Diffusion along interphase boundaries and its effect on retrograde zoning patterns of metamorphic minerals |
| |
Authors: | L M Keller C A Hauzenberger R Abart |
| |
Institution: | 1.Institute for Geological Sciences,Free University Berlin,Berlin,Germany;2.Institute for Earth Sciences,Karl-Franzens University Graz,Graz,Austria |
| |
Abstract: | In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport
along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite
at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are
frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop
when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport
along an interphase boundary depend on the product D
ib
K
ib/s
a, where D
ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K
ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique
belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along
garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these
boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm3/s (575°C) and 1E-20 cm3/s (700°C) from modeling the observed X
Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe2+ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material
that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at
the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities
and solubility of Fe2+ and FeOH+ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange
equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations
from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid
along the interphase boundary at any time during cooling. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|