Quantitative modeling of granitic melt generation and segregation in the continental crust

Abstract

[1] We present a new quantitative model of granitic (in a broad sense) melt generation and segregation within the continental crust. We assume that melt generation is caused by the intrusion of hot, mantle-derived basalt, and that segregation occurs by buoyancy-driven flow along grain edges coupled with compaction of the partially molten source rock. We solve numerically the coupled equations governing heating, melting, and melt migration in the source rock, and cooling and crystallization in the underlying heat source. Our results demonstrate that the spatial distribution and composition of the melt depends upon the relative upward transport rates of heat and melt. If melt transport occurs more quickly than heat transport, then melt accumulates near the top of the source region, until the rock matrix disaggregates and a mobile magma forms. As the melt migrates upward, its composition changes to resemble a smaller degree of melting of the source rock, because it thermodynamically equilibrates with rock at progressively lower temperatures. We demonstrate that this process of buoyancy-driven compaction coupled with local thermodynamic equilibration can yield large volumes of mobile granitic magma from basaltic and meta-basaltic (amphibolitic) protoliths over timescales ranging from $4000 years to $10 Myr. The thickness of basaltic magma required as a heat source ranges from $40 m to $3 km, which requires that the magma is emplaced over time as a series of sills, concurrent with melt segregation. These findings differ from those of previous studies, which have suggested that compaction operates too slowly to yield large volumes of segregated granitic melt.

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

“…This pulse was soon followed by voluminous rhyolite volcanism within the HLP from 7.3 to 6.9 Ma, including many domes and the 280 km 3 RST at 7.1 Ma [Streck and Grunder, 1995;Jordan et al, 2004], representing approximately a quarter of the total volume of silicic magmatism ( Figure 3d). In part, the basaltic flows and voluminous RST may well have covered rhyolitic lavas erupted coeval with basaltic lavas or there may have been a thermal lag between basalt and rhyolite activity [Jackson et al, 2003]. Furthermore, there are limited basalts in the NWBR but still some silicic centers within the NWBR that fall within this age range ( Figure 5).…”

“…Except for Holocene rhyolites at Newberry Volcano (e.g., Big Obsidian Flow), there are no rhyolites associated with this youngest pulse of basaltic activity. We attribute this to a combination of insufficient flux of mafic magmas and a crust that has been made too refractory for partial melting [Ford and Grunder, 2011], or the thermal lag between basalt and rhyolite activity [Jackson et al, 2003].…”

The sources of granitic melt in Deep Hot Zones

“…Models of heat transfer between mafic magma and crust have typically involved a single mafic sill instantaneously emplaced in contact with crustal rocks (Barboza & Bergantz 1996;Bergantz 1989;Huppert & Sparks 1988;Jackson et al 2003;Raia & Spera 1997). The amount of crustal melt generated in these models depends on the thickness of the mafic sill, and on the initial temperatures and physical properties of the mafic magma and crust.…”

Finite element modeling of hydraulic fracturing in 3D

“…It should be mentioned that hydraulic fracturing occurs naturally in the subsurface as a mechanism that releases fluids during pressure build-up over geological time [31,37]. Other examples are melt segregation and eruption in the crust [16], melt intrusion as sills and dykes [32], mud volcanos and hydrothermal megaplumes [18].…”