DRIVING FORCES OF MANTLE PLUME VOLCANISM

Two models currently are favored for generation of mantle plumes at the core mantle boundary (CMB). One invokes catastrophic slab cascade into the deep mantle, ultimately resulting in increased core convection. Another invokes resonance between free core nutations and those forced by solar tidal torques, a process that by inference culminates in increased core convection. If these models are appropriate, mantle plume generation should occur synchronously with a stronger, more stable geomagnetic field.

The annual solar tide was resonant with free core nutation during the Pennsylvanian Period. If plumes responsible for end Permian large igneous provinces (LIPs) were generated during the Kiaman Superchron, a period of proposed high magnetic field strength, then their transits across the mantle required 10s of My. Eruption of both the Siberian and Emeishan Traps was constrained to <10 My, and there is no evidence of plume trails. These observations suggest an energetic, short-lived driving force, consistent with resonant nutations.

A superplume event spans 135-65 Ma, punctuated by peaks in LIP crustal production at about 125, 85 and 65 Ma. Volcanic trails extend from several LIPs of this age. Magnetic paleointensity, markedly low throughout most of this period, was punctuated by high field strength at about 115 and 95 Ma, and the Cretaceous Superchron spanned 120-80 Ma. LIP events at ca. 85 and 65 Ma are consistent with plume generation contemporaneous with both the Cretaceous Superchron and high magnetic field strengths, if 30 My were required for plumes to ascend from the CMB. Simultaneous activation of long-lived plumes, coupled with intermittent trends to high field strength, may reflect sporadic arrival of slabs at the CMB.

Geomagnetic field strength appears to have been comparatively low for more than 80 My before 120 Ma. No significant diminishment in magnetic reversal rate is observed across the same period. Yet several LIPs developed between 200-120 Ma. Whether these observations demand yet another driving force for LIP production, or suggest plumes develop absent core convection of a magnitude requisite for the production of a strong, stable magnetic field, remains debatable.