The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time
scales
Daniel R. Viete
Jörg Hermann
Gordon S. Lister
Iona R. Stenhouse
10.6084/m9.figshare.3454859.v1
https://geolsoc.figshare.com/articles/journal_contribution/The_nature_and_origin_of_the_Barrovian_metamorphism_Scotland_diffusion_length_scales_in_garnet_and_inferred_thermal_time___scales/3454859
<p>In this paper we examine the length scales of major element diffusion in garnet during the Barrovian metamorphism. The role
of diffusion in the flattening of Mn zoning profiles in garnet with increasing metamorphic grade across the Barrovian metamorphic
series is ambiguous. However, the loss of distinct Mn-defined secondary compositional zoning in Barrovian garnets with increasing
metamorphic grade and preservation of geochemical textures around Mn-free inclusions within high-grade Barrovian garnets provide
robust evidence for <em>c</em>. 1000 μm Mn diffusion in sillimanite-zone garnets during the Barrovian regional metamorphism. Sillimanite-grade garnets from
the Barrovian metamorphic series also preserve <em>c</em>. 100 μm Mn diffusion textures between sillimanite-grade rim domains and lower-grade cores. Bimodality in diffusion length
scales requires bimodality in thermal time scales. Length scales of chemical diffusion are considered within the context of
recent duration estimates for the Barrovian metamorphism of a few million years. We conclude that heat associated with the
Barrovian regional metamorphism accumulated following numerous, short time-scale heating events responsible for the smaller-scale
diffusion textures. This self-similar thermal regime was likely accommodated by the operation of shear zones in the highest-grade
regions of the Barrovian metamorphic series, which concentrated small-scale episodic heat sources such as sheeted magmas,
fluids and/or mechanical work.
</p>
2016-06-21 12:24:10
Barrovian garnets
Barrovian metamorphism
diffusion length scales
time scales
Mn zoning profiles
Geology