Peridotite xenoliths from western Grand Canyon and The Thumb: A probe into the subcontinental mantle of the Colorado Plateau

Abstract

Rare earth elements (REE) and Sr and Nd isotopic compositions are presented for spinel peridotite xenoliths from western Grand Canyon, on the margin of the Colorado Plateau (CP), and for garnet peridotite inclusions from The Thumb, central CP. The Grand Canyon spinel peridotites show original textures and geochemical characteristics which do not have counterparts in the xenoliths from the nearby Basin and Range. The restricted range of mineral compositions in these Iherzolites (Mg #Ol = 0.90 ± 0.03; Cr/(Cr+Al)Sp = 0.18 ± 0.02) suggests that they represent a limited series of residues of partial melting. Pyroxene equilibration temperatures are in the range 800–1000°C. The 143Nd/144Nd ratios in clinopyroxene (cpx) are extremely variable (0.5127–0.5202). Sample TRU 2, with εNd = +147, records an ancient depletion event around 1.2 Ga. More than half of the cpx separates show concave downward REE profiles between Gd and Yb, indicating that partial melting occurred in the garnet stability field before final recrystallization of the residues in the spinel peridotite field, probably during decompressional upwelling. The strong light REE (LREE) depletion in some of the cpx cannot be modeled by batch melting of a LREE‐depleted source but requires 5–10% melting with segregation in ∼1% increments. These peridotites could be associated with the production of picritic melts either in a rift environment or beneath a mid‐ocean ridge or oceanic island. A second group of xenoliths, characterized by cpx with strong LREE enrichment, includes amphibole‐bearing peridotites and has a more limited range of 143Nd/144Nd ratios (0.51269–0.51285). Interactions with melts of variable composition (alkali basalt or carbonatite) are inferred on the basis of mineralogical and geochemical evidence. Contrasting with the extreme isotopic heterogeneity of Nd, a high, uniform 87Sr/86Sr ratio around 0.7045 is associated with high Sr/Nd ratios (up to 540). Aqueous fluids derived from a subducted slab could account for this preferential Sr enrichment. Six porphyroclastic garnet Iherzolites from The Thumb sample a deeper part of the subcontinental mantie (120–150 km). One xenolith has a refractory chemistry, low temperature of equilibration (T = 1220°C, P = 3.8 GPa) and Sr‐Nd isotopic compositions (87Sr/86Sr = 0.7037,143Nd/144Nd = 0.51285), suggesting a lithospheric origin. The other xenoliths have a fertile chemistry, higher temperature of equilibration (T = 1260–1280°C, P = 4.5–4.7 GPa), and homogeneous isotopic compositions (87Sr/86Sr = 0.70306–0.70313,143Nd/144Nd = 0.51281–0.51284) which fall slightly to the left of the mantle array in a similar way as St. Helena oceanic basalts. The garnet shows an enrichment in Ti and Fe in the outer 300 μm. Three garnet‐cpx pairs give Sm‐Nd apparent ages between 60 and 100 Ma, which are older than the host minette (∼25 Ma). These observations together show that the cpx has readjusted its composition more completely than garnet during an interaction with melts having a St. Helena‐type isotopic signature. As this signature has not been identified in the CP volcanic rocks, it is inferred that the high‐temperature Iherzolites from The Thumb record the presence of a mantle plume at ∼25 Ma beneath the central CP.