A DEPLETED MANTLE SOURCE BELOW THE NORTHERN EXTENSION OF THE RIO GRANDE RIFT

The Herring Park Basalt (HPb) is composed of two Tertiary basaltic trachyandesite flows in north Fremont and south Park Counties in Colorado. Point-counting on 25 peridotite xenoliths included in the Herring Park Basalt reveal a harzburgitic mantle composition: 49.8% orthopyroxene, 48.2% olivine, 1.3% spinel, and 0.5% clinopyroxene.

Olivine crystals in the xenoliths are forsterite, with Fo numbers ranging from 91.4% to 92.2%. Orthopyroxene data retrieved from Melton (2007) classifies analyzed orthopyroxene grains as enstatite, with Mg numbers ranging from 83.3% to 92.7%. Spinel compositions are generally Mg-Al-Chromite spinels (Mg₀.₇Fe²₀.₃)(Al₁.₆Cr₁.₂ Feᵌ₀.₁) assuming all the Al resides in the M site. Although quite sparse, clinopyroxene in the xenoliths are diopside.

Melton (2007) assumed crystallization pressures at 12 kbar for the Herring Park mantle xenoliths and calculated closure temperatures ranging from 1011°C and 1068°C, with varying degrees of uncertainty. These closure temperatures will be verified by applying a single crystal X-ray diffraction analysis of spinel crystals to determine partitioning of Al between the M and T sites (Perinelli et al., 2014)

Lack of clinopyroxene, along with the occurrence of Al and high Cr numbers in the spinels strongly suggests a highly depleted mantle source. The HPb is one of the few Cenezoic lava flows associated with the northern extension of the Rio Grande Rift and its peridotite xenoliths may record previous mantle depletion events associated with local to regional magmatic occurrences (e.g. Central Colorado Volcanic Field.