Sunday, 15 September 2019

Xenoliths in volcanic rocks from the Cape Verde Oceanic Plateau, and their implication for the structure of the Cape Verde volcanic system.

The Cape Verde Archipelago comprises ten islands and eleven seamounts arising from an oceanic plateau off the coast of West Africa. The volcanic rocks of the region are predominantly alkaline in nature, though a few of the volcanic centres are composed of felsic material. These rocks also contain a range of xenoliths, pieces of rock which are incorporated into volcanic deposits, but which are older in nature, pieces of evidence which can be crucial to the understanding of the structure and evolution of volcanic complexes.

In a paper published in the journal Minerals on 1 February 2019, Abigail Barker of the Department of Earth Sciences at Uppsala University, and the Centre of Natural Hazards and Disaster Sciences, Thor Hansteen of the GEOMAR Helmholtz Centre for Ocean Research Kiel, and David Nilsson, also of the Department of Earth Sciences at Uppsala University, describe the results of a study of xenoliths from Cadamosto Seamount and the Charles Darwin Volcanic Field of the Cape Verde Archipelago by remote operated vehicle. The Cadamosto Seamount is a roughly 2000 m high, large, mature dominantly phonolitic seamount, whereas the Charles Darwin Volcanic Field is comprised of multiple small, young mafic to phonolitic eruption centres on the Cape Verde plateau.

(a) Location map of the Cape Verde archipelago, (b) shows the Cape Verde islands and bathymetry including certain seamounts. The oceanic plateau was extensively sampled by ROV dives and dredging during the Meteor M80/3 expedition to Cape Verde in January 2010. The stars mark the locations of xenolith bearing samples featured in this study. The southwestern star depicts the Cadamosto Seamount located at the southwest of the southern chain of volcanic islands. The other star marks the Charles Darwin Volcanic Field, which lies southwest of the northern volcanic island chain, on the lower submarine flank of Santo Antão. Barker et al. (2019).

The xenoliths found by Barker et al. cam from relatively evolved lavas such as phononephelinites and phonolites. Igneous rocks are considered to be 'evolved' if they have formed from the last phases of a liquid melt that slowly cooled, so that many minerals with high crystallisation temperatures have been lost from the melt earlier; such rocks tend to be dark, dense and hard, with high calcium, magnesium, and iron contents. Such rocks are consistent with magmas with deep origins, probably in the upper oceanic lithospheric mantle, beneath the Mohorovičić discontinuity (the boundary between the Earth's crust and its mantle).

The largest xenoliths found were quite large reaching up to 5 cm in diameter, and are thought to be of sedimentary origin, with clay minerals and distinctive layering, whereas others appear to be of igneous origin, derived from earlier deep Earth processes. These are composed of plagioclase, clinopyroxene, olivine and orthopyroxene crystals of roughly similar size (100 μm).

Photomicrographs of host lavas and xenoliths from Cape Verde. (A) Feldspathoids in a lava from the Charles Darwin Volcanic Field (Sample 068DR14). (B) Phononephelinite from the Cadamosto Seamount (Sample 035ROV10). (C) Sedimentary xenolith with fine grained detrital quartz, feldspar and biotite in a clay matrix. Note the 100 μ m contact zone with the lava. (Sample 035ROV10). (D) Clinopyroxene, orthopyroxene and plagioclase bearing gabbro xenolith (Sample 068DR03). (E) Plagioclase and clinopyroxene in a gabbro xenolith (Sample 067ROV07) (XPL) (F) Olivine, clinopyroxene and plagioclase in a gabbro xenolith (Sample 068DR03) (XPL). (G) Xenolith contact with narrow rim on olivine and wide (100 μ m) rim on plagioclase (Sample 068DR14). (H) Finely recrystallized anorthoclase rim on xenolith plagioclase (Sample 068DR03) (XPL). Abbreviations: plag, plagioclase; ol, olivine; opx, orthopyroxene; cpx, clinopyroxene; di, diopside; aeg, aegirine; bt, biotite; no, nosean; ne, nepheline; le, leucite; sa, sanidine; ancl, anorthoclase. Barker et al. (2019).

Where the mineral olivine is present at the edges of xenoliths, it has begun to dissolve in the melt, then become a centre for the nucleation of clinopyroxene crystals. Barker et al. interpret this to suggest that the minerals had a surface temperature of about 1005°C, and the surrounding melt of about 1083°C, consistent with magma that was cooling during its ascent. The elemental composition of the clinopyroxene rims is similar to that of clinopyroxene in the surrounding matrix, confirming that they are derived from the melt rather than the xenoliths.
 
Barker et al. suggest that this is consistent with a model in which magmas from the lithospheric mantle rise up through oceanic crust gabbros and then shallower sedimentary rocks, collecting sedimentary xenoliths as they rise. Although these magmas are relatively well evolved, suggesting a long time interval between their formation by the melting of older rocks and their eruption at the surface, the presence of xenoliths acquired during their ascent implies that this ascent was fairly rapid, as a slow ascent would have given the xenoliths time to melt completely and be absorbed into the magma.

Schematic image of the lithospheric architecture beneath Cape Verde. The lavas crystallise clinopyroxene (cpx) in the oceanic lithospheric mantle, as they traverse the ocean crust they pick up xenoliths of ocean crust that have crystallised at crustal levels. Subsequently as magma ascent proceeds the magmas cross sediments evidenced by occasional sedimentary xenoliths, before eruption at submarine volcanic centres. Barker et al. (2019).

See also...

https://sciencythoughts.blogspot.com/2019/08/thousands-forces-to-flee-their-homes-as.htmlhttps://sciencythoughts.blogspot.com/2019/07/fifteen-dead-in-morocco-landslide.html
https://sciencythoughts.blogspot.com/2019/02/magnitude-35-earthquake-in-ifrane.htmlhttps://sciencythoughts.blogspot.com/2019/02/magnitude-43-earthquake-in-al-hoceima.html
https://sciencythoughts.blogspot.com/2019/02/collapse-at-gold-mine-feared-to-have.htmlhttps://sciencythoughts.blogspot.com/2019/02/magnitude-35-earthquake-in-figuig.html
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