A Three-Stage Model for Trace Element and Isotope Fractionation During Solidification in Magmatic Veins
Abstract
Magmatic veins are commonly observed in igneous bodies of either crustal or mantle origin. They are formed by injection of melt into surrounding rocks. There is a growing interest in magmatic veins in the petrology community as trace element and isotope ratios in vein minerals can offer potentially important clues to magma source compositions and their tectonic setting. The standard approach is through application of mineral-melt partition coefficient. The goal of this study is to develop a framework for interpreting variations in trace element and isotope ratios in vein minerals.
We consider trace element fractionation during melt flow and crystallization in a vein. We assume that the vein forms instantaneously by fracturing and magma injection. The vein remains open for a period of time during which crystals grow from the surrounding wall inward and additional melt flows through the vein. The crystal + melt vein system is open to mass during this period. In a later stage when the system is waning down, the vein is closed to mass transfer as the melt flux in the vein reduces to zero. We use time-dependent mass conservation equations to characterize the spatial and temporal variations of a trace element in the melt and mineral(s) along the vein. In terms of trace element fractionation, three stages are identified. The first stage is characterized by fractional crystallization following melt injection. Crystallization starts from the wallrock-vein contact and crystals grow toward the center of the vein. This is followed by the second stage when mixing between the flow-through melt and residual melt from early crystallization results in a steady-state. The melt composition becomes constant at a given location in the vein and crystals formed from the melt have constant composition (i.e., no zoning). The length or duration of this constant concentration regime depends on the location in the vein. The third stage starts when melt flow in the vein is shut off. The interstitial melt then undergoes in situ fractional crystallization, resulting in an increase in incompatible trace elements in the growing crystals. Intra- and inter-mineral compositional variations along the vein will be discussed and an application of the three-stage model to peridotite-hosted pyroxenite veins will be presented.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11A..04L
- Keywords:
-
- 1042 Mineral and crystal chemistry;
- GEOCHEMISTRY;
- 1043 Fluid and melt inclusion geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY