Structures Related to the Emplacement of Shallow-Level Intrusions

A systematic view of the vast nomenclature used to describe the structures of shallow-level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions

Efficiency of compaction and compositional convection during mafic crystal mush solidification: the Sept Iles layered intrusion, Canada

Adcumulate formation in mafic layered intrusions is attributed either to gravity-driven compaction, which expels the intercumulus melt out of the crystal matrix, or to compositional convection, which maintains the intercumulus liquid at a constant composition through liquid exchange with the main magma body. These processes are length-scale and time-scale dependent, and application of experimentally derived theoretical formulations to magma chambers is not straightforward. New data from the Sept Iles layered intrusion are presented and constrain the relative efficiency of these processes during solidification of the mafic crystal mush. Troctolites with meso- to ortho-cumulate texture are stratigraphically followed by Fe–Ti oxide-bearing gabbros with adcumulate texture. Calculations of intercumulus liquid fractions based on whole-rock P, Zr, V and Cr contents and detailed plagioclase compositional profiles show that both compaction and compositional convection operate, but their efficiency changes with liquid differentiation. Before saturation of Fe–Ti oxides in the intercumulus liquid, convection is not active due to the stable liquid density distribution within the crystal mush. At this stage, compaction and minor intercumulus liquid crystallization reduce the porosity to 30%. The velocity of liquid expulsion is then too slow compared with the rate of crystal accumulation. Compositional convection starts at Fe–Ti oxide-saturation in the pore melt due to its decreasing density. This process occurs together with crystallization of the intercumulus melt until the residual porosity is less than 10%. Compositional convection is evidenced by external plagioclase rims buffered at An 61 owing to continuous exchange between the intercumulus melt and the main liquid body. The change from a channel flow regime that dominates in troctolites to a porous flow regime in gabbros results from the increasing efficiency of compaction with differentiation due to higher density contrast between the cumulus crystal matrix and the equilibrium melts and to the bottom-up decreasing rate of crystal accumulation in the magma chamber

Chemical Composition and Evolution of the Mantle

A scheme of mantle evolution is proposed that involves extensive (~25%) partial melting of primitive mantle during accretion, followed by cumulate formation in the separated melt and transfer of late-stage fluids, similar to KREEP, from the deeper to the shallower cumulates. Midocean ridge basalts (MORB) form by remelting of incompatible element depleted garnet-rich cumulates; continental and ocean island basalts, including alkali basalts, form by partial melting of a shallow enriched peridotite layer. Forward calculations show that the initial magma and its cumulates have relatively unfractionated Rb/Sr and Sm/Nd and therefore will appear primitive in terms of isotopic ratios. Effective fractionation occurs relatively late in earth history when mantle cooling has reduced the amount of residual fluid in the cumulate layers. The transfer of an intercumulus fluid or partial melt is responsible for depletion of the MO RB reservoir and progressive enrichment of the continental/ocean-island basalt reservoir. The MORB reservoir appears to be an eclogite that earlier had lost a kimberlitic late-state melt. The eclogite, in turn, may have been the result of fractionation of a separated primary melt early in earth history. The large-ion lithophile (LIL) patterns of enriched magmas may be inherited from metasomatic fluids that had been in equilibrium with garnet, rather than indicating a garnet-rich composition for the immediate parent and the residue after partial melting. The composition of the mantle eclogite layer may be picritic. Although the omphacite-pyrope system has not been studied at sufficiently high pressure, results on related systems suggest that the eclogite-garnetite transformation may be responsible for the 400-km discontinuity. The density jump at this discontinuity is about 3%; it seems to be a second-order transition