Petrology of green polished stone axes of the Jomon period from the Sannai-Maruyama site,Japan, investigating the origin of source rock

Numerous green polished stone axes have been excavated from the Sannai-Maruyama site, one of Japan's largest archeological sites in the Jomon period (5.9–4.2 cal kyr BP). The axes are composed of weakly metamorphosed fine-grained volcaniclastic rock having a peculiar texture that includes numerous acicular actinolites growing in random directions within a quartz and albite matrix. Cobbles of Aotora stone found along the Nukabira River, Biratori town, southern Hokkaido, are the most likely raw material for these stone axes. Aotora stones have alternate bands of a soft dark-green picritic layer and a hard SiO₂-rich pale-green layer. The pale-green layer has a texture similar to the stone axes. Basaltic and picritic volcanic rocks of the Sorachi-Yezo Belt occupy the area along the Shidoni River, a tributary of the Nukabira River. Volcaniclastic rocks similar in texture, mineralogy, and bulk rock compositions to the Aotora stone are exposed in the area. These rocks underwent metamorphism under the actinolite-pumpellyite facies conditions. Their protolith is submarine hyaloclastic rocks that are intercalated with laminated picrite detritus. The stone axes, pale-green layers of Aotora stone, and those of the volcaniclastic rocks of the Shidoni River area all have high SiO₂ (~ 55 wt%), Cr (~ 840 μg/g), and Ni (~ 370 μg/g). The rare earth element patterns with abundant light rare earth elements and depleted heavy rare earth elements of stone axes were also consistent with the pale-green layers of the outcrop. These pale-green layers, interleaved with dark-green layers of picritic detritus, were the likely source rock of the stone axes. The high SiO₂ content in the pale-green layer caused the crystallization of quartz and albite in the matrix, which resulted in high-quality raw material for making stone axes.