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Abstract:
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Mafic magma is the most common magma erupted at the surface of the earth . It is generated from partial melting of the mantle , which has been subdivided into end -members based on unique geochemical signatures . One reason these end members , or heterogeneities , exist is subduction of lithospheric plates back into the mantle . The amount of elements , such as Cl and K , removed during subduction and recycled into the deep mantle , is poorly constrained . Additionally , the amount of volatiles , such as Cl , that are recycled into the deep mantle will strongly affect the behavior of the system . I have looked at Cl and K in HIMU source melts to see how it varies . Cl /Nb and K /Nb suggest that elevated Cl /K ratios are the result of depletion of K rather than increased Cl recycled into the deep mantle .
After the mantle has partially melted and mafic melt has migrated to the surface , it usually erupts effusively or with low explosivity because of its low viscosity , but it is possible for larger eruptions to occur . These larger , Plinian eruptions , are not well understood in mafic systems . It is generally thought that basalt has a viscosity that is too low to allow for such an eruption to occur . Plinian eruptions require fragmentation to occur , which means the melt must undergo brittle failure . This may occur if the melt ascends rapidly enough to allow pressure to build in bubbles without the bubbles expanding . To test this , I have done decompression experiments to try to bracket the ascent rate for two Plinian eruptions . One eruption has a fast ascent , faster than those seen in more silicic melts , whereas the other eruption is unable to be reproduced in the lab , however it began with a increased viscosity in the partly crystallized magma .
After fragmentation and eruption , it is generally thought that tephra do not continue to crystallize . We have found that crystallinity increases from rim to core in two basaltic pumice . Textural data along with a cooling model has allowed us to estimate growth rates in a natural system , which are similar to experimental data . |