Abstract
It has been widely recognized that magmas undergoing fractional crystallization can simultaneously assimilate the surrounding crust, as a consequence of heat transfer from hot magmas to the cool crust. Because of the importance and interest in the geochemical and dynamical phenomena associated with assimilation and fractional crystallization (AFC), the underlying physics and the geochemical consequences of the required mass and energy balance, have received considerable attention. However, there is still a shortage of good field examples, constrained by geological observations and detailed geochemisty, which can be used to test the various hypothesis for AFC. Geological approach has a potential to extract direct information on the AFC process involved in individual magmatic systems, through careful examinations of the couplings of the fundamental processes, such as crystallization, solid-melt separation, and transport of crustal melt. However, the previous geological studies have commonly just used geochemical data to evaluate the AFC process in terms of the well-known model of DePaolo (1981), and few studies have elucidated the mechanisms responsible for the AFC process. In this study, we investigate mechanisms of AFC process involved in the evolution of the Kutsugata and Tanetomi lavas, an alkali basalt-dacite suite erupted sequentially from Rishiri Volcano, northern Japan. The Kutsugata and Tanetomi lavas have been the subject of detailed petrologic studies, and the major element variations within the suite have been explained by boundary layer fractionation; i.e. mixing of a magma in the main part of the magma body with a fractionated interstitial melt transported from the mushy boundary layer at the floor. Systematic variations in SiO2 correlate with variations in the Pb, Sr and Nd isotopic compositions of the lavas. It is suggested that crustal melt, transported from the floor crust, was mixed with the fractionated interstitial melt in the floor mush zone, and the mixed melt was further transported to the main magma, causing its geochemical evolution characteristic of AFC process. In order to estimate the ratio of assimilated mass to crystallized mass (r-value) in the magma chamber, a mass balance model including both AFC and boundary layer fractionation is newly developed. The geochemical variations of the lavas can be explained by a constant and relatively low value of the parameter r. Using the estimated r-value, as well as the estimated rate of the inward solidification of the floor boundary layer, the volume flux of the assimilant from the crust to the magma chamber is estimated. The volume flux is suggested to have decreased progressively with time (proportional to t-1/2), and was about 3 x 10-2 m/year at t = 10 years and 1 x 10-2 m/year at t = 100 years.
