Slow changes in lava chemistry at Kamaʻehuakanaloa linked to sluggish mantle upwelling on the margin of the Hawaiian plume

Temporal variations in lava chemistry at active submarine volcanoes are difficult to decipher due to the challenges of dating their eruptions. Here, we use high-precision measurements of 226Ra-230Th disequilibria in basalts from Kamaʻehuakanaloa (formerly Lōʻihi) to estimate model ages for recent eruptions of this submarine Hawaiian pre-shield volcano. The ages range from ca. 0 to 2300 yr (excluding two much older samples) with at least five eruptions in the last ~150 yr. Two snapshots of the magmatic evolution of Kamaʻehuakanaloa (or “Kamaʻehu”) are revealed. First, a long-term transition from alkalic to tholeiitic volcanism was nearly complete by ca. 2 ka. Second, a systematic short-term fluctuation in ratios of incompatible elements (e.g., Th/Yb) for summit lavas occurred on a time scale of ~1200 yr. This is much longer than the ~200-yr-long historical cycle in lava chemistry at the neighboring subaerial volcano, Kīlauea. The slower pace of the variation in lava chemistry at Kamaʻehu is most likely controlled by sluggish mantle upwelling on the margin of the Hawaiian plume.