Sediment-peridotite interaction in a temperature gradient: transport of trace elements and growth of diamonds
Final Report Abstract
In the first part of this project, we attempted to simulate conditions during deep subduction, through a series of experiments with a sediment layer placed above or below a harzburgite layer at pressures of 7.5 and 10.5 GPa in the presence of a controlled temperature gradient. The sediment also contained H2O, CO2 and a variety of trace elements. Graphite (15 wt%) was also added to the harzburgite to simulate an expected redox gradient between the two layers. The “basic” setup had the sediment layer at the bottom of the capsule in the cold zone (400–1200°C) and was overlain by the hotter (900–1500°C) peridotite. The temperature distribution was determined by thermocouples at each end of the capsule and subsequently by orthopyroxene– garnet thermometry across the sample. Multiple layers with different mineralogies developed, along with a melt layer that formed in the hottest part of the capsule. This led to strong enrichment of SiO2 in the peridotite, while the sediment layer gained significant amounts of MgO, FeO and Cr2O3. Potassium is fully extracted into a silico-carbonatitic melt with strongly variable SiO2, MgO, FeO and CaO contents and low Al2O3. Humite-group minerals forms in the solid residue and remain stable to relatively high temperature and pressure, especially when fluorine is present. The trace-element distribution is controlled by temperature and the stable mineral assemblages. Minerals like garnet, zircon, rutile and humite-group minerals can exert an important influence on the trace element signatures of the melts by holding back certain elements in the solid residuum. For example, negative Nb–Ta anomalies are caused by their retention in rutile or (and) humiteminerals. At temperatures < 700°C in the sediment layer, the fluid-mobile elements Ba, Rb, Sr and Li are expelled, resulting in high Ba/La, Ba/Nb, Sr/Nb etc. in the melts created in the peridotite. At T > 700°C (sediment solidus) most of the other incompatible elements are mobile as well, and the produced melt is markedly enriched in Ba, Rb, Sr, LREE and U relative to Ti, MREE and HREE. Thus, with increasing temperature, element ratios characteristic for arcmagmas (i.e. Ba/Nb, Rb/Th) progressively decrease. However, the addition of even 1% of melt or fluid to depleted mantle peridotite is sufficient to produce basaltic melts with incompatible element contents (i.e. a “sedimentary” signature) similar to those observed in natural subduction-related magmas. In the second part of the project, diamond nucleation and growth were studied in two types of experiments: 1) those with a temperature gradient, as described previously, and 2) additional experiments that were run under isothermal conditions with a sediment layer containing diamond seeds (Girnis et al. to be submitted). In all experiments, the sedimentary mixture was partly or completely melted, and the melt percolated and interacted with the harzburgite. The graphite-todiamond transition in the peridotite material was observed as spontaneous diamond nucleation at >1300°C at 7.5 GPa and >1200°C at 10.5 GPa in the temperature-gradient experiments, and at temperatures ~100°C lower in the isothermal experiments with diamond seeds. Textural observations indicate that diamond nucleation occurs via the melt phase following graphite dissolution and recrystallization (i.e. no direct transformation from graphite to diamond). In our study, diamond crystallized from a variety of melt compositions ranging from “carbonatitic” with >50 wt% volatiles to hydrous silicate melts <10 wt% volatiles, indicating that melt composition is not a primary factor in controlling diamond formation.
Publications
-
(2018) Graphite to diamond transformation during sediment–peridotite interaction at 7.5 and 10.5 Gpa. Lithos 310-311 302–313
Walton, E.; Hibar, D. P.; van Erp, T. G. M.; Potkin, S. G.; Roiz-Santiañez, R.; Crespo-Facorro, B.; Suarez-Pinilla, P.; van Haren, N. E. M.; Zwarte, S. M. C. de; Kahn, R. S.; Cahn, W.; Doan, N. T.; Jørgensen, K. N.; Gurholt, T. P.; Agartz, I.; Andreassen,
-
(2018) Subduction factory in an ampoule: Experiments on sediment–peridotite interaction under temperature gradient conditions. Geochimica et Cosmochimica Acta 223 319–349
Woodland AB, Bulatov VK, Girnis AV, Brey GP, Höfer HE
-
(2017) Sediment-peridotite interactions in a thermal gradient: mineralogic and geochemical effects and the “sedimentary signature” of arc magmas. EGU Meeting, Wien, April 2017, EGU2017-6316
Woodland AB, Girnis AV, Bulatov VK, Brey GP, Höfer HE, Gerdes, A
