Collaborative Research: Record of Mafic Inputs to an Arc Magma System: Analysis of Enclaves and Their Host Lavas Cored During Phase I, Unzen Scientific Drilling Program

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EichelbergerEAR-0106834This proposal seeks support for the PIs to participate in internationally collaborative geochemical investigations of samples now being obtained during the first phase of the Unzen Scientific Drilling Project (USDP), Japan. USDP is a developing project of the International Continental Drilling Program (ICDP), which seeks to use drilling as a tool to advance understanding of fundamental processes of continental crust. The United States is a major partner and funding source for ICDP, through the National Science Foundation. The goal of USDP is to understand the eruptive behavior of Unzen as representative of an important class of andesitic arc volcanoes. This is to be done through a program of drilling, borehole observation, and core analysis in complement with ongoing surface geological and geophysical studies as well as the remarkable suite of geophysical and geochemical observations made during the recent eruption. That devastating episode occurred during 1991 to 1994, claimed 44 lives, and did some $2 billion in damage.In the first phase of the project, for which support is requested through this proposal, flank holes of 750 m and 1500 m are being drilled to the base of the edifice in order to obtain a complete physical and chemical record of the volcano's activity. Simultaneously, and also to be supported under this request, planning will be undertaken for the second phase in which multiple core sections through the conduit will be obtained in order to address problems of magma transport and degassing. These observations will aid in understanding the paradox of effusive eruption of water-rich magma. The overall scientific rationale and plan for this undertaking are presented in the proposals to ICDP and will be repeated in condensed form here to provide context. ICDP itself supports drilling only, not the associated science, hence the need for this request.Recent eruptions of Unzen, Pinatubo, and Soufriere Hills volcanoes have graphically demonstrated the concept that injection of mafic magma into a crustal slush is an important petrologic process and a volcanic trigger. The high visibility of these events has served to strengthen a dialog between workers on volcanic and plutonic rocks, and reinvigorated the investigation of mafic enclaves and disequilibrium phenocryst assemblages. A basic question has become: how do we fit these products of open system behavior of magma systems, produced by mafic recharge or replenishment, into the broader picture of crustal plutonism and volcanism? A subset of that question, which this proposal seeks to answer, is: What are the long term-petrologic consequences of repeated mafic inputs to the crustal magma system? Studies to date have elucidated single events as sampled by eruption products or viewed the end product of many such events in the form of mafic and silicic layered intrusions (MASLI). We wish to look at the time series provided by 500,000 years of Unzen eruptions, to characterize the inputs to the system and understand their influence on its chemical evolution. The work will be coordinated with complementary efforts in Japan on dating the sequence and documenting the geologic and chemical evolution of the volcano.