US EPA

6906563 

Journal Article 

Garnet growth as a proxy for progressive subduction zone dehydration 

Baxter, EF; Caddick, MJ 

2013 

Yes 

Geology
ISSN: 0091-7613 

GEOLOGICAL SOC AMER, INC 

BOULDER 

41 

6 

643 

English 

10.1130/G34004.1 

WOS:000323269800005 

http:///www.geosociety.org
Exit
 

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-... cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones. Using thermodynamic analysis including the effects of garnet fractionation and fluid removal, we show the proportional relationship between garnet and water production in two end-member crustal lithologies (pelitic sediment and hydrated mid-oceanic-ridge basalt [MORB]) in three representative subduction geotherms. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ~1.4 and 3.0 GPa where geophysical models and observations predict dehydration. The average production ratio for altered MORB compositions is 0.52 (wt% water as fluid per vol% garnet) in cooler geotherms (Honshu [Japan] and Nicaragua) and 0.27 in hotter geotherms (Cascadia [North America]), whereas for pelite the production ratios are about half (0.24 and 0.13, respectively). Garnet growth correlates with production of 3.3-5.9 wt% water in hydrated MORB and 1.8-3.1 wt% water in pelite, representing 42%-100% of the water lost between 0.5 and 6.5 GPa from a fully saturated starting material. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration. (ProQuest: ... denotes formulae/symbols omitted.) 

Earth Sciences--Geology; Igneous rock; Seismology; Crystals; Minerals