One dimensional diffusion of radiogenic 87Sr and fluid transport of volatile elements across the margin of a metamorphosed Archaean basic dyke from Saglek,Labrador

An example of diffusion in a natural solid rock is described using ⁸⁷Sr as a tracer. The distribution of excess radiogenic ⁸⁷Sr across the contact of a metamorphosed basic dyke in a granodioritic gneiss is shown to fit closely to a mathematically modelled theoretical diffusion curve. It is suggested that the excess radiogenic Sr was generated by the breakdown of older Rb-rich biotite in the country rock to form chlorite during a late-stage greenschist facies metamorphism, and then taken up into a fluid phase migrating along the country rock — dyke interface. This fluid introduced silica, alkalies, light REE, H₂O, CO₂, N and Cl into the marginal zone of the dyke, probably during fluid migration along grain boundaries. The present irregular distribution of the volatile constituents across the contact and in the dyke excludes a diffusion controlled distribution. The distribution of volatiles probably reflects the influence of the existing mineralogy on the geochemical anomaly actually recorded from a post magmatic process. Evidence is presented that the diffusivity of radiogenic Sr was enhanced by the presence of a volatile phase.