bring radiohalos to popular attention, it is important that we realise that his model for their formation is not the same as Snelling’s.
Basically, Gentry proposed that the Po isotopes (which decayed to generate the halos) were instantaneously formed by divine fiat during Creation Week and that the rocks bearing them were thus primordial, created rocks. However, as Snelling and others have shown, this hypothesis is inconsistent with the abundant evidence (from field relationships and petrological data) that Po radiohalo-bearing granites were formed by the crystallization and cooling of magmas. Often the host rocks into which such granites have been intruded (and which are therefore older than the granites themselves) originated as fossil-bearing sediments thought by most creationists to have formed during the global Flood.
Snelling’s model, by contrast, is a hydrothermal fluid transport model, in which the Po isotopes were produced by U-238 decay in zircon crystals found in the same biotite mica flakes as the Po radiohalos (Snelling 2005). The hydrothermal fluids released by the crystallizing and cooling granite magma flowed along the biotite cleavage planes and transported the Po isotopes from the U-238 radiocentres. The Po isotopes were then precipitated in lattice defects along the same biotite cleavage planes where sulphur, chlorine and other atoms chemically attractive to Po were located, within a millimetre or so of the U-238 radiocentres. These Po precipitation sites became the radiocentres for the Po halos.
As the Po in the radiocentres decayed, new Po atoms were supplied from the hydrothermal fluids flowing through the mica. Thus, provided the supply of Po isotopes was sufficient and the hydrothermal fluid flows were sustained and rapid, the required Po concentrations would have been supplied to the radiocentres to generate the Po radiohalos within hours or days, consistent with the fleeting half-lives of the Po isotopes.
A constraining factor on the preservation of the Po radiohalos is that the alpha-particle damage left by the Po decay is retained in the mica flakes only below 150 degrees C. Above this annealing temperature, the damage either doesn’t register or is obliterated. Thus, all the radiohalos in the granites had to form below 150 degrees C, which is relatively late in the cooling history of granite. (Granite magmas when intruded are at temperatures of 650-750 degrees C, and the hydrothermal fluids are released at temperatures of 370-410 degrees C after most of the granite and its constituent minerals have crystallized).
“However, the accessory zircon grains with their contained 238U crystallize very early at higher temperatures, and may have even been already formed in the magma when it was intruded. Thus the 238U decay producing Po isotopes had already begun well before the granite had fully crystallized, before the hydrothermal fluids had begun flowing, and before the crystallized granite had cooled to 150ºC. Furthermore, by the time the temperature of the granite and the hydrothermal fluids had cooled to 150ºC, the heat energy driving the hydrothermal fluid convection would have begun to wane and the vigor of the hydrothermal flow would also have begun to diminish … The obvious conclusion has to be that if the processes of magma intrusion, crystallization, and cooling required 100,000–1 million years, then so much Po would have already decayed and thus been lost from the hydrothermal fluids by the time the granite and fluids had cooled to 150ºC that there simply would not have been enough Po isotopes left to generate the Po radiohalos …”
Thus, taking all these factors into account, Snelling has concluded that the granite intrusion, crystallization, and cooling processes must have occurred together over a timescale of only about 6-10 days. This is helpfully summarized in Figure 8 of Snelling (2008), reproduced here.
So to summarize:
According to Gentry, granites are primordial rocks, formed by divine fiat during the Creation Week, and the radiohalos they contain were generated by primordial polonium created in situ.
According to Snelling, granites were formed by the crystallization and cooling of magmas, including during the global Flood, and the radiohalos they contain were generated by polonium separated by hydrothermal fluids from nearby uranium sources.
Only Snelling’s model is consistent with the variety of geological settings in which the Po halo-bearing rocks are found.
Snelling A. A. 2005. Radiohalos in granites: evidence for accelerated nuclear decay. In: Vardiman L., Snelling A. A. and Chaffin E. F. (editors). Radioisotopes and the Age of the Earth: Results of a Young-Earth Creationist Research Initiative, Institute for Creation Research, El Cajon, California and Creation Research Society, Chino Valley, Arizona, pp.101-207.
Snelling A. A. 2008. Catastrophic granite formation: rapid melting of source rocks, and rapid magma intrusion and cooling. Answers Research Journal 1:11-25.
Snelling A. A. and Gates D. 2009. Implications of polonium radiohalos in nested plutons of the Tuolumne Intrusive Suite, Yosemite, California. Answers Research Journal 2:53-77.