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An Isotopic Perspective into the Magmatic Evolution and Architecture of the Rift Zones of Kīlauea Volcano

2018; Oxford University Press; Volume: 59; Issue: 12 Linguagem: Inglês

10.1093/petrology/egy098

1460-2415

A. J. Pietruszka, J. P. Marske, Daniel E. Heaton, Michael O. Garcia, J. M. Rhodes,

earthquake and tectonic studies

The basic model for the plumbing system of Kīlauea Volcano states that (1) compositionally distinct batches of primitive mantle-derived magma ascend through a primary conduit to the volcano’s summit magma storage reservoir and (2) this olivine-controlled magma is mixed and stored prior to eruption at the summit or lateral intrusion into the East Rift Zone (ERZ) or Southwest Rift Zone (SWRZ) for further storage, mixing, differentiation and/or eruption. In detail, however, the magmatic architecture of Kīlauea’s rift zones and their connection to the summit reservoir and the deeper source(s) of melt from the Hawaiian mantle plume, is more complicated. Here we use the Pb, Sr, and Nd isotopic compositions of historical Kīlauea summit and rift lavas, from 1959 to the recent eruptions at Puʻu ʻŌʻō and Halemaʻumaʻu, to test models for the volcano’s rift zones and decipher their magmatic evolution. These samples (including a 1919 summit lava) record a rapid and systematic variation in 206Pb/204Pb (18·39 to 18·66), 87Sr/86Sr (0·70347 to 0·70364), and εNd (+5·9 to +6·5) that derives from changes in the composition of the parental magma delivered to the volcano. Assimilation of the Pacific oceanic crust and/or the Hawaiian volcanic edifice does not significantly modify the isotopic composition of mantle-derived Kīlauea magmas. Many contemporaneous summit and rift lavas display small, but statistically significant, differences in 206Pb/204Pb and 87Sr/86Sr that are used to (1) identify magma batches from the mantle and (2) track their movement though the ERZ and/or SWRZ. A series of intrusions from the summit reservoir to the ERZ in the 1960s left behind a large amount of magma that became a prominent mixing component in subsequent rift lavas. Variations in the 206Pb/204Pb and 87Sr/86Sr ratios of lavas from the Mauna Ulu rift eruption (1969–1974) are mostly related to mixing within the ERZ between stored 1960s-era magma and at least two new mantle-derived magmas. Lavas from several upper ERZ (UERZ) eruptions in the 1960s and 1970s (including the August 1968 eruption, the Mauna Ulu eruption, and additionally, the eccentric December 1974 eruption near the volcano’s summit) may each contain at least one component of mantle-derived magma that bypassed the summit reservoir. The early differentiated lavas from the Puʻu ʻŌʻō rift eruption (1983–2018) were stored in the middle ERZ (MERZ) since the 1960s. This stored magma was rapidly flushed from the ERZ by olivine-controlled 1982-era summit magma, the last of which erupted at Puʻu ʻŌʻō by ∼1990. Thereafter, Puʻu ʻŌʻō lavas display (1) short-term fluctuations in 206Pb/204Pb on a time scale of <10 years and (2) a gradual increase in 87Sr/86Sr from ∼0·70359 to a maximum of ∼0·70364 between ∼2000 and 2003, and a slight decrease thereafter. These mantle-derived isotopic variations were efficiently transmitted from the source to the surface because Puʻu ʻŌʻō lavas were poorly buffered by the small volume of magma within the summit reservoir and UERZ to MERZ (∼0·2 km3, as inferred from a magma mixing trend). An increase in the maximum duration of pre-eruptive magma storage from the UERZ (<7 yr) to the MERZ (<19 yr) and, on average, the more differenatiated nature of downrift lavas is likely controlled by a decrease in the long-term average rate of magma supply to the more distal portions of the ERZ. Our results support recent models for Kīlauea’s plumbing system: (1) the summit reservoir comprises two magma bodies; (2) the ERZ is connected to the ∼2–4 km deep summit magma body beneath the southern rim of the caldera; (3) the volcanic SWRZ is connected to the < 2 km deep summit magma body beneath the eastern rim of Halemaʻumaʻu.