Compositional variation of density and seismic velocities in natural peridotites at STP conditions: Implications for seismic imaging of compositional heterogeneities in the upper mantle

Abstract

Densities and elastic properties of solid natural spinel- and garnet-peridotite samples (n = 133) at standard temperature (T) and pressure (P) (STP) conditions were calculated for compositions ranging from Mg# (100 ◊ Mg/(Mg + Fe)) of 86–94. The physical properties were used to investigate how natural compositional variations control density and seismic velocity. A corresponding set of compositional derivatives (d/dMg#) of density and seismic velocity is provided. Because the P and T derivatives of elastic moduli are very similar for different compositional end-members of peridotitic minerals, the variation of elastic moduli with Mg# at STP conditions holds at elevated P and T. Increased Mg# leads to a significant increase in V S because of the sensitivity of mineral shear moduli to this parameter (dV S /dMg# = 0.0143 ± 0.0009 km s−1). In contrast, the compressional wave velocity (V P ) is insensitive to Mg# and, instead, correlates weakly with increasing olivine abundance at STP conditions. The ratio V P /V S therefore exhibits a significant negative correlation with Mg# (d(V P /V S )/dMg# = −0.00407 ± 0.00038). Because the temperature dependency of V P /V S is small (