Rapid Mantle-Crust Transit Times Derived From Crystal Zonation Modeling
Abstract
The timescales of magma transit through the entire crustal column in mature volcanic plumbing systems have been elusive, as both geophysical and geochemical techniques are most sensitive to upper crustal processes. Geophysical studies of the lower crust are often hampered by the small signal-to-noise ratio and the absence of intrinsically large earthquakes as a result of limited stress build-up. However, new approaches in seismic studies (Shelly and Hill, 2011; Tarasewicz et al., 2012) use waveform cross correlation techniques to augment existing earthquake catalogs and document hypocenter migration associated with fluid movement within the entire crust and even within the uppermost mantle. This technique is limited to (1) well-monitored volcanic systems and (2) transient magma transport that may or may not be associated with current volcanic activity. Complementary to these seismic approaches, we have developed a geochemical speedometer on the basis of trace element diffusion modeling (primarily Ni) in primitive olivine "phenocrysts" that integrates over the entire crustal ascent path and can be used to extract ascent rates from both current and older eruptions. Underlying the use of the geochemical speedometer is the assumption that primitive (> Fo88) olivines crystallize close to the Moho and then are transported without long-term (> one year) storage in the crust. We present thermal dike models showing that the conditions in the crust are unfavorable for the long-term preservation of primitive olivine crystals as cooling, differentiation, and diffusive re-equilibration inevitably lead to lower Fo olivines. Furthermore, distinct trace element signatures (e.g., Ni, Cr, Ca) in individual phenocrysts require the presence of compositionally distinct mantle-derived melts that most likely pool at the rheologic boundary of the Moho. Crystallizing olivine then captures these distinct melts during a first episode of cooling. Their magmatic origin is supported by high CaO contents (> 0.1 wt%). As the complex trace element zonations are not erased diffusively during ascent they constrain transport from the Moho to the surface. Here we show that such olivine crystals in hybrid magmas with primitive components from various arc settings (Central America, Alaska, Tonga) transit the entire crust (20-40 km) on the order of months to years. Micro-earthquake swarms (at Iceland, Tahoe and Long Valley) have also recently been discovered in the lower crust, some migrating upward at comparable rates. The two approaches are thus providing complementary information about rapid magma recharge directly from the mantle at timescales relevant to eruptions themselves. These advances in volcano seismology and geochemical modeling are new steps towards illuminating magma transport in the lower crust, thus allowing deeper and earlier forecasting of volcanic activity. REFS: [1] Shelly and Hill (2011) GRL, doi: 201110.1029/2011GL049336. [2] Tarasewicz et al. (2012) JGR, doi: 201210.1029/2011JB008751.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V43F..04R
- Keywords:
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- 3613 MINERALOGY AND PETROLOGY / Subduction zone processes;
- 3620 MINERALOGY AND PETROLOGY / Mineral and crystal chemistry;
- 8419 VOLCANOLOGY / Volcano monitoring;
- 8434 VOLCANOLOGY / Magma migration and fragmentation