Ankaramitic Lavas and Clinopyroxene Megacrysts From the Tanganasoga Volcano, El Hierro Island (Canary Archipelago)
Abstract
Sometime between 134 and 21 ka, the volcanic edifice of El Hierro Island, the youngest, smallest and westernmost island of the Canary Archipelago, grew unstable and its north flank collapsed seaward to form the 15-km-wide El Golfo embayment. Since this event, eruptions at El Hierro have concentrated at the base of, and directly on, the landslide headwall and have commonly involved peculiar ankaramitic lavas rich in clinopyroxene and olivine crystals. The most striking examples of such eruptive products are located at the prominent Tanganasoga volcano, where at least ten eruptive vents have produced a large bulge in the centre-west part of the El Golfo embayment. Lava bombs from the northernmost craters reach up to ~1 m across and show extremely high crystal contents of up to 50 vol. %. Loose lapilli deposits found on the slopes of the cones contain large, often intact clinopyroxene crystals that frequently reach 1.5-2 cm, with the largest found being 3 cm across. We analyzed the whole-rock and groundmass composition of the ankaramites, as well as the composition of phenocryst phases. Moreover, four clinopyroxene megacrysts were studied in further detail, with > 300 electron microprobe spot analyses per crystal. Results indicate that groundmass and co-existing olivine and clinopyroxene crystals approach chemical equilibrium in terms of Fe-Mg exchange. Core-to-rim chemical profiles in the smaller crystals (< 1cm) reveal relatively homogeneous compositions of intra-sample crystal interiors. However, steep normal Fe-Mg zoning is common in the outermost 20-40 μm of the crystals. In contrast, some crystals show reverse zoning towards the rim. Clinopyroxene thermobarometry indicate crystallization pressures of 700-900 MPa and temperatures of 1170-1220°C. The megacrysts show complex oscillatory zoning patterns, which, nonetheless, translate into modest chemical variations (Mg# = molar Mg/(Mg + Fetotal) = 76-80). We propose that the formation of Tanganasoga’s ankaramitic magmas and clinopyroxene megacrysts is the result of a complex interplay between prolonged magma storage, regular influx of fresh magma and efficient crystal growth, fractionation and accumulation at upper mantle depth. The eruption of such dense, crystal-rich magmas probably requires forceful triggering, and appears to be facilitated after large-scale landslide events.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51A1662L
- Keywords:
-
- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1042 GEOCHEMISTRY / Mineral and crystal chemistry