Warm and Slightly Reduced Mantle Under the Off-Rift Snæfellsnes Volcanic Zone, Iceland

Abstract

Olivine (Fo75-91) with spinel inclusions (Cr# 10-61) from basaltic lavas/tephras from the off-rift Snæfellsnes Volcanic Zone in Iceland record the chemistry, temperature and oxygen fugacity of fractionating magmas. After a detailed assessment of equilibrium conditions, crystallisation temperatures and oxygen fugacity of basaltic melts can be calculated from the coexisting compositions of homogeneous Cr-spinel and Al-chromite inclusions in olivine phenocrysts. Geologically meaningful results can occasionally be obtained when homogenous spinel is enclosed in mildly zoned olivine and KDMg-Fe [(Mg/Fe)olivine/(Mg/Fe2+)spinel] is within the range for homogenous spinel in homogeneous olivine (3.5-4.3 for our samples). Spinel in normally zoned Fo84.7-90.9 olivine records the primitive stages of magma fractionation and has crystallised from clinopyroxene-free primitive melts, probably at Moho depth and/or below. Discrepancies between Tol-liq (Mg-Fe2+ diffusion sensitive) and TAL (diffusion insensitive) suggest that some primitive olivines experienced magma mixing, completely overprinting their Fo content. Consequentially, Tol-liq in primitive olivines occasionally records residence rather than crystallisation conditions. Temperature (1187 °C – 1317 °C) gradually decreases across normally zoned Fo84.7-90.9 olivine and controls fO2 (Δlog fO2 (QFM) -0.6 - +0.2). Recharge-related primitive Fo83.8-86.8 mantles of reversely zoned olivine contain the most primitive Cr-spinel linked to crustal magma storage zones. These spinels are mostly antecrysts with high Cr# (41.1-47.9) similar to spinel in normally zoned olivines that were captured by olivine and equilibrated in terms of Mg-Fe2+. A rare olivine macrocryst crystallised alongside clinopyroxene (wehrlite) and includes abundant homogeneous Al-rich Cr-spinels. These are unique because they appear to record closed-system fractional crystallisation rather than magma mixing and because they show that Cr-poor, Al-rich spinel crystallised alongside clinopyroxene. The macrocryst olivine-spinel pairs record lower crustal crystal mush conditions with fO2 around the QFM buffer and Tol-liq of ∼1200 °C, similar to recharge-related mantles of reversely zoned olivine. More evolved compositions occur in the cores of reversely zoned olivine (Fo75-85) that contain Cr-spinel, Fe-spinel and Al-magnetite. Contrary to spinel in more primitive olivine, these compositions are diverse and follow increasing 100Fe3+/(Cr+Al+Fe3+) of 12.3 to 54.8 and TiO2 (3.3 to 14.7 wt.%) at decreasing Mg# (57.4 to 24.1) and Cr# (30.4 to 9.9) and rapidly increasing oxygen fugacities (Δlog fO2 (QFM) +0.2 to + 2.0) over only a limited temperature decrease (Tol-liq: 1190 °C to 1145 °C). These compositions span the “spinel gap” and are extremely rare globally. Their preservation is likely related to high temperature crystallisation followed by rapid cooling. These compositions occur at two of the four investigated volcanic centres (Búðahraun and Berserkjahraun) and indicate the strong influence of crustal magmatic processes on crystal composition and fO2, which is absent in the other two locations (Ólafsvíkurenni and Nykurhraun). Spinel and olivine compositions support the tectonically controlled decompression melting of a fertile peridotitic source at elevated mantle temperatures relative to MORB and more reducing conditions than other off-rift magmatism in Iceland.

Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland

“…The oxygen fugacity was calculated based on equilibrated (following recommendations in ref. 48 ) olivine-spinel pairs using the calibration of ref. 49 .…”

Magmatic Controls on Volcanic Sulfur Emissions at the Iceland Hotspot

“…This may suggest that SIVZ magmas are more oxidized than remaining Icelandic magmas and have higher proportion of dissolved S 6+ relative to S 2− , which would increase the total sulfur solubility (e.g., Hughes et al., 2023; Nash et al., 2019; Wallace & Edmonds, 2011). Spinel–olivine oxybarometry of two 500–700 ka ankaramite eruptions of the Eyjafjallajökull volcano (∆FMQ = +0–0.5; Nikkola et al., 2019), and Fe 3+ /Fe T of up to 0.155 in tephra glass of the Surtsey 1963–1967 CE eruption (Schipper & Moussallam, 2017) suggest that SIVZ melts are slightly more oxidized than both rift basalts and the off‐rift SNVZ magmas (van der Meer et al., 2021). However, based on S T CSS calculations, even higher melt Fe 3+ /Fe T of 0.17–0.19 are required for the SIVZ melts to attain maximum observed Δ S max values (Figure S9 in Supporting Information ).…”

“…Sulfur contents of primary basaltic melts are essentially controlled by the S content and the extent of melting of their mantle source, and its oxygen fugacity. The mantle beneath Iceland is thought to have an oxygen fugacity close to, or slightly below the FMQ (Novella et al., 2020; Óskarsson et al., 1994; van der Meer et al., 2021). While more oxidized melts have higher S T CSS and are capable of dissolving more S at high MgO (Figure 5a), the measured MI S contents show good agreement with S 2– CSS calculated assuming Fe 3+ /Fe T of 0.10.…”

“…rift basalts and the off-rift SNVZ magmas (van der Meer et al, 2021). However, based on S T CSS calculations, even higher melt Fe 3+ /Fe T of 0.17-0.19 are required for the SIVZ melts to attain maximum observed ΔS max values (Figure S9 in Supporting Information S1).…”

Supplemental Material: Deep magma mobilization years before the 2021 CE Fagradalsfjall eruption, Iceland

“…Olivine-spinel pairs were assigned pressures of 2 kbar and temperatures of 1200°C for rims with <Fo87 and 2 kbar and 1240°C for cores with >Fo87 based on geothermobarometry results. Compositionally homogeneous olivine-spinel pairs give KD Mg-Fe [(Mg/Fe)olivine/(Mg/Fe 2+ )spinel] between 3.1 and 3.4, heterogeneous spinel and disequilibrium olivine-spinel pairs outside this range in KD Mg-Fe were discarded (Van der Meer et al, 2021). 26 equilibrium olivine-spinel pairs give Δlog fO2 between -0.1 and -0.6 with an average of -0.3, without a clear difference between cores and rims.…”