Compositions and effects of incipient melts of mantle peridotites in the presence of H2O and CO2 - an experimental study
The volatile components H2O and CO2 depress the melting point of the peridotitic mantle considerably. Volatile-controlled melting dominates the initial stages of melting, with small degrees of melt existing over a large temperature range (~300°C) in the peridotitic upper mantle, before major melting begins. However, we have only rudimentary knowledge about these first, incipient melts. Their chemical compositions are poorly constrained by high-pressure experiments, and natural magmas on the surface do not correspond to primitive melts due to fractionation and degassing. These incipient melts are important due to their potential to transport large amounts of volatiles and alkalis through the upper mantle and metasomatize the surrounding rocks. In most cases, these metasomatic reactions are complete, where the metasomatic agent can only be inferred, but not observed directly. Therefore, it is crucial to characterize the incipient melts that can be formed with various volatile contents and in various peridotitic compositions to understand low-degree metasomatic melts and their effect on metasomatism and refertilization processes in the upper mantle. This thesis fills this knowledge gap by determining the compositions of incipient melts under the influence of various mixtures of the volatile components H2O and CO2. This experimental research (using piston cylinder and multi anvil apparatuses) consists of a systematic study to determine the chemical compositions of incipient melts formed in four different peridotitic upper mantle rock compositions (Hawaiian pyrolite, a K2O-enriched pyrolite, harzburgite and depleted lherzolite) between 2.5 GPa and 7 GPa and from 990°C to 1210°C. Volatile-bearing melts do not quench well to form glasses but result in inhomogenous crystal overgrowths with intersertal textures. Therefore, a standardized three dimensional “melt tomography” approach was developed using defocused electron microprobe and calibrated SEM, to standardize measurements of the major element chemistry of these inhomogenously quenched melt pools to minimise subjective (user´s) errors during the estimation of melt compositions. Results show that the lowest degree melts (6-13 %) of the enriched and pyrolitic lherzolites show a carbonate-rich character and coexist in equilibrium with hydrous phases (phlogopite, amphibole) at low pressure (2.5 GPa). At 4-5 GPa, a gradual increase in SiO2 content of the melts suggests that no abrupt changes occur between initial carbonatitic (2.75 wt%) and higher-temperature aillikitic (up to 41 wt%) character as TiO2, Na2O and K2O and volatiles (CO2, H2O) generally decrease with increasing temperature and melt fractions. Low-degree melts of the depleted lherzolite and harzburgite show carbonated silicate melt character (ultramafic lamprophyre) with moderate alkali contents and higher SiO2 contents (26-44 wt%) with no sign of hydrous accessory phases. Melts of depleted lherzolite and harzburgite resemble kimberliteic melts with high MgO contents (wt%), but do not reproduce the classical high K2O/Na2O ratios of natural kimberlites. The general high to moderate K2O and Na2O contents and high volatile contents of the low-degree melts indicate that these would act as reactive metasomatic agents that have the potential to transfer a large amount of heat and induce chemical changes in the base of the lithospheric mantle. This thesis presents a novel experimental approach to characterise the reactive metasomatic behaviour of incipient melts in the lower cratonic lithosphere through redox freezing. During the redox freezing process, the oxidized incipient melts infiltrate into geochemically depleted and reduced cratonic lithospheric wall rock. Experiments at 5 GPa studying the interaction by redox freezing of alkali-rich hydrous carbonatite and alkali-poor CO2-H2O-bearing ultramafic lamprophyre with peridotite highlight the importance of incipient melt infiltration and contrasting oxygen fugacities as the key factors triggering diamond formation. Infiltration of alkali-rich hydrous carbonatite melt produces clinopyroxenite, dunite, and refertillized Mica-Amphibole-Rutile-Ilmenite-Diopside-like assemblages (MARID) with phlogopite and K-richterite. In contrast, redox freezing of alkali-poorer, CO2-H2O-bearing ultramafic lamprophyric melt produces metasomatic rock assemblages resembling bimineralic eclogite, dunite and garnet-bearing websterite. Experimentally produced redox freezing reactions show that one single incipient melt infiltration event into reduced and depleted wall rock allows the creation of most of the dominant rock types sampled from the cratonic lithosphere. Redox freezing highlights a new mechanism to form dunite and harzburgite suites via reactive melt infiltration, contrasting with previous interpretations that dunites are caused by high-degree melting. Furthermore, redox freezing can trigger the formation of garnet and clinopyroxene with the major element signatures of cratonic eclogite xenoliths, indicating that these may be formed via redox freezing in the absence of subducted material. This experimental characterisation of redox freezing mechanisms indicates new, alternative ways to form many rock types in the cratonic lithosphere, so that additional geochemical (trace elements, isotopes) information is necessary to interpret the formation of cratonic rock types.