Metasomatism and the Chemical Transformation of Rock The Role of
Fluid-aided mass transfer and subsequent mineral re-equilibration are the two defining features of metasomatism and must be present in order for metamorphism to occur. Coupled with igneous and tectonic processes, metasomatism has played a major role in th
- PDF / 854,857 Bytes
- 35 Pages / 439.37 x 666.142 pts Page_size
- 87 Downloads / 167 Views
The Chemical Composition of Metasomatic Fluids in the Crust Bruce W.D. Yardley
Abstract Any crustal fluid can give rise to metasomatism when it migrates from one rock type to another, and metasomatism is normally associated with specific fluid flow paths, such as fractures, faults, shear zones or lithologies which were more permeable than those around them. Examples of representative analyses of a wide range of crustal fluids from the literature are therefore presented and discussed. Except in shallow crustal settings, metasomatism is generally associated with brines, and highly concentrated brines are particularly effective metasomatic agents. Leaving aside mid-ocean-ridge geothermal systems, the most abundant sources of metasomatic fluids are sedimentary basin brines and magmatic fluids, but mantle and metamorphic fluids can also give rise to metasomatism. Basinal brines are initially of two types: low-Br, NaCl brines derived by dissolution of halite, and Br, Ca-rich brines evolved from the bitern brines remaining after halite precipitation. Both types are implicated in ore-formation and metasomatism (e.g. albitisation) in both sedimentary basins and their underlying crystalline basement. Many magmas give off significant amounts of acid fluids, often rather saline, as they crystallise, and these may also contain distinctive volatile components derived from the melt (e.g. B, F) that influence the metasomatic effects, such as greisenisation, that result. The high transition metal contents of magmatic brines mean that they are also potential ore-formers. Some deep-derived magmas act as vectors to bring mantle fluids into the crust. These appear to be brines that are also highly charged with CO2, and one effect of degassing at crustal levels is that these fluids become silica undersaturated, for example they may leach silica during fenitisation. Skarns are a particularly important class of metasomatic rock, and owe their origin in part to the transient generation of secondary porosity as infiltrating silica-saturated aqueous fluids trigger decarbonation reactions in carbonate rocks. They may be formed from metamorphic fluids as well as magmatic
B.W.D. Yardley (*) School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK e-mail: [email protected] D.E. Harlov and H. Austrheim, Metasomatism and the Chemical Transformation of Rock, Lecture Notes in Earth System Sciences, DOI 10.1007/978-3-642-28394-9_2, # Springer-Verlag Berlin Heidelberg 2013
17
18
B.W.D. Yardley
ones, but in general metamorphic fluids are not a major cause of metasomatism, except where focussed in subduction, because they are usually released very slowly by endothermic reactions taking place in an over-pressured, and therefore low permeability, environment.
2.1
Introduction
Metasomatism, the process by which rocks change their composition while remaining effectively solid, is self-evidently important in the evolution of a wide range of rock types, but remains quite poorly understood. This is largely because it is complex
Data Loading...
