Garnet in the Earth's mantle
Garnet in the Earth's mantle (in Garnet; common mineral, uncommonly useful, Ethan F. Baxter (editor), Mark J. Caddick (editor) and Jay J. Ague (editor))
Elements (December 2013) 9 (6): 421-426
- chain silicates
- clinopyroxene
- coordination
- crystal chemistry
- depth
- diamond
- eclogite
- garnet group
- high pressure
- high temperature
- igneous rocks
- inclusions
- majorite
- mantle
- melts
- metals
- metamorphic rocks
- mineral inclusions
- native elements
- nesosilicates
- orthosilicates
- partition coefficients
- peridotites
- phase equilibria
- phase transitions
- physical properties
- plutonic rocks
- pressure
- pyroxene group
- rare earths
- silicate melts
- silicates
- temperature
- trace elements
- transformations
- ultramafics
- upper mantle
Aluminous garnet, (Mg,Fe (super 2+) ,Ca) (sub 3) (Al,Cr) (sub 2) Si (sub 3) O (sub 12) , is an important constituent of mantle peridotite ( approximately 10%) and of the other abundant upper mantle rock, eclogite ( approximately 50%). Its unusual crystal chemistry means that it strongly prefers some trace elements and confers a "garnet signature" on mantle melts. As depth increases from 250 to 600 km, garnet increases in abundance in mantle rocks, dissolving large fractions of the other silicates and becoming Si rich (majoritic). These compositional changes are observed in some garnets found as inclusions in diamond. Garnet disappears from mantle assemblages at about 700 km depth, where it is replaced by an even denser silicate, perovskite.
