Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport1–4, important characteristics of mid-ocean ridge magmatism1,5. A consequence of such shallow crustal processing of magmas4,5 is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust6,7. Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal direct sourcing from a near-Moho magma storage zone. Geochemical proxies, which signify different mantle compositions and melting conditions, changed at a rate unparalleled for individual basaltic eruptions globally. Initially, the erupted lava was dominated by melts sourced from the shallowest mantle but over the following three weeks became increasingly dominated by magmas generated at a greater depth. This exceptionally rapid trend in erupted compositions provides an unprecedented temporal record of magma mixing that filters the mantle signal, consistent with processing in near-Moho melt lenses containing 107–108 m3 of basaltic magma. Exposing previously inaccessible parts of this key magma processing zone to near-real-time investigations provides new insights into the timescales and operational mode of basaltic magma systems.

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<p>The recent eruption of the Fagradalsfjall complex in the Reykjanes Peninsula of Iceland represents incompletely mixed basaltic magma directly erupted from a sub-crustal storage region. The eruption comprises olivine tholeiite lava with whole rock MgO between 8.7 and 10.1 wt%. The macrocryst cargo comprises olivine up to Fo<sub>90</sub>, plagioclase up to An<sub>89</sub>, and Cr-rich clinopyroxene up to Mg# 89. Gabbro and anorthosite xenoliths are rare. Olivine-plagioclase-augite-melt (OPAM) barometry of the groundmass glass from tephra collected from 28<sup>th</sup> April to 6<sup>th</sup> May yield high equilibration pressures and suggest that this eruption is originally sourced from a deep (0.48±0.06 GPa) storage zone at the crust-mantle boundary.</p><p> </p><p>Over the course of the eruption, Fagradalsfjall lavas have changed significantly in source signature. The first erupted lavas (mid-March) were more depleted (K<sub>2</sub>O/TiO<sub>2</sub> ­= 0.14, La/Sm = 2.1, <sup>87</sup>Sr/<sup>86</sup>Sr = 0.703108, <sup>143</sup>Nd/<sup>144</sup>Nd = 0.513017, <sup>206</sup>Pb/<sup>204</sup>Pb = 18.730) and similar in composition to basalts previously erupted on the Reykjanes Peninsula. As the eruption continued, the lavas became increasingly enriched and were most enriched in early May (K<sub>2</sub>O/TiO<sub>2</sub> = 0.27, La/Sm = 3.1, <sup>87</sup>Sr/<sup>86</sup>Sr = 0.703183, <sup>143</sup>Nd/<sup>144</sup>Nd = 0.512949, <sup>206</sup>Pb/<sup>204</sup>Pb = 18.839), having unusual compositions for Reykjanes Peninsula lavas and similar only to enriched Reykjanes melt inclusions. From early May until the end of the eruption (18<sup>th</sup> September), the lava K<sub>2</sub>O/TiO<sub>2</sub> and La/Sm compositions displayed a sinuous wobble through time at lower amplitude than observed in the early part of the eruption. The enriched lavas produced later in the eruption are more enriched than lavas from Stapafell, a Reykjanes eruption thought to represent the enriched endmember on the Reykjanes. The full range of compositional variation observed in the eruption is large – about 2.5 times the combined variation of all other historic Reykjanes lavas.</p><p> </p><p>The major, trace, and radiogenic isotope compositions indicate that binary mixing controls the erupted basalt compositions. The mixing endmembers appear to be depleted Reykjanes melts, and enriched melts with compositions similar to enriched Reykjanes melt inclusions or Snaefellsnes alkali basalts. The physical mechanism of mixing and the structure of the crust-mantle boundary magmatic system is a task for future study.</p><p> </p><p>In contrast to the geochemical variations described above, the oxygen isotope composition (δ<sup>18</sup>O) of the groundmass glass (5.1±0.1‰) has little variation and is lower than MORB (~5.5‰). Olivine phenocrysts δ<sup>18</sup>O  values range from typical mantle peridotite values (5.1‰) to lower values (4.6‰), with the lower values in close equilibrium with the host melt. Given the crust-mantle boundary source of the eruption, these low δ<sup>18</sup>O values are unlikely to represent crustal contamination, and are more likely to represent an intrinsically low δ<sup>18</sup>O mantle beneath the Reykjanes Peninsula.</p>

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