Experimental investigation of the interaction of basaltic melts with peridotite with implication for mantle-crust interaction and magma origin

Gorbachev N.S., Kostyuk A.V., Nekrasov A.N.

Institute of Experimental Mineralogy RAS, Chernogolovka, Russia

The geophysical and geochemical data testify to a large-scale exchange of substance between a crust and a mantle. The major mechanism of such interaction is subduction of oceanic slab. Consequence of this process can be formation in a mantle of the chambers containing alongside with peridotite of protolithes ancient subducted of an oceanic slab. For experimental modelling melting of such source the system peridotite-basalt-volatile (๎₂๏, ๎₂๏+๓๏₂) in interval ๔=1250-1400º๓, ๒ = 1.5-4.0 GPa, at content of volatiles in system 3-5 wt.% was investigated.

Experiments were carried out using of apparatus high pressure (piston cilinder and anvil with hole) by a quenching technique. It was used two ampoules (platinum + peridotite) method (fig.1).

Fig.1. Back scattered electron images of the typical experimental sample. On the left –peridotite ampoule filled of silicate glass, sulfide. Sharp grey zone – area of interaction between silicate melt and peridotite; on the right - reaction zone between glass (melt) and peridotite with Opx substitute for Olš

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At interaction with peridotite melts, formed at melting of initial tholeiite or olivine basalt, ("reactionary" melting, the model of the closed system) were formed magnesian melts picritic-basalt type (tabl.1, น 1-2). At interaction of andesite melts with peridotite (the modelš of the open system), were formed SiO₂-rich magnesian melts lake boninites (tabl. 1, น 3). The volume "reactionary" melts was defined by volume of initial basalt.

Tabl. 1. Major elements compositions (wt. % oxides) of the typical hydrous “reaction” (1-3) and intergranular (4) mels. P = 2 GPa, T =1350ºC. šThe types of starting basalts: 1 – tholeiitic, 2 – olivine, 3 – andesitic. 4 - intergranular melt .š

In parallel with "reactionary" melting, there was a partial, "film" melting peridotite which degree did not exceed 5 % . In result of partial melting of peridotite were formed intergranular melts andesite-dacite type (tabl. 1, น 4).

The composition of liquidus associations, equilibrium with magnesian "reactionary" melts answered garzburgites Ol+Opx, and in the some cases - pyroxenites Cpx+Opx (fig. 2). Absence of Ol on the liquidus of magnesian basalts it is possible to explain reactions (1-3) of Ol with formed basalt melts:

šMg2SiO4 (Ol) + SiO2 (m) = 2MgSiO3 (Opx);

Mg2SiO4 (Ol) + Al2O3 (m) = MgSiO3 (Opx) + MgAl2O4 (Sp);

Mg2SiO4 (Ol) + MgSiO3 (Opx) + Al2O3 (m) + SiO2 (m) = Mg3Al2 [SiO4]

Fig. 2. Back scattered electron images for experimental samples, which show of the "reactionary" andš "film" melting peridotite and character interaction between silicate melt and peridotite: Gl₁ - the "reactionary" _šGl₂ – intergranulary melts; on the left side – Ol peridotite-melt equilibrium, on the right side – Opx substitute for Ol .

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Experiments have shown, that melting mantle with protolithes of an oceanic slab at "reactionary" melting for produce magnesian melts so high temperatures not required as at direct partial melting peridotite. At identical ๔ and ๒ volume generated melts, taking into account volume basitic components subduction oceanic slab, are much higher than melts, formed at direct partial melting peridotite. Intraplate magmatism, including formation of huge magmatic provinces can be connected to melting of the chambers, containing protolithes of an ancient oceanic slab.