From partial melting to lava emplacement : the petrogenesis of some Icelandic basalts

This thesis provides insights into the petrogenesis of Iceland basalts via three subprojects. The first uses olivine macrocrysts as a proxy for mantle melting conditions below Iceland, the second utilizes petrological thermobarometry to resolve the crustal storage conditions of the most primitive basaltic rocks (ankaramites) of the Eyjafjallajökull volcano, and the third investigates basalt fractionation processes within the Hafnarhraun pāhoehoe lava lobe. The sub-Icelandic mantle is evidently heterogeneous in composition. Yet olivine major and minor element compositions in Iceland basalts typically concur with common mantle lherzolite as the source of magmas, with the only potential exceptions being the basalts of Eyjafjallajökull and Vestmannaeyjar volcanic systems in South Iceland. These South Iceland basalts have forsteriterich olivine with relatively high Ni and low Mn contents, together with low Sc and V and high Cr, Ti, Zn, Cu and Li contents. Elevated Ni and low Mn in olivine have been attributed to olivine-free pyroxenitic mantle source; however, the South Iceland olivine compositions are best explained by the effect of comparatively high-pressure (Pfinal>1.4 GPa) and high-temperature melting of somewhat enriched olivine-bearing mantle. I conclude this because (i) elevated Ni and low Mn in olivine can also indicate deep, high-temperature, mantle melting, (ii) the abundances of Sc, V, Ti and Zn in the South Iceland olivine are compatible with low-degree partial melts of olivine-rich mantle, and (iii) melts of olivine-free pyroxenite are, according to recent models, easily consumed in reactions with subsolidus mantle peridotite and thus unlikely to migrate to the crust and crystallize olivine. The identified high-Ni/low-Mn olivine macrocrysts suggest final mantle equilibration depths greater than 45 km for South Iceland magmas, and imply effective mantle-to-surface magma transport. Two Eyjafjallajökull ankaramite outcrops (Hvammsmúli and Brattaskjól), rich in olivine (Fo81–90) and clinopyroxene (Mg#cpx 78–90) macrocrysts (~30 vol%) in near equal amounts, have a specifically prominent “deep mantle source signature” (high-Ni/low-Mn) in olivine. To investigate the crustal storage conditions of these and other Eyjafjallajökull basaltic magmas, I analyzed olivine, clinopyroxene, spinel and melt inclusion compositions from these volcanic units. These analyzes revealed that the olivine-hosted spinel inclusions have exceptionally high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical chromian spinels in Iceland, in line with the postulated deep and enriched mantle source of the parental magmas. According to olivine-spinel oxybarometry, these spinels crystallized under a moderate oxygen fugacity (ΔlogFMQ 0–0.5). Furthermore, jadeite-in-clinopyroxene barometry indicates clinopyroxene crystallization at a rather low pressure (1.7–4.2 kbar; external precision ±1.4 kbar), implying a magma storage depth of 10.7±5 km. Additionally, clinopyroxe-liquid, olivine-liquid and liquid only thermometry gives varying crystallization temperatures of 1120–1195 °C, 1136–1213 °C and 1155–1222 °C, respectively, for the compositionally diverse macrocrysts. The scarcity of macrocryst plagioclase and trends in clinopyroxene compositions indicate that the mid-crustal crystallizing assemblage was olivine and clinopyroxene, and plagioclase fractionated later. Diffusive re-equilibration in Brattaskjól olivine grains suggests that this crystal assemblage mobilized and erupted from its storage within a few weeks. To conclude, the Brattaskjól and Hvammsmúli crystal cargoes are agitated wehrlitic or plagioclase-wehrlitic mushes from the mid-crust that ascended to the surface relatively rapidly. Basaltic lavas are practically never primitive mantle melts owing to fractional crystallization in the crust, which, at low pressure, may be aided by volatile exsolution. Deciphering magma fractionation processes from solidified crustal intrusions is hampered by their often complex emplacement history. The emplacement of pāhoehoe lavas, however, is simpler and well understood, and hence I investigated the mechanisms of basalt fractionation from a differentiated pāhoehoe lava lobe in Hafnarhraun lava flow field. Here, volatile exsolution had facilitated separation of basaltic residual melts to form three types of melt segregations: vesicle cylinders (VC) in the core of the lobe and two types of horizontal vesicle sheets (HVS1 and HVS2) in the upper part of the lobe. Interestingly, the VC do not match chemically with the modelled residual melts of the lobe, and their formation seems to have included two stages: volatile-aid-ed melt separation from crystallizing base of the lobe and later contamination by primitive macro-and microphenocrysts in the lava core. HVS1, which resemble VC, were formed as the ascending VC diapirs accumulated to the upper solidification front of the lava lobe. HVS2, in turn, are distinctly evolved in compositions compared to other units in the lobe and were formed as highly fractionated residual melts seeped to voids in the upper crust of the lobe. Processes analogous to segregation formation at Hafnarhraun may contribute to genesis of evolved basalts and silicic rocks in shallow magmatic systems. Overall, my work highlights the exceptional nature of South Iceland among other volcanically active regions in Iceland. Furthermore, analyses of the Hafnarhraun pāhoehoe lava reveal the processes of melt segregation formation in pāhoehoe lava lobes. I hope future research will expand on these findings, further resolving the nature of mantle melting below South Iceland and the significance of volatile-aided processes in crustal magma differentiation.Tämä väitöskirja käsittelee Islannin basalttisten laavojen syntyä ja kehitystä. Väitöskirjan ensimmäisessä osassa käytän laavoissa esiintyvien oliviinihajarakeiden koostumuksia Islannin alaisen maapallon vaipan sulamisolosuhteiden indikaattorina, toisessa selvitän Eyjafjallajökull-tulivuoren ankaramiitti-laavojen purkautumista edeltäviä säilytysolosuhteita maankuoressa, ja kolmannessa tutkin basalttisen magman differentiaatioprosesseja pāhoehoe-laavapatjasta Hafnarhraun-laavakentällä. Islannin basalttien oliviinihajarakeiden koostumus viittaa yleensä oliviinirikkaiden (lherzoliittisten) vaipan kivien sulamiseen Islannin alla. Tähän ainoa poikkeus ovat eteläisen Islannin Eyjafjallajökull- ja Vestmannaeyjar-tulivuorten laavoissa esiintyvät oliviinihajarakeet, joissa on paitsi korkea nikkeli- ja matala mangaanipitoisuus, myös alhainen skandium ja vanadium, sekä korkea kromi, titaani, sinkki, kupari ja litium. Oliviinihajarakeiden korkea nikkeli- ja matala mangaanipitoisuus voidaan nähdä oliviinista köyhän pyrokseniittisen vaipan indikaattorina. Tästä huolimatta eteläisen Islannin poikkeavat oliviinikoostumukset selittyvät parhaiten vaipan syvällä sulamisella korkeassa lämpötilassa. Esitän näin sillä (i) korkea nikkeli ja matala mangaani oliviinissa voi olla myös seurausta vaipan korkeasta sulamislämpötilasta, (ii) Sc-, V-, Ti- ja Zn-pitoisuudet eteläisen Islannin oliviinihajarakeissa viittaavat oliviinirikkaaseen vaipan lähteeseen, ja (iii) uusimpien vaipan sulamisen mallien mukaan pyrokseniittivaipan sulat reagoivat helposti ympäröivän lherzoliittivaipan kanssa ja näin ne tuskin säilyvät muuttumattomina noustessaan kohti maapallon kuorta. Eteläisen Islannin magmat ovat luultavasti lähtöisin yli 45 km syvyydestä maan vaipasta ja ne nousevat tehokkaasti litosfäärin läpi. Erityisen voimakas syvän vaipan sulamisen signaali (korkea nikkeli ja matala mangaani oliviinissa) havaittiin kahdesta ankaramiitti-laavasta Eyjafjallajökull-tulivuoren rinteiltä. Selvittääkseni näiden laavojen purkautumista edeltävät säilytysolosuhteet maankuoressa, analysoin niistä oliviini-, klinopyrokseeni-, spinelli- ja sulasulkeumakoostumuksia. Näiden analyysien perusteella arvioin, että tutkittujen ankaramiittien hajarakeet kiteytyivät yllättävän matalassa 3±1,4 kbar paineessa, mikä vastaa 10,7±5 km syvyyttä maapallon kuoressa. Nämä hajarakeet ovat luultavasti peräisin keskikuoren wehrliittisistä tai plagioklaasi-wehrliittisistä kidepuuroista ja ne nousivat maanpintaan vain muutaman viikon sisällä. Jotta ymmärtäisin basalttisen magman differentiaatioprosesseja, analysoin pāhoehoe-laavapatjan sisäisiä rakenteita Hafnarhraun-laavakentältä. Selvisi, että laava sisältää kolmen tyyppisiä segregaatiorakenteita (sulasegregaatioita): rakkulapiippuja (VC) laavapatjan ytimessä ja kahden tyyppisiä horisontaalisia rakkulapatjoja (HVS1 ja HVS2) laavapatjan yläosissa. VC koostumukset eivät vastannut laavan mallinnutettuja jäännössulia, mutta ne voitiin selittää kaksivaiheisella syntyhistorialla, jossa VC ensin erottuvat isäntämagmasta lähellä laavan pohjaa ja tämän jälkeen niihin kertyy oliviini- ja plagioklaasikiteitä laavapatjan keskiosassa. HVS1 puolestaan muodostuivat, kun nousevat VC levittäytyivät laavan kiteytyvän yläkuoren alapintaa vasten, ja HVS2 ovat pitkälle fraktioituneita isäntälaavan jäännösulia, joita tihkui laavan yläkuoreen muodostuneisiin aukkoihin ja rakoihin. Näiden sulasegregaatioiden erottumista muistuttavat prosessit voivat edesauttaa kehittyneiden basalttisten magmojen muodostumista myös maanpinnan läheisissä magmasäiliöissä. Väitöskirjatutkimukseni korostaa eteläisen Islannin ainutlaatuisuutta verrattuna Islannin muihin magmaattisesti aktiivisiin alueisiin ja valaisee sulasegregaatioiden kehitystä pāhoehoe-laavoissa. Toivon, että löydökseni inspiroivat Islannin vaipan sulamisolosuhteita ja magmaattisen differentiaation prosesseja selvittävää tutkimusta

Formation of segregation structures in Hafnarhraun pāhoehoe lobe, SW Iceland: a window into crystal–melt separation in basaltic magma

To gain insights into crystal–melt separation processes during basalt differentiation, we have studied an 8-m-thick pāhoehoe lava lobe from the Hafnarhraun lava flow field in SW Iceland. The lobe has abundant melt segregations, porous cylindrical and sheet-like structures, generally interpreted as separated residual melts of a lava lobe. We divide these melt segregations into three types based on morphology and composition: vesicle cylinders (VC), type 1 horizontal vesicle sheets (HVS1), and type 2 horizontal vesicle sheets (HVS2). Remarkably, the studied VC are not simple residual melts generated by fractional crystallization, but their composition points to removal of plagioclase from the parental lava. HVS1 resemble VC, but have fractionated more olivine (ol) + plagioclase (plg) ± augite and have lost most, if not all, of their olivine phenocrysts. HVS2 are Fe-rich and evolved, corresponding to residual melts after 50–60% fractional crystallization of the lobe. We suggest that the Hafnarhraun VC formed in a two-stage process. Firstly, VC forming residual melt and vapor detached as rising diapirs from ol+plg+melt+vapor mush near the lava base, and later, these VC diapirs accumulated ol phenocrysts and minor plg microphenocrysts in the lava core. HVS1 represent accumulations of VC to the viscous base of the solidifying upper crust of the lobe, and HVS2 formed as evolved vapor-saturated residual melts seeped into voids within the upper crust. Such vapor-aided differentiation, here documented for the Hafnarhraun lava, may also apply to shallow crustal magma storage zones, contributing to the formation of evolved basalts.Peer reviewe

Signature of deep mantle melting in South Iceland olivine

We present new high-precision major and trace element data on olivine macrocrysts from various volcano-tectonic settings in Iceland and use these data as a proxy for mantle mode and melting conditions. Within individual sampling sites examined (seven lavas and one tephra) olivine-dominated fractional crystallization, magma mixing and diffusive re-equilibration control observed variability in olivine composition. High-pressure fractional crystallization of clinopyroxene may have lowered Ca and increased Fe/Mn in one olivine population and subsolidus diffusion of Ni and Fe-Mg affected the mantle-derived Ni/Fo ratio in some compositionally zoned olivine macrocrysts. Interestingly, magmas erupted at the southern tip of the Eastern Volcanic Zone (SEVZ), South Iceland, have olivines with elevated Ni and low Mn and Ca contents compared to olivines from elsewhere in Iceland, and some of the SEVZ olivines have relatively low Sc and V and high Cr, Ti, Zn, Cu and Li in comparison to depleted Iceland rift tholeiite. In these olivines, the high Ni and low Mn indicate relatively deep melting (P-final>1.4GPa,similar to 45km), Sc, Ti and V are compatible with low-degree melts of lherzolite mantle, and elevated Zn may suggest modal (low-olivine) or geochemical (high Zn) enrichment in the source. The SEVZ olivine macrocrysts probably crystallized from magmas derived from olivine-bearing but relatively deep, enriched and fertile parts of the sub-Icelandic mantle, and indicate swift ascent of magma through the SEVZ lithosphere.Peer reviewe

Formation of segregation structures in Hafnarhraun pāhoehoe lobe, SW Iceland: a window into crystal–melt separation in basaltic magma

Publisher's version (útgefin grein).To gain insights into crystal–melt separation processes during basalt differentiation, we have studied an 8-m-thick pāhoehoe lava lobe from the Hafnarhraun lava flow field in SW Iceland. The lobe has abundant melt segregations, porous cylindrical and sheet-like structures, generally interpreted as separated residual melts of a lava lobe. We divide these melt segregations into three types based on morphology and composition: vesicle cylinders (VC), type 1 horizontal vesicle sheets (HVS1), and type 2 horizontal vesicle sheets (HVS2). Remarkably, the studied VC are not simple residual melts generated by fractional crystallization, but their composition points to removal of plagioclase from the parental lava. HVS1 resemble VC, but have fractionated more olivine (ol) + plagioclase (plg) ± augite and have lost most, if not all, of their olivine phenocrysts. HVS2 are Fe-rich and evolved, corresponding to residual melts after 50–60% fractional crystallization of the lobe. We suggest that the Hafnarhraun VC formed in a two-stage process. Firstly, VC forming residual melt and vapor detached as rising diapirs from ol+plg+melt+vapor mush near the lava base, and later, these VC diapirs accumulated ol phenocrysts and minor plg microphenocrysts in the lava core. HVS1 represent accumulations of VC to the viscous base of the solidifying upper crust of the lobe, and HVS2 formed as evolved vapor-saturated residual melts seeped into voids within the upper crust. Such vapor-aided differentiation, here documented for the Hafnarhraun lava, may also apply to shallow crustal magma storage zones, contributing to the formation of evolved basalts.The Nordic Volcanological Center funded this work 2015–2017. We thank Robert A. Askew and Leó Kristjánsson for the aid in sampling, Atli Hjartarson and Guðmundur H. Guðfinnsson for the help in sample preparation and microprobe analyses, and Richard J. Brown for the editorial handling. Comments from Kaisa Nikkilä, Scott Rowland, and anonymous reviewer significantly improved the manuscript. Funding information: Open access funding provided by University of Helsinki including Helsinki University Central Hospital.Peer Reviewe

Mid-crustal storage and crystallization of Eyjafjallajokull ankaramites, South Iceland

Our understanding of the long-term intrusive and eruptive behaviour of volcanic systems is hampered by a relatively short period of direct observation. To probe the conditions of crustal magma storage below South Iceland, we have analysed compositions of minerals, mineral zoning patterns, and melt inclusions from two Eyjafjallajokull ankaramites located at Brattaskjol and Hvammsmuli. These two units are rich in compositionally diverse macrocrysts, including the most magnesian olivine (Fo(88)(-)(90)) and clinopyroxene (Mg#(cpx)( )89.8) known from Eyjafjallajokull. Olivine-hosted spinel inclusions have high Cr# (spl )(52-80) and TiO2 (1-3 wt%) and low Al2O3 (8-22 wt%) compared to typical Icelandic chromian spinel. The spinel-olivine oxybarometer implies a moderate oxygen fugacity of Delta logFMQ 0-0.5 at the time of crystallization, and clinopyroxene-liquid thermobarometry crystallization at mid-crustal pressures (1.7-4.2 kbar, 3.0+1.4 kbar on average) at 1120-1195 degrees C. Liquid-only thermometry for melt inclusions with Mg#(melt) 56.1-68.5 and olivine-liquid thermometry for olivine macrocrysts with Fo(80.7-88.9) yield crystallization temperatures of 1155-1222 degrees C and 1136-1213 degrees C, respectively. Diffusion modelling of compositional zonations in the Brattaskjol olivine grains imply that the Brattaskjol macrocusts were mobilized and transported to the surface from their mid-crustal storage within a few weeks (at most in 9-37 days). Trends in clinopyroxene macrocryst compositions and the scarcity of plagioclase indicate that the mid-crustal cotectic assemblage was olivine and clinopyroxene, with plagioclase joining the fractionating mineral assemblage later. In all, the crystal cargoes in the Brattaskjol and Hvammsmtili ankaramites are composed of agitated wehrlitic or plagioclase wehrlitic crystal mushes that crystallized over a large temperature interval at mid-crustal depths.Peer reviewe

From partial melting to lava emplacement: the petrogenesis of some Icelandic basalts

This thesis provides insights into the petrogenesis of Iceland basalts via three subprojects. The first uses olivine macrocrysts as a proxy for mantle melting conditions below Iceland, the second utilizes petrological thermobarometry to resolve the crustal storage conditions of the most primitive basaltic rocks (ankaramites) of the Eyjafjallajökull volcano, and the third investigates basalt fractionation processes within the Hafnarhraun pāhoehoe lava lobe. The sub-Icelandic mantle is evidently heterogeneous in composition. Yet olivine major and minor element compositions in Iceland basalts typically concur with common mantle lherzolite as the source of magmas, with the only potential exceptions being the basalts of Eyjafjallajökull and Vestmannaeyjar volcanic systems in South Iceland. These South Iceland basalts have forsteriterich olivine with relatively high Ni and low Mn contents, together with low Sc and V and high Cr, Ti, Zn, Cu and Li contents. Elevated Ni and low Mn in olivine have been attributed to olivine-free pyroxenitic mantle source; however, the South Iceland olivine compositions are best explained by the effect of comparatively high-pressure (Pfinal>1.4 GPa) and high-temperature melting of somewhat enriched olivine-bearing mantle. I conclude this because (i) elevated Ni and low Mn in olivine can also indicate deep, high-temperature, mantle melting, (ii) the abundances of Sc, V, Ti and Zn in the South Iceland olivine are compatible with low-degree partial melts of olivine-rich mantle, and (iii) melts of olivine-free pyroxenite are, according to recent models, easily consumed in reactions with subsolidus mantle peridotite and thus unlikely to migrate to the crust and crystallize olivine. The identified high-Ni/low-Mn olivine macrocrysts suggest final mantle equilibration depths greater than 45 km for South Iceland magmas, and imply effective mantle-to-surface magma transport. Two Eyjafjallajökull ankaramite outcrops (Hvammsmúli and Brattaskjól), rich in olivine (Fo81–90) and clinopyroxene (Mg#cpx 78–90) macrocrysts (~30 vol%) in near equal amounts, have a specifically prominent “deep mantle source signature” (high-Ni/low-Mn) in olivine. To investigate the crustal storage conditions of these and other Eyjafjallajökull basaltic magmas, I analyzed olivine, clinopyroxene, spinel and melt inclusion compositions from these volcanic units. These analyzes revealed that the olivine-hosted spinel inclusions have exceptionally high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical chromian spinels in Iceland, in line with the postulated deep and enriched mantle source of the parental magmas. According to olivine-spinel oxybarometry, these spinels crystallized under a moderate oxygen fugacity (ΔlogFMQ 0–0.5). Furthermore, jadeite-in-clinopyroxene barometry indicates clinopyroxene crystallization at a rather low pressure (1.7–4.2 kbar; external precision ±1.4 kbar), implying a magma storage depth of 10.7±5 km. Additionally, clinopyroxe-liquid, olivine-liquid and liquid only thermometry gives varying crystallization temperatures of 1120–1195 °C, 1136–1213 °C and 1155–1222 °C, respectively, for the compositionally diverse macrocrysts. The scarcity of macrocryst plagioclase and trends in clinopyroxene compositions indicate that the mid-crustal crystallizing assemblage was olivine and clinopyroxene, and plagioclase fractionated later. Diffusive re-equilibration in Brattaskjól olivine grains suggests that this crystal assemblage mobilized and erupted from its storage within a few weeks. To conclude, the Brattaskjól and Hvammsmúli crystal cargoes are agitated wehrlitic or plagioclase-wehrlitic mushes from the mid-crust that ascended to the surface relatively rapidly. Basaltic lavas are practically never primitive mantle melts owing to fractional crystallization in the crust, which, at low pressure, may be aided by volatile exsolution. Deciphering magma fractionation processes from solidified crustal intrusions is hampered by their often complex emplacement history. The emplacement of pāhoehoe lavas, however, is simpler and well understood, and hence I investigated the mechanisms of basalt fractionation from a differentiated pāhoehoe lava lobe in Hafnarhraun lava flow field. Here, volatile exsolution had facilitated separation of basaltic residual melts to form three types of melt segregations: vesicle cylinders (VC) in the core of the lobe and two types of horizontal vesicle sheets (HVS1 and HVS2) in the upper part of the lobe. Interestingly, the VC do not match chemically with the modelled residual melts of the lobe, and their formation seems to have included two stages: volatile-aided melt separation from crystallizing base of the lobe and later contamination by primitive macro- and microphenocrysts in the lava core. HVS1, which resemble VC, were formed as the ascending VC diapirs accumulated to the upper solidification front of the lava lobe. HVS2, in turn, are distinctly evolved in compositions compared to other units in the lobe and were formed as highly fractionated residual melts seeped to voids in the upper crust of the lobe. Processes analogous to segregation formation at Hafnarhraun may contribute to genesis of evolved basalts and silicic rocks in shallow magmatic systems. Overall, my work highlights the exceptional nature of South Iceland among other volcanically active regions in Iceland. Furthermore, analyses of the Hafnarhraun pāhoehoe lava reveal the processes of melt segregation formation in pāhoehoe lava lobes. I hope future research will expand on these findings, further resolving the nature of mantle melting below South Iceland and the significance of volatile-aided processes in crustal magma differentiation.Í þessari ritgerð er ljósi varpað á uppruna basaltkviku á Íslandi með þremur rannsóknarþemum. Í fyrsta þemanu er snefilefnainnihald ólívíndíla notað til að skýra hvaða skilyrði ríkja við hlutbræðslu möttulsins undir Íslandi. Í öðru þemanu er stuðst við jarðefnafræðilega hita- og þrýstimæla til þess að meta eiginleika kvikuhólfa/-þróa sem innihalda frumstæða basaltkviku (þ.e. ankaramít) í skorpunni undir Eyjafjallajökli. Þriðja þemað varðar þróunarferli basaltbráðar í helluhraunssepa í Hafnarhrauni við Þorlákshöfn. Möttulinn undir Íslandi er að samsetningu misleitur. Samt samræmist aðal- og snefilefnasamsetning ólívíndíla í íslensku basalti kviku sem á uppruna sinn að rekja til hlutbræðslu á venjulegum lherzólítmöttli. Hugsanleg undantekning frá þessari reglu er basaltkvikan sem kemur upp í eldstöðvakerfunum Eyjafjallajökli og Vestmannaeyjum, sem innihalda mjög forsterítríka ólivíndíla með tiltölulega háan Ni-styrk og lágan Mn-styrk, ásamt lágum styrk Sc og V og háum styrk Cr, Ti, Zn, Cu og Li. Þrátt fyrir að hár Ni-styrkur og lágur Mn-styrkur í frumstæðum ólivíndílum sé gjarnan rakinn til kviku sem myndast við hlutbráðnun á ólivínlausum pyroxenítmöttli, þá er samsetning umræddra ólivíndíla best skýrð með háhitabráðnun á auðguðum, ólivínríkum perídótítmöttliviðháanþrýsting(Pfinal> 1,4 GPa). Ég dreg þessa ályktun vegna þess að (i) hár Nistyrkur og lágur Mn-styrkur í ólivíni samræmist einnig bráðnun við háan hita djúpt í möttlinum, (ii) tiltölulega hár styrkur Sc, V, Ti og Zn í ólivíndílunum er í samræmi við litla hlutbráðnun á ólivínríkum möttli, og (iii) samkvæmt nýjustu líkönum hvarfast pýroxenítbráð auðveldlega við möttulperidótít og því ólíklegt að slík bráð komist upp í jarðskorpuna og kristalli ólivín. Þessi hái styrkur Ni og lágur styrkur Mn í ólivíndílunum bendir til þess að kvikan hafi síðast verið í jafnvægi við möttulefnið á meira en 45 km dýpi ogaðkvikanhafiflusthrattfrámöttlitilyfirborðs. Tvær ankaramítmyndanir í Eyjafjöllum, Hvammsmúli og Brattaskjól, sem auðugar eru af ólivíndílum (Fo81–90) og klínópýroxendílum (Mg#cpx 78–90) (~ 30%), nokkurn veginn í sama magni,sýnamerkiumupprunadjúptímöttlinum, þ.e. hátt Ni-magn / lágt Mn-magn í ólivíndílunum. Efnasamsetningólivíns,klínópýroxens,spínilsog bráðarinnlyksna var greind í sýnum frá þessum myndunum til þess að meta dýpi kvikuþróa þar sem þessar kvikur safnast fyrir í skorpunni fyrir gos. Þessar greiningarsýna að spínilkristallarnir, sem eru til staðar sem innlyksur í ólivíndílum, hafa óvenju hátt Cr#spl (52–80) og mikið af TiO2 (1–3 þ.%) og lítið af Al2 O3 (8–22 þ.%) í samanburði við dæmigerða krómspínla í íslensku basalti. Þetta er í takt við ályktunina um djúpstæðan uppruna móðurkvikunnar frá auðguðum möttli. Samkvæmt ólivín-spínil súrefnisþrýstingsmælinum kristölluðust þessir spínlar við hóflegan súrefnisstyrk (ΔlogFMQ 0–0,5).Jafnframtbendirþrýstimælirsembyggirá magnijaðeítþáttaríklínópýroxenitilaðkristöllun klínópýroxens hafi átt sér stað við frekar lágan þrýsting (1,7–4,2±1,4 kbar),semgefurtil kynna 10,7 ± 5 km dýpi fyrir kvikuþróna.Að auki gefa hitamælarfyrir klínópýroxen-gler, ólivín-gler og gler eingöngu mismunandi kristöllunarhitastig fyrir hinar mismundi gerðir fasa, nefnilega 1120– 1195 °C, 1136–1213 °C og 1155–1222 °C. Lítið magn plagíóklasdíla ásamt samsetningu klínópýroxendílanna bendir til þess að þessar kvikur hafi kristallað ólivín og klínópýroxen í kvikuþrónni og myndun plagíóklasdíla hafist síðar. Líkanreikningar á efnasveimi í ólivíni frá Brattaskjóli bendir til að þessi dílafarmur hafi farið að stað úr geymslurýminu og borist til yfirborðs í eldgosi innan nokkurra vikna. Niðurstaðan er að dílafarmurinn í Brattaskjóli og Hvammsmúla sé að uppruna kristalríkur massi í miðskorpunni með steindafylki wehrlíts eða plagíóklas-wehrlíts sem reis tiltölulega hratt til yfirborðs. Basalthraun eru nær aldrei með sömu samsetningu og frumstæðar möttulbráðir vegna hlutkristöllunar sem á sér stað í jarðskorpunni, en hlutkristöllunin getur við lágan þrýsting orðið fyrir áhrifum af aðskilnaði gastegunda. Erfitt getur verið að greina áhrif þróunarferla í kviku með því að skoða storknuð innskot vegna flókinnar sögu þeirra oft á tíðum. Hins vegar er myndun helluhrauna vel skilin og hef ég því rannsakað kvikuþróunarferli basalts með því að skoða þróaðan helluhraunssepa í Hafnarhrauni. Í þessu tilfelli átti aðskilnaður gass þátt í aðskilnaði afgangsbráðar með samsetningu basalts. Bráðaraðskilnaðurinn var af þrennu tagi: blöðrusívalningar (VC) í kjarna hraunsepans og tvær gerðir láréttra blöðrulaga (HVS1 og HVS2) í efri hluta sepans. Áhugavert er að efnasamsetning VC fellur ekki að líkanreikningum fyrir sepann og myndun þeirra virðist hafa orðið í tveimur þrepum: bráðaraðskilnaður með hjálp gass í botni sepans þar sem kristöllun átti sér stað og síðar mengun af frumstæðum stór- og smádílum í kjarna hraunsins. HVS1 líkjast VC og mynduðust þegar VC-sívalningar risu og söfnuðust fyrir á storknunarmörkum hraunsepans. Hins vegar eru HVS2 greinilega þróaðir í samanburði við hinar gerðirnar í sepanum og mynduðust við að mjög þróaðar afgangsbráðirseytluðu í holrými í efri skorpu sepans. Ferli lík þeim sem mynduðu afgangsbráðirnar í Hafnarhrauni gætu komið við sögu við myndun þróaðs basalt og súrs bergs í grunnstæðum kvikukerfum. Heiltyfirlitiðþásýnarannsóknirmínarhversu einstakt Suðurgosbeltið er meðal virkra gosbelta á Íslandi. Þá hafa rannsóknirnar á Hafnarhrauni leitt í ljós ferli bráðaraðskilnaðar í helluhraunssepum. Ég vona að framtíðarrannsóknir muni byggja á þessum uppgötvunum og leiða til betri skilnings á eðli möttulbráðnunar undir Suðurlandi og því hversu mikilvægt gas er fyrir kvikuþróunarferli í skorpunni

U–Pb zircon dating endeavors on the host rocks of the Juomasuo gold deposit, Kuusamo supracrustal belt, eastern Finland

In situ U–Pb zircon dating was done for the host rocks of the Juomasuo gold deposit, located in the Paleoproterozoic Kuusamo supracrustal belt, eastern Finland. For dating purposes, five samples were chosen to represent the typical volcano-sedimentary rock types of the deposit in their mineralized and unmineralized form. The samples were studied petrologically and dated using the LA–MC–ICP/MS technique at the research laboratory of the Geological Survey of Finland and the Finland Geoscience Laboratory (SGL). The U–Pb data revealed heterogenic detrital zircon populations even for the felsic volcanic rocks, and thus, all the samples were delineated as sedimentary/volcano-sedimentary sequences. The maximum deposition ages fall in the 2.75–2.65 Ga window, and not a single Paleoproterozoic age was measured in the bulk of 159 measurements. The pervasively altered samples from the mineralized sequence of the Juomasuo deposit hosted a younger population of homogenized, BSE-pale/CL-dark zircons and domains within zircon grains, with 2.62–2.60 Ga ages. This age can be linked to the Neoarchean crustal growth event at 2.7–2.6 Ga, described from the Archean terrains all over the world, including the Archean of the Karelia Province in Finland. Unfortunately, in the limits of this thesis, the homogenization age cannot be interpreted as the minimum depositional age of the studied metasedimentary rocks, as it remains unclear, whether the homogenization of the zircons took place before or after the deposition. Although the majority of the precursors of the studied supracrustal rocks from Kuusamo were presumably deposited between 2.43 and 2.21 Ga, there are no corresponding ages of volcanism in the area. The lack of U–Pb ages of that time frame is a global phenomenon, and the topic is thus pertinent to modelling of Paleoproterozoic plate tectonics. It may be that the Kuusamo supracrustal belt represents a deposition during a global slowdown in plate tectonics, thus recording an anomalous period in the history of planet Earth.Kuusamon liuskejaksoon kuuluvan Juomasuon kultaesiintymän isäntäkivien ikä määritettiin in situ U-Pb zirkoni-iänmääritysmenetelmää käyttäen. Iänmääritystä varten valittiin viisi näytettä, jotka edustavat alueelle tyypillisiä vulkaanis-sedimentäärisiä kivilajeja niiden malmiutuneessa ja malmiutumattomassa muodossa. Näytteiden petrologia tutkittiin ja niiden esikäsittely suoritettiin Geologian tutkimuskeskuksen tutkimuslaboratoriossa. Varsinainen iänmääritys suoritettiin LA–MC–ICP/MS laitteella Suomen Geotieteen Laboratoriossa (SGL). Saatu U-Pb aineisto paljasti detritaalisia, sedimenteille tyypillisiä zirkonpopulaatioita jopa näytteistä, jotka oltiin aiemmin tulkittu felsisiksi vulkaanisiksi kiviksi. Tästä johtuen kaikki tutkitut näytteet todettiin sedimentäärisiksi tai vulkaanis-sedimentäärisiksi yksiköiksi. Yhtään paleoproterotsooista ikää ei löydetty 159 mittauksen joukosta, ja kaikkien näytteiden maksimikerrostumisiät määritettiin aikavälille 2.75-2.65 Ga. Tämän lisäksi Juomasuon kultaesiintymän malmiutunut osa sisälsi hydrotermisessa tapahtumassa homogenisoituneita zirkoneita ja zirkoni osueita, jotka pystyttiin iättämään aikavälille 2.62-2.60 Ga. Tämä ikä voidaan yhdistää neoarkeeisen (2.7-2.6 Ga) kuoren syntyyn ja siihen liittyvään vuoripoimutukseen. Valitettavasti ei kyetty selvittämään, tapahtuiko zirkonien homogenisoitumiseen johtanut hydrothermaalinen tapahtuma ennen vai jälkeen tutkittujen sedimenttikivien kerrostumisen. Näin homogenisoitumisikää 2.62-2.60 Ga ei voida pitää tutkittujen sedimenttikivien minimi-ikänä. Vaikka Kuusamon liuskejakson muodostumisiäksi on aikaisemmin määritetty 2.43-2.21 Ga, kerrostumisen aikaisia vulkaanisia kiviä ei ole löydetty. U-Pb ikien puuttuminen aikavälillä 2.43-2.21 Ga on maailmanlaajuinen ilmiö, ja tämän vuoksi Kuusamon liuskealueen kehitystä on hyvä verrata maailmanlaajuista paleoproterotsooista laattetokniikkaa käsitteleviin teorioihin. Kuusamon sedimenttikivien kerrostuminen on mahdollisesti tapahtunut laattatektonisen pysähtyneisyyden aikana ja näin se saattaa edustaa tektonisesti ainutlaatuista aikaa maapallon historiassa

U-Pb zircon ages of the host rocks of the Juomasuo Au-Co-Cu deposit, northeastern Finland

U-Pb zircon single grain dating using the LA-MC-ICP-MS technique was utilized to determine the age of the host rocks of the Juomasuo Au-Co-Cu deposit located in the late Archean Kuusamo supracrustal belt. Even though the dated samples have diverse geochemical signatures that imply felsic volcanic and sedimentary precursors, the U-Pb data revealed heterogeneous detrital zircon populations for all samples. The host rocks are thus considered to belong to reworked sedimentary/volcano-sedimentary sequences. The maximun depositional ages of the samples fall in the 2.75-2.65 Ga age window, and no Paleoproterozoic ages were recovered in the examined zircon grains (151 analytical spots in total). In addition, a younger population (2.65-2.60 Ga) of internally featureless, BSE-pale/CL-dark zircon and zircon domains, was found in the mineralized sequences. These homogenized zircon grains resemble zircon formed in postmagmatic solid-state processes, in which zircon is recrystallized in metamorphic-hydrothermal conditions. This metamorphic-hydrothermal event most probably occurred in the original provenance area of the metasedimentary rocks.Peer reviewe