The ⁴⁰Ar/³⁹Ar and K/Ar geochronometers are valuable tools for defining the eruption histories of Pleistocene arc volcanoes in Japan and around the world. It is particularly important to determine the timing of eruption events over the last 100,000 years to enable assessments of recent trends in magma compositions and volumes at potentially active volcanoes. However, measuring accurate and precise ⁴⁰Ar/³⁹Ar and K-Ar ages for late Pleistocene eruptive products is challenging due to the low yields of radiogenic ⁴⁰Ar in young volcanic rocks.

For this study, we have characterized the textural and compositional features of lava flow groundmass samples for which ⁴⁰Ar/³⁹Ar and K-Ar ages have been determined. Studied samples were collected from Ruapehu volcano in New Zealand, and Chokai, Zao, and Ontake volcanoes in Japan. The sizes and K-contents of groundmass feldspars in these samples were measured and compared with the precision of their age measurements. These datasets have provided preliminary benchmark values for the optimal textures and compositions required for high-precision ⁴⁰Ar/³⁹Ar and K-Ar age measurements of late Pleistocene andesite lava flows.

Active volcanoes are key natural laboratories to study in detail -and monitoring- the history of magmas before, during and after eruptions. The scientific interest to advance our knowledge of how magma reservoirs behave and evolve is a fundamental and long-lasting target in Earth Sciences in general and in volcanology-petrology-geochemistry in particular, as it is directly related to the type of hazards that volcanoes may generate. When an eruption is already inevitable, the fundamental information about the magmatic evolution under the volcano, which strongly depends on how the plumbing system behaves at depth, is challenging, infrequent and often too late for mitigating volcanic hazards. Hence, combining exhaustive petrological studies of erupted products with volcano monitoring data recorded prior to -and during- a volcanic eruption becomes extremely useful to achieve such purpose, which its first step is integrating petrologic and geochemical research. Studies of petrography, mineral and bulk chemistry, stable isotopes, thermodynamics, micro-CT and microRaman techniques, of erupted materials provide essential information about the plausible mechanisms controlling the next eruption initiation through the physical and chemical changes that occur at depth prior to eruptions. In this talk, we will visit volcanoes within different geological settings (eg. Mt. Zao, Deception Island, Ruapehu, Hofsjökull, Tagoro) connecting from the petrologic and geochemical perspectives the magma plumbing system with its evolution up to eruption at surface. The results of this magma evolution can be considered in further studies of volcanic monitoring to improve the capability to interpret geochemical and geophysical data and signals recorded during volcanic unrest episodes, and hence, forecast volcanic eruptions and related hazards.

The Ryoke belt records the Cretaceous magmatic flare-up in East Asia and along arc variation of the magmatic pulses is studied in detail. Recently, U-Pb zircon ages of the granitoids accumulated in the Mikawa [1, 2] and Yanai [3] areas revealed a variation in pulsed magmatism within the Ryoke belt. Meanwhile, in the central Mie prefecture, such scheme is yet to be established despite several U-Pb zircon ages of the granitoids are reported [4, 5]. This study reports new U-Pb zircon ages of the Ryoke granitoids (nomenclature after [6]) of this area and compared its magmatic scheme with other regions of the Ryoke belt.

Weakly mylonitised Yokono granodiorite consists mainly of Qtz+Pl+Kfs+Bt. Zircon is present in the matrix or included in Pl and Kfs porphyroclasts. Weakly foliated Fukudayama granodiorite mainly consists of Qtz+Pl+Bt+Hbl, and zircon is found in the matrix and included in Pl and Bt. In both granitoids, CL-dark oscillatory-zoned domain is located in the outer part of zircon. The weighted mean U-Pb ages ±2σ error of these domains are 90±1 Ma for Yokono granodiorite, and 88±1 Ma for Fukudayama granodiorite. These are interpreted as magmatic ages since they are obtained from the oscillatory-zoned domain.

Compilation of the U-Pb zircon ages of the Ryoke granitoids from the central Mie prefecture [4, 5] and this study revealed three magmatic pulses at ca. 110-100 Ma, ca. 90 Ma, and ca. 80-70 Ma. The similarity of the magmatic scheme of this area and the Mikawa area is shown by the wide exposure of the youngest pulse plutons from the north to the vicinity of the MTL in the south. Meanwhile, the differences of the two areas are shown by the presence of ca. 90 Ma pulse and the older-aged nature of the oldest pulse in central Mie. The areal distribution of the oldest pulse plutons is also different. The ca. 110-100 Ma pulse plutons are widely exposed to the south of the Grt-Crd zone in central Mie, while ca. 99-95 Ma pulse plutons occur to the north of the Grt-Crd zone in Mikawa. Oscillatory-zoned ca. 100 Ma inherited cores from ca. 70 Ma granite [5] may imply that the ca. 110-100 Ma pulse plutons are wider-spread than currently exposed in central Mie.

References

[1] Takatsuka+ (2018a) Island Arc [2] Takatsuka+ (2018b) Lithos [3] Skrzypek+ (2016) Lithos [4] Higashino+ (2023) JAMS abstract [5] Kim+ (2024) JpGU abstract [6] Hayama+ (1982) J. Geol. Soc. Japan