Permeability generation and resetting of tracers during metamorphic fluid flow: implications for advection-dispersion models |
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| Authors: | Ian Cartwright |
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| Institution: | (1) Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Clayton 3168, Australia, AU |
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| Abstract: | Advection-dispersion fluid flow models implicitly assume that the infiltrating fluid flows through an already fluid-saturated
medium. However, whether rocks contain a fluid depends on their reaction history, and whether any initial fluid escapes. The
behaviour of different rocks may be illustrated using hypothetical marble compositions. Marbles with diverse chemistries (e.g.
calcite + dolomite + quartz) are relatively reactive, and will generally produce a fluid during heating. By contrast, marbles
with more restricted chemistries (e.g. calcite + quartz or calcite-only) may not. If the rock is not fluid bearing when fluid
infiltration commences, mineralogical reactions may produce a reaction-enhanced permeability in calcite + dolomite + quartz
or calcite + quartz, but not in calcite-only marbles. The permeability production controls the pattern of mineralogical, isotopic,
and geochemical resetting during fluid flow. Tracers retarded behind the mineralogical fronts will probably be reset as predicted
by the advection-dispersion models; however, tracers that are expected to be reset ahead of the mineralogical fronts cannot
progress beyond the permeability generating reaction. In the case of very unreactive lithologies (e.g. pure calcite marbles,
cherts, and quartzites), the first reaction to affect the rocks may be a metasomatic one ahead of which there is little pervasive
resetting of any tracer. Centimetre-scale layering may lead to the formation of self-perpetuating fluid channels in rocks
that are not fluid saturated due to the juxtaposition of reactants. Such layered rocks may show patterns of mineralogical
resetting that are not predicted by advection-dispersion models. Patterns of mineralogical and isotopic resetting in marbles
from a number of terrains, for example: Chillagoe, Marulan South, Reynolds Range (Australia); Adirondack Mountains, Old Woman
Mountains, Notch Peak (USA); and Stephen Cross Quarry (Canada) vary as predicted by these models.
Received: 3 February 1997 / Accepted: 26 June 1997 |
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