Magma chamber processes in central volcanic systems of Iceland: constraints from layered gabbro of the Austurhorn intrusive complex

New field work and petrological investigations of the largest gabbro outcrop in Iceland, the Hvalnesfjall gabbro of the 6–7 Ma Austurhorn intrusive complex, have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units. The bases of the macrorhythmic units are composed of 2–10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine, relative to the thicker overlying leucocratic oxide gabbros. While the overall compositional variation is limited (Mg# clinopyroxene 72–84; An% plagioclase 56–85), the melanocratic bases display spikes in Mg# and Cr₂O₃ of clinopyroxene and magnetite indicative of magma replenishment. Some macrorhythmic units show mineral trends indicative of up-section fractional crystallisation over up to 100 m, whereas others show little variation. Two populations of plagioclase crystals (large, An-rich and small, less An-rich) indicate that the recharge magma carried plagioclase xenocrysts (high An-type). The lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge, tapping and fractionation. Modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from Iceland that had undergone about 20% crystallisation of olivine, plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200–300 m. Such a “mushy” magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid-ocean ridges and present-day magma chambers over the Iceland mantle plume. The Austurhorn central volcano likely formed in an off-rift flank zone proximal to the Iceland mantle plume during a major rift relocation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Hekla eruption 1980–1981

The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m³ of lava and about 60 million m³ of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m³ of lava flowed from a summit crater and crater rows on the north slope. The lavas and tephra are of uniform intermediate chemical composition similar to that of earlier Hekla lavas. Although the repose time was short the eruptions fit well into the behaviour pattern of earlier eruptions. Distance changes in a geodimeter network established after the eruptions are interpreted as due to inflation of magma reservoirs at 7–8 kilometers depth.     

On the rate of lava- and tephra production and the upward migration of magma in four icelandic eruptions

The rate of the lava production of three Icelandic fissure eruptions (Lakagigar 1783, Hekla 1947, Askja 1961) is calculated and an attempt is made at a reasonable approximation of the rate of lava production per length and width of the feeding fissures and the rate of upward migration of the magma through the fissures. The results are summed up in Table 2. The figures for the maximum upward migration there presented should be regarded as minimum figures. As a comparison with the mainly lava producing fissure eruptions the tephra production of the acid and highly explosive Askja eruption of 1875 is discussed. It has hitherto been assumed that the enormous amount of tephra, about 2 km₃, which was produced by this eruption in 8 1/2 hours, came entirely from the Viti crater, but with regard to its small diameter it seems likely that the very fine grained tephra produced during the first hours of the eruption was partly expelled from nearby fissures now covered by Lake Öskjuvatn.

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Zusammenfassung Die Rate der Lavaförderung bei drei isländischen Spalteneruptionen (Lakagigar 1783, Hekla 1947, Askja 1961) wird errechnet, und es wird versucht, einen Annäherungswert zu gewinnen über die Rate der Lavaförderung, bezogen auf Länge und Weite der Zuführungsspalten und die Rate der Aufwärtsbewegung des Magma durch die Spalten. Die Ergebnisse sind in Tafel 2 zusammengefaßt. Die darin angegebenen Zahlen für die maximale Aufwärtsbewegung sollten als Minimum aufgefaßt werden. Als Vergleich mit den hauptsächlich Lava führenden Spalteneruptionen wird die Tephraproduktion der sauren und hochexplosiven Askja Eruption von 1875 behandelt. Bisher wurde angenommen, daß die enorme Menge Tephra — ungefähr 2 km₃, die diese Eruption in 8 1/2 Stunden hervorbrachte — ausschließlich aus dem Viti Krater kam; aber in Anbetracht seines geringen Durchmessers scheint es wahrscheinlich, daß die sehr feinkörnige Tephra, die während der ersten Stunden der Eruption entstand, teilweise von nahegelegenen Spalten ausgeworfen wurde, die jetzt vom Öskjuvatn-See bedeckt sind.

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Résumé La mesure de production de lave de trois éruptions fissurales de l'Islande (Lakagigar 1783, Hekla 1947, Askja 1961) est calculée et il est essayé de rapprocher la mesure de cette production à l'égard de longueur et largeur des fissures d'alimentation, et la mesure de la migration vers le haut du magma par les fissures. Les résultats sont présentés au tableau 2. Les chiffres de la migration maximum vers le haut doivent être entendus comme minimum. En comparaison avec l'éruption fissurale produisant principalement de la lave, la téphra production de l'acide et fort explosive éruption de l'Askja de 1875 est discutée. On a supposé jusqu'à présent que l'énorme masse de téphra, environ 2 km₃, qui fut produite lors de cette éruption en 8 1/2 heures, provenait seulement du cratére Viti; mais considérant son petit diamètre il est probable que la téphra de grain fin fut en partie lancée par des fissures voisines et qui à présent sont couvertes par le Lac Öskjuvatn.

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(Lakagígar 1783; Hekla 1947; Askja 1961). . Askja 1875 , . .

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday