Igneous processes within crustal magma reservoirs - a geochemical investigation of igneous layering within the lower oceanic crust (Hess Deep) and The Rum igneous breccia


Thea Hatlen Heimdal

CEED PhD Student



Geochemical investigations of the first in-situ sampled layered oceanic 
crust (Hess Deep), and whole rock geochemical data of the systematically 
under-sampled magmatic breccias from Rum (Scotland), shed new light on 
intrinsic igneous processes within crustal magma reservoirs. 
Petrogenetic modeling postulates that 10 % melting of a mantle source is 
capable to produce the primitive parental melts responsible for 
crystallizing large mafic layered lithologies after passing the 
mantle-crust transition. In both of the studied areas, initial melts 
have high Mg# (> 70), and up to komatiitic compositions (MgO > 27 wt.%) 
at Rum. The olivine oversaturation of these melts is best illustrated by 
the observed crystallization of harrisitic (skeletal/spinifex) olivine. 
Transport of these low viscous, “hot” (>> 1200 °C) melts within the 
crust can be porous and pervasive on a local scale, however major melt 
passage is observed to occur within fractures. A key process within this 
transport seems to be “magmatic fracturing,” which can be understood as 
a procedure with similar physics to hydrofracturing during oil 
exploration. The increasing permeability fragmentation process results 
in magmatic brecciation of lower crustal cumulates, which now allows 
focused melt transport within dykes and veins through a formerly 
impermeable layer. After the melt has been emplaced within a lower 
crustal reservoir, in situ crystallization and melt evolution is 
restricted to sills. Such sills are modally layered with primitive 
troctolitic cumulates at the base, overlain by more evolved gabbros, and 
anorthositic gabbros at the top. The gabbros are unlikely to be 
differentiation products of a single parental melt, but probably 
attributed to mixing of residual melt with replenished mantle melt. In 
contrast, the upper anorthositic layer is petrogenetically linked to the 
accumulation of buoyant plagioclase crystals on top of the sill. 
Finally, the studied layered lithologies are supporting that a 
sheeted-sill like model is most plausible for the formation of fast 
spread oceanic crust. Correspondingly, such a petrogentic sill model is 
most likely also describing continuously replenished layered mafic – to 
ultramafic intrusions.

Published Aug. 25, 2015 1:29 PM - Last modified June 28, 2016 3:58 PM