Formation of andesite melts and Ca‐rich plagioclase in the submarine Monowai volcanic system, Kermadec arc

Abstract

Andesites are typical rocks of island arcs and may either form by fractional crystallization processes or by mixing between a mafic and a felsic magma. Here we present new petrographic and geochemical data from lavas of the submarine Monowai volcanic system in the northern Kermadec island arc that display a continuous range in composition from basalt to andesite. Using petrology, major, trace, and volatile element data, we show that basaltic magmas mostly evolve to andesitic magmas by fractional crystallization. Our thermobarometric calculations indicate that the formation of the large caldera is related to eruption of basaltic‐andesitic to andesitic magmas from a magma reservoir in the deeper crust. Small variations in trace element ratios between the caldera and the large active cone imply a homogeneous mantle source. Contrastingly, resurgent dome melts of the caldera stagnated at shallower depths are more depleted and show a stronger subduction input than the other edifices. The Monowai basaltic glasses contain less than 1 wt % H2O and follow typical tholeiitic fractionation trends. High‐An plagioclase crystals observed in the Monowai lavas likely reflect mixing of H2O‐saturated melt batches with hot and dry tholeiitic, decompression melt batches. The result is a relatively H2O‐poor mafic magma at Monowai implying that partial melting of the mantle wedge is only partly due to the volatile flux and that adiabatic melting may play a significant role in the formation of the parental melts of the Monowai volcanic system and possibly other arc volcanoes.

Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation

“…The lavas range from tholeiitic basalts through to andesite, with their geochemical variation being consistent with fractional crystallisation under relatively anhydrous conditions (<1.1 wt% H 2 O) acting as the major process based on petrography and patterns of major and trace element variation (Timm et al, 2011;Kemner et al, 2015). The SiO 2 contents of these samples range from 51 to 61 wt%, 6.9 to 1.0 wt% for MgO, with TiO 2 contents encompassing a range of 0.57 to 1.17 wt%, reaching a peak in TiO 2 contents at Mg# of 44.20.…”

“…A brief summary of the geologic context of each sample suite is provided below. (Field et al, 2013); Ascension Island (Chamberlain et al, 2019), Heard Island (Barling, 1990); Santorini (Andujar et al, 2015; Alarcon Rise (Clague et al, 2018) and Monowai (Kemner et al, 2015) differentiation suites measured for their Ti isotopic composition in this study. * indicates intensive variables and Fe-Ti oxide compositions that are derived from Rhyolite MELTS (Gualda et al, 2012).…”

“…Evidence for the general absence of magma mixing in the studied suites comes from the scarcity of reverse zoning and disequilibrium textures e.g. Ascension Island (Chamberlain et al, 2016;Chamberlain et al, 2019) and Santorini (Klaver, 2016), lack of radiogenic isotope variation in Agung (Dempsey, 2013), Heard Island (Barling et al, 1990a(Barling et al, ,b, 1994a and Monowai (Timm et al, 2011) or the preservation of the FeO*, TiO 2 , and incompatible trace element enrichment along a single liquid line of descent in magmas recording the early stages of magma differentiation in Alarcon Rise (Clague et al, 2018), Monowai (Kemner et al, 2015) and the Afar Rift (Field et al, 2013). Whilst magma mixing is often invoked as the main mechanism of generation of intermediate arc magmas e.g.…”

“…The SiO 2 contents of these samples range from 51 to 61 wt%, 6.9 to 1.0 wt% for MgO, with TiO 2 contents encompassing a range of 0.57 to 1.17 wt%, reaching a peak in TiO 2 contents at Mg# of 44.20. Petrography indicates the onset of titanomagnetite crystallisation at ~ 4 wt % MgO (Kemner et al, 2015). Magnetite is present in basaltic and basaltic andesite lavas as a groundmass phase, but major element variation and least squares modelling suggest it is Ti-poor (Kemner et al, 2015).…”

“…Petrography indicates the onset of titanomagnetite crystallisation at ~ 4 wt % MgO (Kemner et al, 2015). Magnetite is present in basaltic and basaltic andesite lavas as a groundmass phase, but major element variation and least squares modelling suggest it is Ti-poor (Kemner et al, 2015).…”