Abstract

Abstract: 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 c . 1000 μm Mn diffusion in sillimanite-zone garnets during the Barrovian regional metamorphism. Sillimanite-grade garnets from the Barrovian metamorphic series also preserve c . 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. Supplementary material: A table of quantitative electron microprobe analyses, an example of the calibration curve used to produce quantitative electron microprobe maps from count maps, and Y maps are available at http://www.geolsoc.org.uk/SUP18415 .

Highlights

  • MethodsTo study major and trace element zoning preserved in Barrovian garnets, we employed a combined analytical approach

  • We report on the results of a systematic investigation into the nature of major element and rare earth element (REE) zoning preserved in garnets from the Barrovian metamorphic series and the manner by which geochemical zoning of garnet varies with increasing metamorphic grade

  • Because of issues relating to the effect of growth timing on initial Mn and Y zoning patterns, such observations provide no assurance of grain-scale diffusion during the Barrovian metamorphism

Read more

Summary

Methods

To study major and trace element zoning preserved in Barrovian garnets, we employed a combined analytical approach. We chose representative profiles to show the main features of the major element zoning These profiles are of high resolution, they are of relatively low sensitivity, because of the relatively short counting time employed for the mapping. Rather than reproducing the profiles with fully quantified electron microprobe spot profiles, we chose to obtain highresolution, high-sensitivity quantitative composition profiles for the garnets by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This approach has an advantage over the electron microprobe in that it is capable of simulta-

Results
Discussion
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call