Iceland's volcanic activity is attributed to a mantle plume intersecting the Mid-Atlantic Ridge. The Reykjanes Peninsula (RP), a volcanically active transform zone, experienced its first eruption in 781 years with the 2021 Fagradalsfjall eruption. This eruption, preceded by intense seismic activity and inflation, is likely the start of a new eruption period. Understanding the mantle sources feeding this eruption is crucial for assessing magma and hydrothermal fluid production in the region. A long-standing debate exists concerning the Icelandic geodynamic setting and the plume components involved in magma genesis. Geochemical studies suggest enriched and depleted components from a deep plume source. Stable oxygen isotopes (δ¹⁸O) are powerful tracers of crustal and mantle inputs due to their minimal fractionation at high temperatures and distinct ratios in mantle and crustal components. Previous studies reveal significant oxygen isotope diversity in Icelandic basalts, ranging from +2 to +6‰, much higher than the relatively uniform δ¹⁸O values of ~5.5‰ in normal upper mantle (MORB). The cause of this diversity – original mantle heterogeneity, crustal assimilation, or a combination – remains unresolved. This study aims to characterize the 2021 Fagradalsfjall eruption products to understand the mantle sources and test for oxygen isotope diversity in relation to different mantle components. The study covers a time span of 160 days from the eruption's onset, allowing for analysis of temporal changes in magma composition.

Decades of research on Iceland's geodynamic setting have led to ongoing debates about the exact nature and contribution of mantle plume components to magma genesis. Studies using trace element ratios (Zr/Y, Nb/Zr, Nb/Y) and radiogenic isotopes have suggested the involvement of both enriched and depleted mantle components in Icelandic magmatism. Stable oxygen isotopes (¹⁸O/¹⁶O ratios, expressed as δ¹⁸O) offer a robust method to trace crustal and mantle contributions because oxygen is a major component of magmatic rocks, isotopic ratios are minimally fractionated at high mantle temperatures, and mantle and crust have distinct oxygen isotope ratios. Previous oxygen isotope studies on Icelandic rocks have revealed a wide range of δ¹⁸O values (+2‰ to +6‰), far exceeding the variation expected from magmatic processes and significantly different from the uniform δ¹⁸O values of ca. 5.5‰ found in typical MORB. The origin of this diversity – whether it reflects primary mantle heterogeneity, crustal assimilation of low-δ¹⁸O hydrothermally altered basaltic crust, or both – is a central unresolved issue in understanding Icelandic volcanism.

The study analyzed 30 lava and tephra samples for major and trace elements, using ICP-OES and ICP-MS. 47 glassy samples were analyzed for oxygen isotopes (δ¹⁸O and Δ¹⁷O) using laser fluorination and gas-source mass spectrometry at the University of Oregon. Three samples underwent Δ¹⁷O analysis and ten were analyzed for water content and hydrogen isotopes (δD) using a TCEA-MAT-253 system. Additionally, electron probe microanalysis (EPMA) was used to analyze groundmass glasses and minerals. The data were analyzed to investigate temporal changes in magma composition, evaluate the extent of oxygen isotope variability, and assess the potential influence of fractional crystallization, crustal assimilation, variations in mantle melting degree, and the involvement of multiple mantle sources on the observed geochemical patterns. Data quality was verified by repeated analysis of internal reference materials. Analytical precision was better than 5% for major elements and 10% for trace elements. Oxygen isotope precision was ±0.1‰, and the natural variability of glass oxygen isotope data is estimated as 0.2‰. Hydrogen isotope precision on water content <0.1 wt.% was ±8‰, and at higher concentration it was ±5%.

The Fagradalsfjall lavas are olivine tholeiites with variations in incompatible trace element ratios (Nb/Zr, Zr/Y, Th/Yb), indicating involvement of multiple mantle sources. Temporal changes in these ratios were observed, with an overall increase in Nb/Zr over time. Despite this trace element variability, the δ¹⁸O values of the Fagradalsfjall samples are remarkably homogeneous (mean 5.4 ± 0.3‰, 2SD, N = 47) and consistent with "normal" upper mantle (MORB). This homogeneity persists across different sampling times (March-August 2021), suggesting that any variability is not linked to changes in crustal contamination or degassing. The Δ¹⁷O values are also within the mantle range. Hydrogen isotope (δD) values overlap with the canonical MORB range but are slightly higher than North Atlantic depleted MORB and lower than enriched MORB. Water contents are lower than typical MORB, suggesting degassing during eruption. The trace element variations suggest mixing of depleted mantle, E-MORB, and enriched Icelandic plume components (Eldgjá and Snæfellsnes), possibly reflecting a heterogeneous mantle source. The invariant δ¹⁸O values suggest a lack of interaction with low-δ¹⁸O crust and an absence of ¹⁸O-depleted mantle sources beneath the Reykjanes Peninsula. The data span roughly half the entire recorded trace element variability for the RP, demonstrating that despite the source diversity, the oxygen isotope composition is consistent with MORB.

The consistent δ¹⁸O values in the Fagradalsfjall basalts, despite the varied trace element ratios, strongly suggest that the mantle source beneath the Reykjanes Peninsula has a relatively uniform primary δ¹⁸O value. This contrasts with other parts of Iceland, such as Holuhraun (δ¹⁸O = 3.8‰), where lower δ¹⁸O values have been attributed to assimilation of low-δ¹⁸O crust. The homogeneous oxygen isotope composition rules out crustal assimilation as a primary control on the Fagradalsfjall geochemical variations. Fractional crystallization and variations in mantle melting degree are also unlikely to explain the observed trace element variability. The most likely explanation is a heterogeneous mantle source with closely spaced and simultaneously tapped multiple mantle components: depleted mantle, E-MORB, and enriched Icelandic plume components. The invariant δ¹⁸O values indicate rapid magma ascent from near-Moho levels, with limited time for interaction with crustal materials. The slightly lower-than-average MORB δ¹⁸O values may be due to minor degassing during the eruption.

The 2021 Fagradalsfjall eruption reveals a heterogeneous mantle source beneath the Reykjanes Peninsula, characterized by a mixture of depleted mantle, E-MORB, and enriched Icelandic plume components. Remarkably, the oxygen isotope composition remains remarkably homogeneous and consistent with "normal" MORB, providing a useful mantle reference value for this region. The lack of low-δ¹⁸O values suggests an absence of ¹⁸O-depleted mantle or significant interaction with low-δ¹⁸O crust. Future research could explore the spatial extent of this homogeneous δ¹⁸O signature across Iceland and further investigate the processes leading to the observed oxygen isotope homogeneity despite the source heterogeneity.

The study focuses on a single eruption, and the findings may not be fully generalizable to other eruptions on the Reykjanes Peninsula or elsewhere in Iceland. The relatively short sampling period (160 days) may not fully capture the entire range of compositional variability. While the consistent δ¹⁸O values suggest limited crustal interaction, minor localized assimilation cannot be entirely excluded.