ESTIMATION OF FLUID FUGACITIES IN OROGENIC MANTLE PERIDOTITES
Abstract
Water (Hv2O and H-species in minerals) has a strong influence on many physio-chemical properties of upper mantle rocks (e.g. viscosity, melting temperature, thermal conductivity). Estimates of mantle water contents are often based on measuring the concentration of H in nominally anhydrous minerals (NAMs) contained in peridotites. However, these NAMs may suffer diffusive loss of H during emplacement at the Earth’s surface, and the relationship between NAM H contents and Hv2O fugacities are not well known for all mantle NAMs. In this study, fluidbuffering mineral equilibria are used to estimate the fugacities of various fluid species in peridotites. The compositions of co-existing minerals from nine orogenic peridotites were determined via electron microprobe. These samples were from six different locations (New Caledonia; Bestiac, Lherz, and Turon de Técouère, France; Almklovdalen, Norway; and the Trinity Ophiolite, CA USA), and contain amphibole + olivine (ol) + orthopyroxene (opx) + clinopyroxene (cpx) + spinel (spl), one sample contains garnet, and one contains plagioclase. Location selection was purposefully varied in an attempt to capture mantle conditions at different locations as well as potentially capturing different tectonic settings and emplacement mechanisms. Two pyroxene thermometry yields temperatures of 660 to 900°C at pressures (P) ranging from 5.8 – 15 kbar. Estimates of water activities (aHv2O), based on amphibole equilibria, range from 0.07 to 0.4, except for samples from New Caledonia which record values between 0.87 and 1.0. The compositions of co-existing spl + ol + opx yield oxygen fugacities (ƒOv2) ranging from 0.0 to 1.9 ∆logƒOv2(FMQ).
Low values of aHv2O (< 0.4) are similar to values recorded in other amphibole-bearing peridotites. Amphibole growth in peridotites is often attributed to rock interaction with a metasomatic fluid. This may be the case in some samples, however, the resulting low water activities suggest that any water present in a metasomatic fluid was likely consumed entirely by the growth of amphibole, resulting in relatively “dry” rocks. However, it is possible that the water needed to stabilize amphiboles present in some of the samples here may have been derived from H present in the co-existing NAMs. Regardless of amphibole formation mechanism it is clear that amphibole bearing peridotites are not necessarily “hydrous” or “wet” as compared to nonamphibole bearing peridotites.
The samples from New Caledonia are the first mantle derived samples to record high values of aHv2O (0.87 -1.0) relative to other studies that have employed similar methods. These are the only samples in this suite that are derived from a paleo oceanic transform fault setting. High values of aHv2O in these rocks are consistent with the idea that fluid may infiltrate oceanic transform faults to depths that corresponding to temperatures between 600 °C and 1100 °C . Ultimately, these samples provide important insights into fluid conditions at a variety of mantle settings, as well as mechanisms for amphibole growth in mantle peridotites. Additionally, these samples (particularly those that record of high aHv2O) should permit future insight into the amount of H-loss experienced by NAM’s during emplacement, and the role of metasomatism in amphibole growth in mantle derived peridotites.
Citation
Cummings, Thomas (2019). ESTIMATION OF FLUID FUGACITIES IN OROGENIC MANTLE PERIDOTITES. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /189011.