Characterization of magmatic and diffusional processes in fractionally differentiated ultramafic systems: Examples from the Grand Canyon, USA
A 50 meter thick unit of Proterozoic cumulate layered wehrlite, lherzolite, and olivine websterite exposed at river mile 91 in the Ninetyone Mile Creek Canyon, a side canyon to the Grand Canyon, Arizona, USA, hosts numerous spheroid to ellipsoidal, dunite enclaves that range in size from 1 to 20 cm in diameter. Major-element geochemical analyses suggest that the dunite enclaves and cumulate peridotites are co-genetic. A harzburgite enclave does not appear to be related by fractionation to either the surrounding cumulate peridotite or the dunite enclaves. The unusual major element composition of the harzburgite enclave is also not a product of reaction with a percolating melt. It is suggested that the harzburgite enclave formed as a residue of partial melting of a less primitive peridotite. The ellipsoid and tear-drop shapes of many of the enclaves suggests that they were emplaced while hot enough for ductile deformation. Diffusion profiles also suggest emplacement while both the enclaves and the surrounding cumulate peridotite were still at high enough temperatures for rapid diffusion. The restitic harzburgite enclave(s) were probably dislodged from their original location by eruptive events, tectonic events, or convective flow followed by gravitational settling and deposition before being subsequently covered by more evolved cumulate peridotite. The dunite enclaves were either dislodged and transported in a similar fashion or moved, transported, and ultimately deposited by a within-pluton mass wasting event before being covered by more evolved cumulate peridotite.
Chromite, hercynite, and magnetite grains included in olivine, serpentinized olivine, and pyroxene from the Ninetyone Mile Peridotite. With the exception of a single sample, spinel grains observed in the Ninetyone Mile Peridotite retain very little of their original igneous composition. Subsequent overprinting by post-cumulus, subsolidus, and metamorphic processes dominates the eventual compositions providing some insights into the evolution of the Ninetyone Mile Peridotite. Interaction between spinel grains and trapped intercumulus melt (or liquid) during the later stages of crystallization resulted in a slight increase in Cr/[Cr+Al] and large increases in Fe3+/[Fe 3++Cr3++Al3+] and TiO2. The extent of these changes suggests a large shift in melt composition and a long of time between the crystallization of earlier mineral phases and the remainder of the melt in the Ninetyone Mile Peridotite magma chamber. Sub-solidus re-equilibration of Mg and Fe between spinel grains and surrounding olivine resulted in a substantial decrease in Mg/[Mg+Fe2+] in spinel. Subsolvus unmixing textures (hercynite + magnetite) in some of the better-preserved grains, and re-equilibration of Fe3+ between spinel grains and surrounding clinopyroxene and serpentine suggest slow cooling as well. Hydrothermal metamorphism led to decreases in spinel TiO2 and increases in Cr/[Cr+Al]. Amphibole occurring in both inclusions with spinel and silicate grains in olivine formed as devitrified hydrous melt inclusions.
In the most evolved lherzolite samples in the Ninetyone Mile Peridotite, coronas of orthopyroxene and orthopyroxene + amphibole symplectite are ubiquitous in separating cumulate grains of olivine from interstitial amphibole (with minor amounts of magnetite, spinel, and clinopyroxene), and occasional, albitic plagioclase. Compositional (low-Al coronitic orthopyroxene and albitic plagioclase) and textural (cuspate, convex inward olivine-orthopyroxene grain boundaries) evidence suggests that the olivine corona microstructures in the Ninetyone Mile Peridotite formed during a multistage process involving reaction between cumulate olivine and surrounding interstitial volatile-rich liquid. SiO 2 in the liquid reacted with olivine resulting in the inward dissolution of olivine to form orthopyroxene. Simultaneously, O2 from the liquid oxidized the fayalite component in the olivine, producing magnetite and SiO 2. The fayalite consumption resulted in a wide layer (reaction rim) of low-Al orthopyroxene with higher Mg/(Mg+FeTotal) than the original olivine and the production of small amounts of magnetite near the new orthopyroxene grain boundary. A change in the composition of the liquid then resulted in the partial replacement of orthopyroxene by amphibole and the formation of an orthopyroxene + amphibole symplectite layer. Finally, highly fractionated interstitial liquid crystallized to form albitic plagioclase. The availability of H2O was an important factor in the development of these coronas. The reactions depicted in the coronas in the Ninetyone Mile Peridotite occurred at pressures greater than 0.6 GPa and at ∼900-1000°C, which is consistent with regional metamorphic history.