Selected elastic moduli and their temperature derivatives for olivine and garnet with different Mg/(Mg+Fe) contents: Results from GHz ultrasonic interferometry

Selected P‐wave acoustic travel times through single crystal samples of olivine and garnet with different Mg/(Mg+Fe) ratios were measured at elevated temperatures (from 293 K to 363 K) using a newly developed GHz ultrasonic interferometer. These high precision data allow us to examine aspects of the elasticity‐composition systematics of olivine (forsterite‐fayalite) and garnet (pyrope‐almandine) solid solution series. For every 10% decrease in the Mg/(Mg+Fe) content, the P‐velocity of olivine along the b‐axis decreases about 3% and ∂lnC 22 /∂T decreases about 7%; for garnet ∂ln⌊C 44 + (C 11 + C 12 ) / 2⌋/∂T decreases about 6%. If such velocity‐composition systematics are typical and hold at the P‐T regime of the mantle, only a 3–4% variation in the Mg/(Mg+Fe) content of olivine is required to explain the lateral seismic velocity variation in the upper portion of the mantle within one tectonic region (d ln Vp∼l%), and 7–8% variation in the Mg/(Mg+Fe) content to explain the 2% lateral seismic velocity variation of the upper mantle globally.