Timescales between mantle metasomatism and kimberlite ascent indicated by diffusion profiles in garnet crystals from peridotite xenoliths

Date

2018

Authors

Jollands, Michael
Hanger, Brendan
Yaxley, Gregory
Hermann, Joerg
Kilburn, Matt R

Journal Title

Journal ISSN

Volume Title

Publisher

Elsevier

Abstract

Rare garnet crystals from a peridotite xenolith from the Wesselton kimberlite, South Africa, have distinct zones related to two separate episodes of mantle metasomatism. The garnet cores were firstly depleted through melt extraction, then equilibrated during metasomatism by a potentially diamond-forming carbonate-bearing or proto-kimberlitic fluid at 1100–1300 °C and 4.5–5.5 GPa. The garnet rim chemistry, in contrast, is consistent with later overgrowth in equilibrium with a kimberlite at around 1025±25°C and 4.2±0.5 GPa. This suggests that the rock was physically moved upwards by up to tens of kilometres between the two metasomatic episodes. Preserved high Ca, Al and Cr contents in orthopyroxenes suggest this uplift was tectonic, rather than magmatic. Diffusion profiles were measured over the transitions between garnet cores and rims using electron microprobe (Mg, Ca, Fe for modelling, plus Cr, Mn, Ti, Na, Al) and nano Secondary Ion Mass Spectrometry (NanoSIMS; 89Y, along with 23Na, Ca, Cr, Fe, Mn and Ti) analyses. The short profile lengths (generally <10 μm) and low Y concentrations (0.2–60 ppm) make the NanoSIMS approach preferable. Diffusion profiles at the interface between the zones yield constraints on the timescale between the second metasomatic event and eruption of the kimberlite magma that brought the xenolith to the surface. The time taken to form the diffusion profiles is on the order of 25 days to 400 yr, primarily based on modelling of Y diffusion along with Ca, Fe and Mg (multicomponent diffusion) profiles. These timescales are too long to be produced by the interaction of the mantle xenolith with the host kimberlite magma during a single-stage ascent to the crust (hours to days). The samples offer a rare opportunity to study metasomatic processes associated with failed eruption attempts in the cratonic lithosphere.

Description

Keywords

diffusion, kimberlite, mantle metasomatism, garnet, NanoSIMS

Citation

Source

Earth and Planetary Science Letters

Type

Journal article

Book Title

Entity type

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Restricted until

2099-12-31
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