Volcanic plumbing filters on ocean-island basalt geochemistry

Ocean-island basalts (OIBs) are considered to be messengers from the deep mantle, yet the filtering effect of the plumbing systems that bring OIB melts to the surface remains poorly assessed. We investigated volcanic products from El Hierro island (Canary Islands) from textural and chemical perspectives. The majority of geochemical data cluster at relatively fractionated basaltic compositions of 5 wt% MgO. Compositions ≥10 wt% MgO are porphyritic whole rocks that accumulate mafic minerals. Near-primary melts do not erupt. Instead, we show that carrier melts (crystal-free whole rocks, glasses, and melt inclusions) are consistently buffered to low-MgO compositions during passage through the plumbing system. We tested our model of melt fractionation and crystal accumulation on a global compilation of OIBs. Similar to El Hierro, the majority of data cluster at evolved compositions of 5 wt% MgO (alkaline) to 7 wt% MgO (tholeiitic). Modeling the fractionation of OIB parental melts, we show that with 50% crystallization, OIB melts reach 5 wt% MgO with reduced density, increased volatile content, and overall low viscosity, becoming positively buoyant relative to wall rocks and highly eruptible when reaching volatile saturation at depths around the crust-mantle boundary. Under these conditions, 5 wt% MgO OIB “sweet spot” melts are propelled to the surface and erupt carrying an assortment of recycled crystals. This mechanism is consistent with the petrography and chemistry of erupted products and suggests OIB volcanoes are dominated by low-MgO basaltic melts.

Supplemental Material: Volcanic plumbing filters on ocean-island basalt geochemistry

Data (Tables S1–S8), methods, and Figures S1–S3.<br>

Supplemental Material: Volcanic plumbing filters on ocean-island basalt geochemistry

Data (Tables S1–S8), methods, and Figures S1–S3.<br>

Magmatic architecture below the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand, inferred from basalt geochemistry

The Okataina Volcanic Centre (OVC) is the most recently active rhyolitic volcanic centre in the Taupō Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are numerous examples of basaltic to basaltic-andesite contributions to OVC eruptions, ranging from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. Plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions within the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that at least five different magma types (two basalts, two dacites, one rhyolite) are sampled during basaltic eruptions. The most abundant basaltic magma type is generated by cooling-induced crystallisation of a common, oxidised, basaltic melt at various depths throughout the crust. The volatile content of this melt was increased by protracted fluid-undersaturated crystallisation. All eruptions display abundant evidence for syn-eruptive mixing of the different magma types. Rotomakariri, consisting of a mafic crystal cargo mixed into a dacitic magma is the most extreme example of this process. Despite similar bulk compositions, comparable to other basaltic deposits in the region, these four OVC eruptions are texturally distinct as a consequence of their wide variation in eruption style.