## Intercrystalline stable isotope diffusion - A fast grain-boundary model

### Détails

ID Serval

serval:BIB_02F4C45C8227

Type

**Article**: article d'un périodique ou d'un magazine.

Collection

Publications

Fonds

Titre

Intercrystalline stable isotope diffusion - A fast grain-boundary model

Périodique

Contributions to Mineralogy and Petrology

ISSN-L

0010-7999

Statut éditorial

Publié

Date de publication

1992

Peer-reviewed

Oui

Volume

112

Pages

543-557

Langue

anglais

Résumé

We formulated a numerical model for stable isotope interdiffusion which

predicts the temperatures recorded between two or more minerals, and the

intra-granular distribution of stable isotopes in each mineral, as

functions of mineral grain sizes and shapes, diffusivities, modes,

equilibrium isotopic fractionations, and the cooling rate of a rock. One

of the principal assumptions of the model is that grain boundaries are

regions of rapid transport of stable isotopes. This Fast Grain Boundary

(FGB) model describes interdiffusion between any number of mineral

grains, assuming that local equilibrium and mass balance restrictions

apply on the grain boundaries throughout the volume modeled. The model

can be used for a rock containing any number of minerals, any number of

grain sizes of each mineral, several grain shapes, and any thermal

history or domain size desired. Previous models describing stable

isotope interdiffusion upon cooling have been based on Dodson's equation

or an equivalent numerical analogue. The closure temperature of Dodson

is the average, bulk temperature recorded between a mineral and an

infinite reservoir. By using Dodson's equation, these models have

treated the closure temperature as an innate characteristic of a given

mineral, independent of the amounts and diffusion rates of other

minerals. Such models do not accurately describe the mass balance of

many stable isotope interdiffusion problems. Existing models for cation

interdiffusion could be applied to stable isotopes with some

modifications, but only describe exchange between two minerals under

specific conditions. The results of FGB calculations differ considerably

from the predictions of Dodson's equation in many rock types of

interest. Actual calculations using the FGB model indicate that closure

temperature and diffusion profiles are as strongly functions of modal

abundance and relative differences in diffusion coefficient as they are

functions of grain size and cooling rate. Closure temperatures recorded

between two minerals which exchanged stable isotopes by diffusion are a

function of modal abundance and differences in diffusion coefficient,

and may differ from that predicted by Dodson's equation by hundreds of

degrees C. Either or both of two minerals may preserve detectable

zonation, which may in some instances be larger in the faster diffusing

mineral. Rocks containing three or more minerals can record a large span

of fractionations resulting from closed system processes alone. The

results of FGB diffusion modeling indicate that the effects of diffusive

exchange must be evaluated before interpreting mineral fractionations,

concordant or discordant, recorded within any rock in which diffusion

could have acted over observable scales. The predictions of this model

are applicable to thermometry, evaluation of open or closed system

retrogression, and determination of cooling rates or diffusion

coefficients.

predicts the temperatures recorded between two or more minerals, and the

intra-granular distribution of stable isotopes in each mineral, as

functions of mineral grain sizes and shapes, diffusivities, modes,

equilibrium isotopic fractionations, and the cooling rate of a rock. One

of the principal assumptions of the model is that grain boundaries are

regions of rapid transport of stable isotopes. This Fast Grain Boundary

(FGB) model describes interdiffusion between any number of mineral

grains, assuming that local equilibrium and mass balance restrictions

apply on the grain boundaries throughout the volume modeled. The model

can be used for a rock containing any number of minerals, any number of

grain sizes of each mineral, several grain shapes, and any thermal

history or domain size desired. Previous models describing stable

isotope interdiffusion upon cooling have been based on Dodson's equation

or an equivalent numerical analogue. The closure temperature of Dodson

is the average, bulk temperature recorded between a mineral and an

infinite reservoir. By using Dodson's equation, these models have

treated the closure temperature as an innate characteristic of a given

mineral, independent of the amounts and diffusion rates of other

minerals. Such models do not accurately describe the mass balance of

many stable isotope interdiffusion problems. Existing models for cation

interdiffusion could be applied to stable isotopes with some

modifications, but only describe exchange between two minerals under

specific conditions. The results of FGB calculations differ considerably

from the predictions of Dodson's equation in many rock types of

interest. Actual calculations using the FGB model indicate that closure

temperature and diffusion profiles are as strongly functions of modal

abundance and relative differences in diffusion coefficient as they are

functions of grain size and cooling rate. Closure temperatures recorded

between two minerals which exchanged stable isotopes by diffusion are a

function of modal abundance and differences in diffusion coefficient,

and may differ from that predicted by Dodson's equation by hundreds of

degrees C. Either or both of two minerals may preserve detectable

zonation, which may in some instances be larger in the faster diffusing

mineral. Rocks containing three or more minerals can record a large span

of fractionations resulting from closed system processes alone. The

results of FGB diffusion modeling indicate that the effects of diffusive

exchange must be evaluated before interpreting mineral fractionations,

concordant or discordant, recorded within any rock in which diffusion

could have acted over observable scales. The predictions of this model

are applicable to thermometry, evaluation of open or closed system

retrogression, and determination of cooling rates or diffusion

coefficients.

Création de la notice

02/10/2012 20:34

Dernière modification de la notice

18/11/2016 12:32