Sr isotopes indicate millennial-scale formation of metal-rich layers by reactive melt percolation in an open-system layered intrusion

&amp;lt;p&amp;gt;In order to test whether the crystal mushes that form layered mafic intrusions can behave as open systems, we investigated mineral-scale textural, chemical and Sr isotopic heterogeneity in the c. 60 Ma Rum intrusion, Scotland. Within Unit 10 of the Rum intrusion, intercumulus plagioclase and clinopyroxene crystals in peridotite 1-2 cm above and below millimetric Cr-spinel seams exhibit complex optical and chemical zoning (Hepworth et al. 2017). These Cr-spinel seams are closely associated with sulphide and platinum-group element (PGE) mineralization. High precision Sr isotopic analyses (undertaken by thermal ionisation mass spectrometry) of individual intracrystal zones (sampled by micromilling) in intercumulus plagioclase and clinopyroxene from within the PGE-enriched Cr-spinel seams have revealed significant intra-crystalline heterogeneity. &amp;lt;sup&amp;gt;87&amp;lt;/sup&amp;gt;Sr/&amp;lt;sup&amp;gt;86&amp;lt;/sup&amp;gt;Sr heterogeneity is present between plagioclase crystals, between clinopyroxene and plagioclase, and within plagioclase crystals, throughout the studied section. The preservation of Sr isotope heterogeneities at 10-100 &amp;amp;#181;m length-scales implies cooling of the melts that formed the precious metal-rich layers at rates &amp;gt;1 &amp;amp;#176;C per year, and cooling to diffusive closure within 10s-100s of years. The combined textural observations and intra-crystal plagioclase &amp;lt;sup&amp;gt;87&amp;lt;/sup&amp;gt;Sr/&amp;lt;sup&amp;gt;86&amp;lt;/sup&amp;gt;Sr data also highlight the importance of repeated cycles of dissolution and recrystallization within the crystal mush, and together with recent documentation of &amp;amp;#8216;out-of-sequence&amp;amp;#8217; layers in other layered intrusions (Mungall et al. 2016; Wall et al. 2018), raise the prospect that basaltic magmatic systems may undergo repeated self-intrusion during solidification.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Hepworth, L.N., O&amp;amp;#8217;Driscoll, B., Gertisser, R., Daly, J.S. and Emeleus, H.C. 2017. Journal of Petrology 58, 137-166; Mungall, J. E., Kamo, S. L. &amp;amp; McQuade, S. 2016. Nature Communications 7, 13385; Wall, C. J., Scoates, J. S., Weis, D., Friedman, R. M., Amini, M. &amp;amp; Meurer, W. P. 2018. Journal of Petrology 59, 153&amp;amp;#8211;190.&amp;lt;/p&amp;gt; </jats:p

Felsites and breccias in the Northern Marginal Zone of the Rum Igneous Complex:changing views c. 1900-2000

As in several other parts of the British Tertiary Igneous Province, breccias and felsite sheets are closely associated on the Isle of Rum. This association has been described and interpreted by several workers over the last 125 years. Opinion has divided into an intrusive origin for both rock types, as explosion breccias and felsite intrusions, versus a sedimentary origin for the breccias and an extrusive origin for the felsite. Evidence is reviewed for both opinions and it is concluded that the latter is substantially correct, as indicated by the presence of sedimentary structures and interbedded tuffs in the breccias and eutaxitic textures in the felsites. The breccias formed by inwards slumping of rocks from the oversteepened walls of a caldera, whereas the felsites formed by eruption of pyroclastic flows which were thick and hot enough to weld. It is inferred that the caldera formed initially and subsided progressively without any accompanying eruptions, and this is attributed to growth of the underlying magma chamber. The breccias accumulated during this stage. There followed a resurgent stage in which caldera collapse occurred in response to repeated ignimbrite eruptions partially emptying the magma chamber. The chamber is inferred to have been chemically and mineralogically zoned.</p

Caldera formation in the Rum Central Igneous Complex, Scotland

The Northern Marginal Zone of the Rum Central Igneous Complex in NW Scotland represents part of the early, felsic phase of the volcano. The marginal zone is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks. Its formation has been explored through field examination of the ring fracture system of the Complex and its pyroclastic and epiclastic intracaldera facies. A sequence of magmatic tumescence and chamber growth caused initial doming, followed by the formation of a collapse structure without accompanying volcanism. This collapse structure, circular in plan, is akin in origin to a salt basin formed by crustal stretching above a rising diapir. We call this the proto-caldera. Collapse breccias, which represent the slumping and sliding of megablocks, blocks and boulders of the Torridonian sandstones which form the walls of the basin, were the original infilling. Logs of these deposits reveal considerable variation in thickness of the breccias (from 80-170 m) in the Complex, indicating an uneven floor to the proto-caldera, consistent with piecemeal collapse. Following accumulation of up to &gt;70 m thickness of breccia, thin interbedded rhyodacitic crystal tuffs (10-30 cm) record the earliest eruptions of felsic magma in the caldera. Caldera formation was then interrupted by a period of quiescence, recorded by the presence of an epiclastic sandstone of locally several metres thickness, formed by washout of fines from the breccias. Subsequent resurgence created a fracture pattern characteristic of doming, along which rhyodacite magma rose in dykes and erupted up to perhaps 10 km(3) of rhyodacitic intracaldera ignimbrites. This major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber. Mafic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma.</p

Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum igneous centre, Scotland

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (DeltaAn greater than or equal to 10%) followed by gradual decrease in An-content (DeltaAn less than or equal to 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe-Ti-P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.</p