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Cumulate piles are modified by reactive liquid flow

The Rum layered intrusion testifies to modification by injection of hot magma and remobilization of pre-existing cumulate rocks.

The Isle of Rum in the Inner Herbrides of Scotland is a classic and much-studied example of an igneous layered intrusion. Dated at 60 Ma, it’s emplacement was related to the development of the proto-Icelandic plume.

New work by Leuthold et al. focusses on a particular layer within the intrusion, ‘Unit 9’, which shows a progression from peridotite (olivine-rich) through troctolite (olivine + plagioclase) to gabbro (plagioclase + clinopyroxene).

By integrating field and geochemical observations, this study challenges the idea that Unit 9 was formed through progressive fractional crystallization of a single parental liquid. Instead, the authors hypothesise that multiple generations of rimmed clinopyroxenes with sharp boundaries in Cr2O3 and REE indicate that Unit 9 underwent two separate episodes of partial melting in response to the intrusion of hot picritic magma.

Crystal mush

Some of the processes that may occur in a cumulate pile in response to the injection of chemically distinct magmas

This upward and lateral migration of melts and the reactive remobilisation of a cumulate pile may be an important process in all layered intrusions and open magma chambers.

Leuthold J, Blundy JD, Holness MB, & Sides R (2014) ‘Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)’. Contributions to Mineralogy and Petrology, 168(1), 1-27. http://dx.doi.org/10.1007/s00410-014-1021-7

SCIENTIFIC ABSTRACT

We present a detailed microstructural and geochemical study of reactive liquid flow in Unit 9 of the Rum Eastern Layered Intrusion, Scotland. Unit 9 comprises an underlying lens-like body of peridotite overlain by a sequence of troctolite and gabbro (termed allivalite), with some local and minor anorthosite. The troctolite is separated from the overlying gabbro by a distinct, sub-horizontal, undulose horizon (the ‘major wavy horizon’). Higher in the stratigraphy is another, similar, horizon (the ‘minor wavy horizon’) that separates relatively clinopyroxene-poor gabbro from an overlying gabbro. To the north of the peridotite lens, both troctolite and gabbro grade into poikilitic gabbro. Clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro. The poikilitic gabbros contain multiple generations of clinopyroxene, with Cr-rich (~1.1 wt% Cr2O3) anhedral cores with moderate REE concentrations (core1) overgrown by an anhedral REE-depleted second generation with moderate Cr (~0.7 wt% Cr2O3) (core2). These composite cores are rimmed by Cr-poor (~0.2 wt% Cr2O3) and REE-poor to -moderate clinopyroxene. We interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine-phyric picrite to form the discontinuous lenses that comprise the Unit 9 peridotite. Loss of clinopyroxene-saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene-poor gabbro. The spatial extent of clinopyroxene loss is marked by the minor wavy horizon. A second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite, with the major wavy horizon marking the extent of melting during this episode. The poikilitic gabbro formed from clinopyroxene-saturated melt moving upwards and laterally through the remobilized cumulate pile and precipitating clinopyroxene en route. This process, called reactive liquid flow, is potentially important in open magma chambers.

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