Massiivi-tyypin anortosiittien jäännössulien tasapainoinen kiteytyminen: tapaustutkimus 1.64 miljardia vuotta vanhasta Ahvenisto-kompleksista Kaakkois-Suomessa

Fe–Ti–P-rich mafic to intermediate rocks (monzodiorites and oxide–apatite–gabbronorites, OAGNs) are found as small intrusions in most AMCG (anorthosite–magnerite–charnokite–granite) suites. The origin of the monzodioritic rocks is still debated, but in many studies, they are presumed to represent residual liquid compositions after fractionation of anorthositic cumulates. In the 1.64 Ga Ahvenisto complex, SE Finland, monzodioritic rocks occur as minor dike-like lenses closely associated with anorthositic rocks. We report new field, petrographic, and geochemical (XRF, ICP-MS, EMPA) data complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) for the monzodioritic rocks, apatite–oxide–gabbronorite, and olivine-bearing anorthositic rocks of the Ahvenisto complex. The presented evidence suggest that the monzodioritic rocks closely represent melt compositions while the apatite–oxide–gabbronorite and olivine-bearing anorthositic rocks are cumulates. The monzodioritic rocks seem to form a liquid line of descent (LLD) from primitive olivine monzodiorites to more evolved monzodiorites. Petrological modeling suggests that the interpreted LLD closely corresponds to a residual melt trend left after fractional crystallization (FC) and formation of the cumulate anorthositic rocks and minor apatite–oxide–gabbronorite in shallow magma chambers. Consequent equilibrium crystallization (EC) of separate monzodioritic residual magma batches can produce the observed mineral assemblages and the low Mg numbers measured from olivine (Fo25–45) and pyroxenes (En48–63, Mg#cpx 60–69). The monzodioritic rocks and apatite–oxide–gabbronorites show similar petrological and geochemical characteristics to corresponding rock types in other AMCG suites, and the model described in this study could be applicable to them as well.Peer reviewe

Korkea Ni ja alhainen Mn/Fe Karoon meimechiittien oliviinihajarakeissa eivät indikoi pyrokseniittisia vaippalähteitä

Nickel contents and Mn/Fe in olivine phenocrysts have been suggested to reflect the mineral composition of the mantle source of the host magma. This hypothesis is tested here against a well-characterized suite of meimechitic (or Ti-rich komatiitic) dikes from the Antarctic extension of the Jurassic ~180 Ma Karoo large igneous province. The presented trace element data on Fo82–92 olivines show relatively high Ni (2430–3570 ppm) and low 100*Mn/Fe (1.32–1.5; Mn = 890–1570 ppm), compatible with pyroxenite-rich sources (Xpx = 37–75%). Many other mantle source indicators (parental melt MgO and whole-rock Zn/Fe, MgO/CaO, FC3MS, Zr/Y vs. Nb/Y, and radiogenic isotope compositions) suggest dominantly or solely peridotitic mantle sources, however. Therefore, the measured high Ni and low Mn/Fe are likely to reflect high temperatures and pressures of melting and possibly high water contents in such peridotite sources. We recommend considerable caution when using Ni and Mn contents of olivine as source indicators, as they may only serve for qualitative comparison of primitive volcanic rocks that originated under fairly similar mantle conditions.Peer reviewe

Mineraalikemiallisia todisteita erittäin magneisum-rikkaista kivisulista Karoon magmaprovinssissa

Peer reviewe

Lineaaristen geokemiallisten trendien mallinnus Magmakammiosimulaattorilla: esimerkkitutkimus Jindabyne-graniiteista, Lachlan Fold Belt, Australia

Understanding the origins of major and trace element variations and the isotopic character of granite samples in terms of sources and magmatic processes is, arguably, the core of granite petrology. It is central to attempts to place these rocks in the context of broader geologic processes and continent evolution. For the granites of the Lachlan and New England Fold Belts (LFB and NEFB) of Australia there has been great debate between competing petrogenetic models. The open-system view is that the isotopic variability and within-suite compositional trends can be accounted for by magma mixing, assimilation and fractional crystallisation (FC). In contrast, the restite unmixing model views the isotope compositions of diverse granites as a feature inherited from individual protoliths that underwent partial melting to produce magmas entraining varying proportions of residual material in a felsic melt. Reconciling all aspects of the geochemical data in a mixing model is contingent on a plausible fractionation regime to produce the observed consistently linear (or near-linear) trends on Harker diagrams; however, published FC models lack phase equilibria constraints on the liquidus assemblage and do not account for the likely changes in trace element partitioning across the modelled compositional range. The Magma Chamber Simulator (MCS) can be used to model fractional crystallisation alone (FC) or with assimilation (AFC), constraining phase equilibria and accounting for the thermal budget. Here, this tool was used to conduct a case study of the I-type Jindabyne Suite of granites from the LFB, testing whether thermodynamically feasible geochemical trends matching the observed linear variations can arise through FC (with or without assimilation of supracrustal material). The results of 112 MCS models show: (1) that major element liquid lines of descent (LLDs) may be sensibly linear over limited compositional ranges, (2) that the involvement of assimilation extends the range in which trends are relatively simple and near-linear, and (3) that, despite these observations, neither FC nor AFC are able to correctly reproduce the geochemical evolution of the I-type Jindabyne Suite granitoids as an LLD (contrary to existing models)—instead, these processes persistently produce curved and kinked trends. The output of these simulations were further refined to explore models in which: (1) crystal-bearing magmas evolve via FC or AFC (with chemical isolation assumed to be achieved through crystal zoning) and undergo varying degrees of melt-crystal segregation at different stages to produce the sample compositions, and (2) in situ crystallisation occurs via FC within the crystallisation zone, driving the evolution of a liquid resident magma, which the samples represent. These models are able to reproduce the Jindabyne Suite trends reasonably well. The modelling implies that FC, or some variant thereof, is a viable explanation for the linear trends in Jindabyne; however, tendency for grossly non-linear LLDs highlights that it should not be assumed that FC can generally explain linear trends in granites without careful modelling such as shown here.Peer reviewe

Partial melting processes of layered intrusions ‒ a newly funded Academy of Finland project launches at the University of Helsinki

Englanninkielinen abstraktiMagma-wallrock interaction in crustal magma chambers (a process known as crustal assimilation) is critical to the evolution of a magmatic system and the formation of many economically important base and precious metal deposits. Although the generalized crustal assimilation model is largely accepted, the details of the interactions remain relatively poorly characterized. One of the major issues has been the lack of models that integrate mass and energy exchange, thermodynamics and geochemistry. The project titled “Partial melting processes at the contact zones of layered intrusions” proposes to explore magma-wallrock interaction at three major intrusive complexes in Antarctica (Muren and Utpostane, 180 Ma), USA (Duluth, 1100 Ma), and Finland (Fennoscandian LIP, 2440 Ma). The study is a multidisciplinary effort which will include state-of-the-art computational modeling with recently developed energy-constrained equations. The modeling will be tested against existing and new geochemical data, as well as partial melting experiments of wallrock. The results are expected to provide unique insight into the generation of layered intrusions and associated ore deposits. The project is conducted at the University of Helsinki in collaboration with the Geological Survey of Finland, University of California Santa Barbara, Central Washington University, and Swiss Federal Institute of Techonology Zürich. The project is funded by the Academy of Finland from September 2016 to August 2021. Proposals for potential collaboration (e.g. additional case studies) are very welcome

Granite huggers - rock climbing in Finland

Englanninkielinen abstraktiClimbing is not usually associated with Finland, because of lack of mountains, and even the highest hills are generally quite gently sloping. Some impressively steep and tens of meters high rock faces have, however, formed in places that have been strongly affected by shearing, faulting or glacial erosion during the last glacial maximum. Finnish climbing community has been increasingly active since the late 1950’s in developing some of these faces for different climbing styles. Unlike in mountaineering, the main goal in rock climbing is not to conquer the hill, but to climb it via as difficult route as possible. As the bedrock of Finland is characterized by Precambrian granites and gneisses, most of the climbing takes place along crack lines or thin edges and crystal faces, which means that the routes are often rather demanding. At current, about 100 rope climbing and over 400 bouldering destinations can be found in Finland. Climbers have traditionally respected nature and Finnish climbers and landowners have built relatively good relationships over the years

Popularizing geology in the American way - lecturer Nick Zentner has conquered television and YouTube

Englanninkielinen abstraktiNick Zentner is a senior lecturer at the Department of Geological Sciences of the Central Washington University (CWU), United States. After accidentally ending up with studying geology, he built up interest in its narrative aspects and started working at the CWU in 1992. Initially he was a technician, but he was also responsible for community outreach. He started gaining wider popularity among locals via the University’s own television channel, on which he hosted a geology talk show with other CWU geologists as guests. Today, he has released several highly viewed videos in YouTube, he runs his own geology show on a regional television channel, and guides open-to-all geological excursions in Washington State and its surroundings. The University has acknowledged his contributions in the popularization of geology as a part of his job description. Nick highlights that general interest, rehearsing of public performance, genuineness, and humbleness are the most important characteristics of a science popularizer. An e-mail list that anyone can subscribe to is an important tool in informing his audiences about upcoming public events. Nick’s examples can help those who are interested in popularizing geology in Finland. YouTube would probably be one of the most easily accessible platforms for such projects in the future. It is worth noting, however, that when narrating stories about billion-year-or-more old rocks, the approach should be somewhat different than in the geologically active northwestern United States

Geochemistry and petrology of the ferropicrite dikes and associated rocks of Vestfjella, western Dronning Maud Land, Antarctica

Mantereiset laakiobasalttiprovinssit ovat suurimpia tunnettuja ilmanalaisia vulkaanisia muodostumia (alkuperäinen tilavuus jopa 2 × 106 km3). Laakiobasaltteja esiintyy kaikilla mantereilla ja niitä tiedetään muodostuneen miltei läpi maapallon historian. Laakiobasalttien purkautumisella on ollut huomattava vaikutus maapallon ilmastoon ja elämän kehitykseen, mutta niiden synnystä tiedetään edelleen varsin vähän. Tämä johtuu osaltaan siitä, että suurin osa laakiobasalttien kantasulista on reagoinut voimakkaasti mantereisen litosfäärin kanssa ja niiden alkuperäinen, mahdollisesti litosfäärin alaisesta vaipasta peritty geokemiallinen sormenjälki on vaikeasti tunnistettavissa ja tulkittavissa. Ferropikriitit ovat poikkeuksellisen rautarikkaita (FeOtot >13–14 p. %) ja primitiivisiä (MgO ≈ 12–18 p. %) laavakiviä, joita on kuvattu muutamista laakiobasalttiprovinsseista. Toisin kuin tavalliset laakiobasaltit, ferropikriitit eivät yleensä ole merkittävästi reagoineet litosfäärin kanssa ja siksi ne tarjoavat arvokasta tietoa suoraan laakiobasalttimuodostumien alkulähteiltä. Ferropikriitit on usein yhdistetty anomalisen korkeisiin vaipan lämpötiloihin ja vaippapluumeihin, mutta näiden erikoisten kivien syntyyn liittyy useita kysymyksiä: Mistä niiden korkea rautapitoisuus on peräisin? Miten ne kytkeytyvät laakiobasalttien syntyyn? Tässä väitöskirjatyössä käsitellään Vestfjellan (Kuningatar Maudin maa, Etelämanner) ferropikriittien (FeOtot = 13–17 p. %; MgO = 13–19 p. %) sekä niihin liittyvien muiden primitiviisten magmakivien – pikriittien, meimechiittien, pikrobasalttien ja basalttien – geokemiaa ja petrologiaa. Nämä suureksi osaksi aikaisemmin tuntemattomat kivet leikkaavat juonina Karoon suuren magmaprovinssin laakiobasaltteja, jotka purkautuivat jurakaudella noin 180 miljoonaa vuotta sitten Gondwana-supermantereen repeämisprosessin alkuvaiheiden aikana. Valikoiduista näytteistä (yhteensä 31 vähintään yhdestätoista juonesta) analysoitiin mineraalien koostumuksia sekä pääalkuaine-, hivenalkuaine-, ja Sr-, Nd-, Pb- ja Os-isotooppikoostumuksia. Analysoidut näytteet voidaan jakaa hivenalkuaine- ja isotooppikoostumuksensa perusteella kahteen magmatyyppiin: (1) Köyhtynyt magmatyyppi (24 näytettä vähintään yhdeksästä juonesta) on köyhtynyt sopeutumattomimmista alkuaineista ja muistuttaa isotooppikoostumukseltaan valtamerten keskiselänteiden basaltteja; (2) Rikastunut magmatyyppi (7 näytettä vähintään kahdesta juonesta) on suhteellisen rikastunut sopeutumattomimmista alkuaineista ja muistuttaa hivenalkuaine- ja isotooppikoostumukseltaan merellisten saarten basaltteja. Kumpikaan magmatyyppi ei ole merkittävästi saastunut kuorellisella aineksella. Köyhtynyt magmatyyppi on peräisin hyvin MgO-rikkaista (jopa 25 p. %) kantasulista, jotka muodostuivat Karoon päävaiheen aikana korkeissa vaipan lämpötiloissa (>1600 °C) ja paineissa (n. 5–6 GPa) pääosin vesipitoisesta, köyhtyneestä ylävaipan peridotiitista. Rikastuneen magmatyypin lähteenä ovat vaipan heterogeeniset pyrokseenipitoiset komponentit, jotka muodostuivat joko subduktoituneen merellisen kuoren reagoidessa vaipan peridotiitin kanssa tai sulametasomatoosin seurauksena. Geokemiallisen mallinnuksen perusteella monet litosfäärin aineksilla saastuneista Karoon laakiobasalteista ovat saaneet alkunsa samasta vaippalähteestä kuin köyhtynyt magmatyyppi. Tästä lähteestä ovat todennäköisesti peräisin myös isotooppikoostumukseltaan hyvin samankaltaiset Intian Valtameren keskiselänteen basaltit. Nämä havainnot ja köyhtyneelle magmatyypille arvioidut korkeat lähdelämpötilat viittaavat siihen, että Karoon laakiobasaltit saivat suurimmaksi osaksi alkunsa Gondwana-supermantereen alaisen vaipan sisäisen lämpenemisen, ei niinkään ylävaippaan tunkeutuneen syvän vaippapluumin, seurauksena. Tutkimukseni tukee näkemystä siitä, että ferropikriittiset sulat voivat syntyä monin eri tavoin: poikkeuksellisen korkea rautapitoisuus saavutetaan todennäköisimmin, jos lähdemateriaali vaipassa sulaa (1) alhaisella asteella (2) ja/tai korkeassa paineessa ja/tai (3) lähdemateriaali sisältää rikastuneita komponentteja (esim. pyrokseniittia tai metasomatoitunutta peridotiittia). On kuitenkin huomionarvoista, että ferropikriittinen kokokivikoostumus voi olla myös seurausta akkumulaatiosta, sekundaarisesta muuttumisesta tai fraktioivasta kiteytymisestä, ja erityistä huomiota on siksi kiinnitettävä kantamagman koostumuksen määrittämiseen.This study provides insights into the composition and origin of ferropicrite dikes (FeOtot = 13–17 wt. %; MgO = 13–19 wt. %) and associated meimechite, picrite, picrobasalt, and basalt dikes found at Vestfjella, western Dronning Maud Land, Antarctica. The dikes crosscut Jurassic Karoo continental flood basalts (CFB) that were emplaced during the early stages of the breakup of the Gondwana supercontinent ~180 Ma ago. Selected samples (31 overall from at least eleven dikes) were analyzed for their mineral chemical, major element, trace element, and Sr, Nd, Pb, and Os isotopic compositions. The studied samples can be divided into two geochemically distinct types: (1) The depleted type (24 samples from at least nine dikes) is relatively depleted in the most incompatible elements and exhibits isotopic characteristics (e.g., initial εNd of +4.8 to +8.3 and initial 187Os/188Os of 0.1256–0.1277 at 180 Ma) similar to those of mid-ocean ridge basalts (MORB); (2) The enriched type (7 samples from at least two dikes) exhibits relatively enriched incompatible element and isotopic characteristics (e.g., initial εNd of +1.8 to +3.6 and initial 187Os/188Os of 0.1401–0.1425 at 180 Ma) similar to those of oceanic island basalts. Both magma types have escaped significant contamination by the continental crust. The depleted type is related to the main phase of Karoo magmatism and originated as highly magnesian (MgO up to 25 wt. %) partial melts at high temperatures (mantle potential temperature >1600 °C) and pressures (~5–6 GPa) from a sublithospheric, water-bearing, depleted peridotite mantle source. The enriched type sampled pyroxene-bearing heterogeneities that can be traced down to either recycled oceanic crust or melt-metasomatized portions of the sublithospheric or lithospheric mantle. The source of the depleted type represents a sublithospheric end-member source for many Karoo lavas and has subsequently been sampled by the MORBs of the Indian Ocean. These observations, together with the purported high temperatures, indicate that the Karoo CFBs were formed in an extensive melting episode caused mainly by internal heating of the upper mantle beneath the Gondwana supercontinent. My research supports the view that ferropicritic melts can be generated in several ways: the relative Fe-enrichment of mantle partial melts is most readily achieved by (1) relatively low degree of partial melting, (2) high pressure of partial melting, and (3) melting of enriched source components (e.g., pyroxenite and metasomatized peridotite). Ferropicritic whole-rock compositions could also result from accumulation, secondary alteration, and fractional crystallization, however, and caution is required when addressing the parental magma composition