Abstract
Ian Carmichael wrote of an “andesite aqueduct” that conveys vast amounts of water from the magma source region of a subduction zone to the Earth’s surface. Diverse observations indicate that subduction zone magmas contain 5 wt % or more H2O. Most of the water is released from crystallizing intrusions to play a central role in contact metamorphism and the genesis of ore deposits, but it also has important effects on the plutonic rocks themselves. Many plutons were constructed incrementally from the top down over million-year time scales. Early-formed increments are wall rocks to later increments; heat and water released as each increment crystallizes pass through older increments before exiting the pluton. The water ascends via multiple pathways. Hydrothermal veins record ascent via fracture conduits. Pipe-like conduits in Yosemite National Park, California, are located in or near aplite–pegmatite dikes, which themselves are products of hydrous late-stage magmatic liquids. Pervasive grain-boundary infiltration is recorded by fluid-mediated subsolidus modification of mineral compositions and textures. The flood of magmatic water carries a large fraction of the total thermal energy of the magma and transmits that energy much more rapidly than conduction, thus enhancing the fluctuating postemplacement thermal histories that result from incremental pluton growth. The effects of water released by subduction zone magmas are central not only to metamorphism and mineralization of surrounding rocks, but also to the petrology and the thermal history of the plutons themselves.
Highlights
It has long been appreciated that water released by arc plutons plays a central role in the genesis of ore deposits as well as in contact metamorphism
Effects of the prodigious amount of water carried by arc magmas on the plutonic rocks themselves have generally been underappreciated
We argue that passage this water promotesoftextural modification crystallizing magmaoftherefore pass temperatures through already solidified plutonic rock and by some equilibration minerals must to lower via dissolution–precipitation net-transfer magma must passflow, through already solidified plutonic rockbefore by some combination of fracture combination of fracture grain-boundary flow, and diffusion it encounters wall rocks reactions, and that such postmagmatic modification is ubiquitous and a key element in flow, grain-boundary flow,we and diffusion before it wall rocks
Summary
It has long been appreciated that water released by arc plutons plays a central role in the genesis of ore deposits as well as in contact metamorphism. Carmichael [1] used phase equilibria and thermodynamic arguments to estimate the water contents of volcanic arc magmas, and concluded that subduction-related intermediate magmas typically contain several percent water He estimated the plutonic/volcanic ratio in arcs to be approximately 5 and suggested that more than 80% of the water carried upward by arc magmas is released beneath the Earth’s surface by crystallization of intrusive bodies. Magma bodies solidify predominantly from the top down and from the outside in, and geochronology indicates that granitic plutons commonly grow incrementally by downward stacking of sheets (Figure 1; [2,3]). 1. Schematic diagram illustrating incremental pluton growth by downward of intrusive. 1. Schematic diagram illustrating incremental pluton growth by downward of water released by crystallization. Accumulates at the top of the active magma body [4,5]
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