Methods for resolving the origin of large igneous provinces from crustal seismology

Abstract

[1] We present a new quantitative framework to understand the process of mantle melting based on the velocity structure of igneous crust. Our approach focuses on the lower crustal section, which is expected to be least affected by porosity and seawater alteration, especially for thick igneous crust. Our methodology is thus best for constraining the origin of large igneous provinces. First, a quantitative relation between bulk crustal velocity and mantle melting parameters is established on the basis of data from mantle melting experiments. Second, we show how lower crustal velocity can be used to place bounds on the expected range of bulk crustal velocity, despite ambiguity in crustal emplacement processes. By modeling fractional crystallization processes at a range of crustal pressures, these bounds are derived as a function of the proportion of lower versus upper crust. Finally, a simple mantle melting model is constructed to illustrate the effects of potential temperature, active upwelling, and a preexisting lithospheric lid on predicted crustal thickness and velocity. As an example, this new interpretation is applied to a seismic transect across the southeast Greenland margin to constrain mantle dynamics during the opening of the North Atlantic. Some complicating factors in seismological inference on mantle melting process, such as the possibilities of subcrustal igneous fractionation and mantle source heterogeneity, are also discussed with this example.

Formation and evolution of Shatsky Rise oceanic plateau: Insights from IODP Expedition 324 and recent geophysical cruises

“…Because hotter mantle temperature should cause greater melting (and thus greater crustal thickness), this finding can be taken as evidence that the source mantle was cool instead (Korenaga and Sager, 2012). This interpretation depends on the entire crustal column being formed at the same time (Korenaga et al, 2002), which may not be true if the volcano was built by long-traveled lava flows from the central region .…”

“…In addition, seismic refraction data from Tamu Massif display a curious negative correlation between seismic velocity and crustal thickness, not the positive correlation expected from a hotter thermal anomaly (Korenaga and Sager, 2012). This interpretation, however, is weakened because it depends on the assumption that the entire vertical crustal column formed simultaneously (Korenaga et al, 2002) and this may not be true for the location of the seismic transect across the center of Tamu Massif if the edifice formed by flows emanating from its center. These observations could argue instead for a thermochemical plume (e.g.…”

Are ‘hot spots’ hot spots?

“…The seismological demonstrations that the 'superplumes', and the body beneath the Ontong Java plateau, are not hot are salutary cases. Combining seismology with other disciplines, e.g., petrology, is a powerful approach that is able to distinguish between thermal and chemical origins for thick sequences of melt in flood basalts and at oceanic plateaus [Korenaga et al, 2002]. Seismic tomography cross sections of the mantle cannot be viewed as maps of the temperature.…”

“…Igneous crust formed at higher temperatures will not only be thicker, but also richer in olivine and MgO and as a result have higher V P . Crustal thickness and V P will thus be correlated for a hot source ( Figure 7) [Korenaga & Kelemen, 2000;Korenaga et al, 2002]. For a source at normal temperature, enhanced melt volume is expected to be related to source fusibility, not temperature, and thus a negative correlation, or no correlation at all, between crustal thickness and V P is predicted.…”

“…Korenaga et al [2002] studied the Greenland-Iceland ridge, and found that crustal thickness and V P are negatively correlated. This is consistent with the excess melt there arising from a source with enhanced fusibility and not high temperature ( Figure 7).…”