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 periodotitic 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 VS because of the sensitivity of mineral shear moduli to this parameter (dVS/dMg# = 0.0143 ± 0.0009 km s-1). In contrast, the compressional wave velocity (VP) is insensitive to Mg# and, instead, correlates weakly with increasing olivine abundance at STP conditions. The ratio VP/VS therefore exhibits a significant negative correlation with Mg# (d(VP/VS)/dMg# = -0.00407 ± 0.00038). Because the temperature dependency of VP/VS is small (