Home – Home – Jornals – Russian Geology and Geophysics 2006 number 12

We present a synthesis of numerical modeling data for the evolution of mantle-crust systems in oceanic and continental spreading zones from decompression melting with the associated generation of mafic magmas and fluid release in their crystallization to mineral deposition in the crust. Model parameters were chosen to match those appropriate for natural magmatic-fluid systems in slow-spreading mid-ocean ridges (MOR) and the Siberian trap province. The evolution of a melting region was modeled for two cases: (i) a hot spot beneath a mid-ocean ridge, with 7-10 km thick oceanic crust underlain by metasomatized lithosphere, and (ii) a melting region beneath anomalously thick crust. Magmatic systems beneath thick crust were found out to be more compact and symmetrical and undergo a longer evolution.
Numerical modeling for continental melting zones with regard to the lithospheric structure and the size of the juxtaposed cratons and plates allowed the following inferences: (1) the extent of the predicted lithospheric melting region slightly exceeds the length of the respective lava field, (2) the melting zone has a layered structure (therefore, melts derived from a relatively homogeneous substrate should be homogeneous and of the same type), (3) magma chambers are relatively independent, which provides a qualitative explanation for the known cyclicity of lava compositions and the spatial distribution of major-element compositions of rocks in igneous provinces.
The behavior of the compositions of fluids outgassing at the solidus boundary from the crystallizing basaltic melt were computed using the Selektor software in a flow reactor and a step source modifications. Modeling shows that a quasi-steady temperature profile of a fluid-magmatic system related to a 30-40 km deep magma source sets up for 0.5 to 1 Myr. We infer that uncondensed reduced fluids vent on the seafloor and produce graphite and Fe, Ti, and Mn ferrite deposits found in the crest of the Mid-Atlantic Ridge. The numerical results were supported by physical modeling of carbon precipitation during the interaction of synthetic gas with mafic and ultramafic minerals. Carbon-related mineralization associated with gas condensation is controlled by the relative contents of C, H, Cl, F, and S in magmatic fluids. The composition of outgassing fluids changes notably within the liquidus-solidus range in crystallizing magma. During retrograde boiling, fluid separates into a low-density fraction and a brine.
Retrograde boiling in magma chambers and auto-metasomatism of igneous rocks are similar in mid-ocean ridges and in the Siberian trap province in the case of very low wall-rock assimilation and contamination of mafic melts. Assimilation of crustal material, especially carbonates and salt-bearing rocks, coal beds, hydrocarbons, or oil water, by mafic melts produces anomalous magmatic fluids with up to 60 rel.% total hydrocarbons, including 45-50% CH4, and total H₂O, H₂S, N₂, and with H₂ two orders of magnitude higher than CO₂ and CO. These very fluids promote the formation of mineral deposits hosted by igneous rocks.