The Surtsey volcano geothermal system: An analogue for seawater-oceanic crust interaction with implications for the elemental budget of the oceanic crust

Pre-print (óritrýnt handrit)Surtsey is a young volcanic island in the offshore extension of Iceland's southeast rift zone that grew from the seafloor during explosive and effusive eruptions in 1963–1967. In 1979, a cored borehole (SE-1) was drilled to 181 m depth and in 2017 three cored boreholes (SE-2a, SE-2b and SE-3) were drilled to successively greater depths. The basaltic deposits host a low-temperature (40–141 °C) seawater-dominated geothermal system. Surtsey provides an ideal environment to study water-rock interaction processes in a young seawater geothermal system. Elemental concentrations (SiO2, B, Na, Ca, Mg, F, dissolved inorganic carbon, SO4, Cl) and isotope contents (δD, δ18O) in borehole fluids indicate that associated geothermal waters in submarine deposits originated from seawater modified by reactions with the surrounding basalt. These processes produce authigenic minerals in the basaltic lapilli tuff and a corresponding depletion of certain elements in the residual waters. Coupling of measured and modelled concentrations investigates the effect of temperature and associated abundance of authigenic minerals on chemical fluxes from and to the igneous oceanic crust during low-temperature alteration. The annual chemical fluxes calculated at 50–150 °C range from −0.01 to +0.1×1012 mol yr−1 for SiO2, +0.2 to +129×1012 mol yr−1 for Ca, −129 to −0.8×1012 mol yr−1 for Mg and −21 to +0.4 × 1012 mol yr−1 for SO4 where negative values indicate chemical fluxes from the ocean into the oceanic crust and positive values indicate fluxes from the oceanic crust to the oceans. These flux calculations reveal that water-rock interaction at varying water-rock ratios and temperatures produces authigenic minerals that serve as important sinks of seawater-derived SiO2, Mg and SO4. In contrast, water rock interaction accompanied by dissolution of basaltic glass and primary crystal fragments provides a significant source of Ca. Such low-temperature alteration could effectively influence the elemental budget of the oceanic igneous crust and ocean waters. The modeling provides insights into water chemistries and chemical fluxes in low temperature MOR recharge zones. Surtsey also provides a valuable young analogue for assessing the chemical evolution of fluid discharge over the life cycles of seamounts in ridge flank systems.Funding for this project was provided by the University of Iceland Recruitment fund, the International Continental Scientific Drilling Program (ICDP) through a grant to the SUSTAIN project, the Icelandic Science Fund, ICF-RANNÍS, the Bergen Research Foundation and K.G. Jebsen Centre for Deep Sea Research at University of Bergen, Norway, the German Research Foundation (DFG), and DiSTAR, Federico II, University of Naples, Federico II, Italy. The University of Utah, USA and the two Icelandic power companies Reykjavík Energy and Landsvirkjun, contributed additional funds. The authors would like to thank P. Bergsten, A.M. di Stefano, C.F. Gorny, J. Gunnarsson-Robin, G.H. Guðfinsson, Þ. Högnadóttir, E.W. Marshall, R. Ólafssdóttir, D.B. Ragnarsson and Þ.M. Þorbjarnardóttir for their contribution and assistance during sampling, sample preparation, analyses and data evaluation. The authors would like to thank M. E. Böttcher for careful editorial handling. Two anonymous reviewers and J. Alt are thanked for their thoughtful and valuable reviews

Analysis on mineralogical composition of marine sediments on the North Icelandic shelf: Gravity core B05-2006-GC04

Borkjarni B05-2006-GC04 var tekinn í Eyjafjarðarál árið 2006 og hefur verið rannsakaður með það að markmiði að fá upplýsingar um umhverfisbreytingar svo sem loftslagsbreytingar og eldgos. Til þess voru notaðar eðlisþyngdarmælingar, kornastærðargreiningar, greining á steindasamsetningu og vatnsinnihaldi. Kjarninn var tekin sem hluti af alþjóðlegu rannsóknarverkefni (European Climate of the last Millennium (Millennium), 017008-2 styrkt af 6. rammaáætlun Evrópusambandsins). Millennium verkefnið miðar að því að skoða fornar loftslagsbreytingar, til þess að skilja betur þær hröðu loftslagsbreytingar sem eiga sér stað nú á tímum. Eyjafjarðaráll er á landgrunninu norðan við Ísland og er svæðið því mjög viðkvæmt fyrir loftslagsbreytingum. Vegna nálægð svæðisins við eldvirk svæði er hægt er að nota gjóskulög til þess að fá upplýsingar um eldvirkni og aldursgreininga. Hægt er að tengja gjóskulögin við skrifaðar heimildir sem og við jarðlagasnið á landi. Við rannsóknir kom í ljós að aukning varð á kvars kristöllum á 21-22 cm dýpi sem og á 51-52 cm dýpi. Aukning í núnu gleri varð einnig og kom fram toppur á 57-58 cm dýpi og svo annar minni á 51-52 cm dýpi. Það bendir til þess að hafís hafi verið meira viðvarandi á ákveðnum tímabilum heldur en öðrum og við það hafi þessi korn sest til. Við talningu á steindasamsetningu komu einnig fram toppar í súru og basísku gosefni. Sem bendir til þess að gjóskulög sé að finna í kjarnanum Engar aldurs- eða efnagreiningar hafa verið gerðar á kjarnanum og er því ekki hægt að tengja einstaka toppa í hafísbornu efni við skriflegar heimildir. Né héldur er hægt að tengja möguleg gjóskulög við þekkt söguleg gos. Þær breytingar sem sjást í steindasamsetningu gefa til kynna að kjarni B05-2006-GC04 hafi möguleika á að gefa góðar upplýsingar um umhverfisbreytingar á svæðinu.The gravity core B05-2006-GC04 was drilled in Eyjafjarðaráll in 2006 and has been investigated with the objective to get information on environmental changes such as changes in climate and volcanism. Following measurements were applied during the investigation: density measurements, analysis on mineralogical composition, grain size analysis and water content. The core was taken as a part of an international research project, The Millennium project. The Millennium project is founded by the 6th Framework Program of the European Commission (grant 017008-2). The goal of The Millennium project is to investigate palaeoenvironmental changes in order to understand better the fast climatic changes that are occurring now. Eyjafjarðaráll is located on the North Icelandic shelf, close to the oceanographic polar front, and is therefore very sensitive to climatic changes. Due to the area’s close proximity to active volcanoes, tephra layers can be used to get information about age and eruptions. The tephra layers can be correlated to written resources and stratigraphic sections. Investigations showed that an increase in quarts crystals appeared at 21-22 cm and 51-52 cm depth. Altered glass also increased, first a small rise at 51-52 cm depth and another larger rise at 57-58 cm. This indicates that sea ice was more ongoing at some point than another. While counting the mineral composition of the sediments tops could both be seen in acidic and basaltic volcanic material, which indicates that tephra layers can be found in the core. No age or chemical analyses have been made on the core, and therefore it’s not possible to correlate individual increases in ice rafted debris to known sea ice events. Neither is it possible to correlate tephra layers to known volcanic eruptions. The changes that are seen in the mineral composition indicate that the core B05-2006-GC04 has the possibility of providing good information on environmental changes in the area

Carbon isotopes and systematics of Icelandic low-temperature geothermal waters

The carbon chemistry and stable carbon isotope systematics of low-temperature geothermal waters in Iceland was studied. The waters had temperatures ranging from 3 to 97°C, pH between 6.18 and 10.15 as well as dissolved inorganic carbon concentration from 1.8 to 2853 ppm. The carbon isotopes were found to be in the range δ¹³C -1.46 to -13.96‰. The geochemistry of CO₂ and its sources and reactions in low-temperature geothermal water was approached in three ways; by carbonate mineral saturation, a component mixing model and by reaction path isotope modelling. Low-temperature geothermal waters were observed to be calcite saturated, suggesting that calcite may possibly form in low-temperature ground water systems. The source of the carbon in the water and the possible formation of calcite, along with the stable carbon isotope systematics cannot be explained simply by rock dissolution, atmospheric CO2 input and organic matter decay. Instead, progressive basalt dissolution, aqueous speciation, and calcite formation play a major role in carbon isotope systematics and the carbon concentration of the low-temperature geothermal water. For waters that contain low CO₂ concentrations (<50 ppm) and low δ13C values (-5 to -15‰) the CO₂ is thought to be derived from both atmospheric sources and primary rock dissolution. This is due to the variations of CO₂ concentrations and δ13C values generated by the concentration of CO₂ and the exact δ¹³C content of the basalt, and due to carbon isotope fractionation upon water-rock interaction. However, waters that contain high CO₂ concentrations and high δ¹³C values cannot be explained without the introduction of a highly concentrated CO₂ source with a δ13C value of less than -3‰. This source cannot be carbonate dissolution at shallow depth within the crust, as this would result in too low CO₂ concentrations. Mantle degassing through the crust is also unlikely as this would result in too low δ¹³C values. Presently, an alternative CO₂ source of unknown origin has therefore been introduced as the cause of elevated CO₂ low-temperature geothermal waters in Iceland.Efnafræði kolefnis og virkni kolefnis samsæta í lághita jarðhitavatni á Íslandi var rannsakaður. Vatnið var á hitabilinu 3° - 97°C, pH vatnsins var 6.18-10.15 og magn uppleysts ólífræns kolefnis var á bilinu 1.8-2853 ppm. Kolefnis samsæturnar δ¹³C voru á bilinu -1.46 til -13.96‰. Jarðefnafræði CO2, uppruni og efnahvörf í lághita jarðhitakerfum var rannsakaður á þrjá vegu, með mettun kolefnis steinda, blöndunar líkani og með jarðefnafræðilegum samsætulíkanreikningum. Lághita jarðhita vatnið var kalsít mettað, sem bendir til þess að kalsít geti mögulega myndast í lághita grunnvatnskerfum. Uppruni kolefnis í vatninu og myndun kalsítsins ásamt virkni stöðugu samsætanna er ekki hægt að útskýra með uppleysingu á bergi, CO₂ frá andrúmsloftinu eða með niðurbroti lífræns efnis. Í staðinn spila stigvaxandi uppleysing basalts, efnasambönd í vatnslausn og myndun kalsíts stórann þátt í virkni kolefnis samsæta og styrk kolefnis í lághita jarðhita vatni. Uppruni CO₂ í vatni með lágan styrk CO₂ < 50ppm og lág δ¹³C gildi (-5 til -15‰) er talinn vera bæði uppleysing á frumbergi og andrúmsloftið, breytileiki í styrk CO2 og δ¹³C er talinn stafa styrk CO2 og gildi δ¹³C í berginu sjálfu. Aftur á móti er ekki hægt að útskýra vatn með háan CO₂ styrk og há δ13C gildi, án þess að kynna til sögunnar upptök með háan styrk CO₂ og δ¹³C gildi lægri en -3‰. Þessi uppruni getur ekki verið vegna uppleysingar á karbónati á litlu dýpi í jarðskorpunni, þar sem það myndi leiða til hárra δ13C gilda og lágs CO₂ styrks. Afgösunum möttulsins í gegnum jarðskorpuna er einnig ólíklegur þar sem það myndi líklega leiða til of lágra δ¹³C gilda. Önnur CO₂ uppspretta af ókunnum uppruna hefur þ.a.l. verið kynnt sem orsakavaldur hækkaðs CO₂ styrks í lághita jarðhitavatni á Íslandi.Orkurannsóknasjóður Landsvirkjuna

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

AbstractRecent 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.</jats:p