MELT INCLUSIONS IN FORSTERITIC OLIVINE FROM HISTORICAL KILAUEA LAVAS REVEAL DIVERSE MANTLE MELT COMPOSITIONS

Melt inclusions were examined in higher forsteritic olivine (85-90%) from Kilauea Volcano to better understand the nature and cause of the systematic geochemical variations in whole-rock, major and trace elements, and Pb and Sr isotopes (Pietruszka and Garcia 1999). High forsterite olivines offer the potential to preserve the signatures of the primitive mantle-derived inputs in this frequently active volcano (since 1823), which has received nearly continuous and increasing magma recharge from the mantle during the 20^th and 21^st centuries. Anomalous melt inclusion compositions (compared to host matrix) are likely to survive at Kilauea due to the short residence time of magma in the summit reservoir (<10 year) because of its small size (<1 km³; Pietruszka et al. 2015).

We examined euhedral, unstrained, forsteritic olivine from rapidly quenched, historical lava and tephra erupted in 1820, 1832, 1840, 1921, 1959, 1971, 1974, 1998 CE. Microprobe analyses were made of major elements and LA-ICPMS analyses of trace elements in naturally quenched melt inclusions and host matrix glasses. Most olivine have forsterite compositions in equilibrium with the whole-rock composition. To avoid complications from post-entrapment crystallization, ratios of major (e.g., Ca/Al, Ti/K) and trace elements (e.g., La/Yb, Zr/Nb) were examined. Melt inclusions show large ranges in these ratios with averages offset from the matrix glass in most samples. The compositional ranges of melt inclusions from individual samples are commonly larger than observed for all Kilauea lavas erupted over the last 200 years. Thus, the diversity of Kilauea mantle melt compositions was buffered by mixing with earlier magmas in the summit reservoir. Inclusions with depleted melt compositions (flatter REE patterns) and normal (matrix-like enriched patterns) were found in the same sample for some eruptions. The flat REE patterns may be related to a recently melted source (Pietruszka et al., 2006) or a long-term depleted source. Isotopic compositions (Pb, Sr) of the melt inclusions may help to resolve this issue.

Pietruszka et al. 2015. Earth Planet. Sci. Lett. 413, 90-100.

Pietruszka et al. 2006. Earth Planet. Sci. Lett. 244, 155-169.

Pietruszka & Garcia 1999. J Petrol 40, 1,321-1,342