日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-VC 火山学

[S-VC34] Connecting magma dynamics in vent-conduit system with surface expression of volcanic eruption

2019年5月30日(木) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:三輪 学央(防災科学技術研究所)、下司 信夫(産業技術総合研究所 活断層・火山研究部門)

[SVC34-P02] Kinetic delay of crystal growth controls plagioclase-basaltic melt apparent disequilibrium

*種田 凌也1石橋 秀巳1安田 敦2外西 奈津美2 (1.静岡大学理学部地球科学専攻、2.東京大学地震研究所)

キーワード:斜長石、結晶作用、カイネティクス、玄武岩、地質温度計

Plagioclase-melt compositional relations sensitively depend on temperature, T, and water content, XW, of melt. Therefore, the relations are formulated as functions of T and XW and used as geothermohygrometer (e.g., Putirka, 2008). Plagioclase-melt equilibrium is postulated for applying the thermohygrometer models. However, validity of the assumption is unobvious for microlite crystallization driven by degassing and/or cooling during eruption. Can we apply the thermohygrometers to the plagioclase-melt pairs formed by dynamic crystallization? To examine the issue, the cooling crystallization experiments of the high-Al basaltic melt were carried out and textural and chemical analyses were performed the run samples.

We used the high-Al basalt lava from Waianae, Hawaii Oahu as the starting material for the cooling crystallization experiments. This experiment was conducted using the 1atm fO2-controlled furnace at University of Hawaii at Manoa. After 3 hours pre-heating at 1180℃, ~30K higher than the liquidus, the samples were cooled at cooling rates, Rc, of 0.1, 0.3, 1, 3, and 10K/min, and then quenched in water at four target temperatures, Tq, of 1150, 1120, 1090, and 1060℃. We used FE-EPMA (JEOL-JXA-8530FPlus) and EPMA (JEOL8800R) at Earthquake Research Institute, University of Tokyo, for textural and chemical analyses of the run samples.

As Rc and/or Tq increase, the abundance and size of plagioclase crystals decrease. As Rc increases, the shape of plagioclase changes from euhedral to dendritic. The Rc-dependent shape change is more obvious at lower Tq. Pyroxene crystallization was suppressed in the run samples coolied at higher Rc. In addition, melt boundary layers are observed around pyroxene in the samples cooled at 0.3-1K/min, quenched at 1090℃ and cooled at 0.3-3K/min, quenched at 1060℃, and around plagioclase in the samples cooled at >3K/min, quenched at <1120℃. Lower diffusivity of Al2O3 compared to those of FeO and MgO is responsible for lower threshold cooling rate of melt boundary layer formation around pyroxene.

The maximum An# [=100Ca/(Ca + Na)] of plagioclase was almost the same among all run samples. We applied both plagioclase liquidus and plagioclase-melt An-partitioning thermometers of Putirka (2008) to the pairs of plagioclase rim-boundary layer melt (BLM) and plagioclase rim-far field melt (FFM) to estimate temperatures recorded in their phase compositions. At Tq=1150℃, the estimated temperatures well represent Tq for both BLM and FFM. However, at lower Tq, both of the thermometers estimate temperatures higher than Tq; difference between estimated temperature and Tq, ΔT, increases as decreasing Tq. The two thermometers estimate similar temperatures for each sample, and the differences between the two estimated temperatures do not depend on Rc. These results suggest that the plagioclase-melt partition coefficient of An component does not depend on the Rc. Therefore, the increase of ΔT at lower Tq is not due to the Rc-dependence of the partition coefficient. It is attributed to the kinetic delay of the crystal growth at lower temperature. On the other hand, the coincidence between calculated temperatures and Tq at 1150℃ suggests that kinetic delay of plagioclase crystal growth is small at near liquidus temperature.

Present results show that plagioclase-melt thermometers can be applied regardless of the Rc at higher temperatures, while magmatic temperature tends to be overestimated due to kinetic delay of crystal growth at lower temperature.