Open Session - SWISS GEOSCIENCE MEETINGs

2
Symposium 2: Mineralogy-Petrology-Geochemistry
2.1
Bubble- and crystal-size distributions in the emergent Capelas tuff cone,
São Miguel (Azores): insights into magma-ascent and fragmentation
Mattsson Hannes B.*, Solgevik, H.**
*Institute for Mineralogy and Petrology, ETH Zurich (hannes.mattsson@erdw.ethz.ch)
**Department of Geology and Geochemistry, Stockholm University
The late Holocene Capelas eruption started in a shallow marine environment off the north coast of São Miguel (Azores). As
the eruption progressed, a tuff cone grew to a subaerial setting connecting with the main island of São Miguel, and a scoria
cone formed inside the main crater of the tuff cone.
In this study, we sampled different depositional facies which were subject to combined analysis of the bubble- and crystalsize
distributions. With a total of 13705 bubbles and 4719 plagioclase crystals analyzed, a clear pattern emerges. During the
eruption, the amount of plagioclase remain fairly constant (7±2 vol%) whereas the bubble content of the samples are much
more variable (varying from 51 to 8 vol%). Inititally, in the thick fall deposits at the base of the tuff cone, bubbles are abundant
(average=39.9 vol%) and individual clasts show fluidal textures consistent with jetting or fire-fountaining type of activity.
The later deposits show a decrease in the total amount of bubbles (average=26.8 vol% in fall deposits and 15.7 vol% in
surge-deposits) but also in the absolute size of individual bubbles.
Our textural data suggests that the Capelas magma was only briefly ponded in a crustal magma chamber, allowing bubbles
to accumulate/coalesce near the top of the magmatic reservoir but not long enough to allow flotation of plagioclase laths to
occur (as indicated by the constant crystal content of the deposits). The first phase of the eruption drained the upper part of
this reservoir (by sustained jetting characterized by limited magma-water interaction). The decreasing amount, and size, of
the bubbles with time during the eruption can be interpreted as reflecting either (i) an increase of the decompression rate,
or (ii) a systematic deepening of the fragmentation level within the conduit. We favour the latter alternative as most basaltic
volcanic eruptions show a clear decrease in the eruption rate over time.
2.20
Petrology of the Lake Natron – Engaruka monogenetic volcanic field,
northern Tanzania
Nandedkar Rohit, Dr. Mattsson Hannes
Institute for Mineralogy and Petrology, Clausiusstrasse 25, CH-8092 Zurich (rohit@student.ethz.ch)
During fieldwork in 2006 a total of 97 monogenetic volcanic cones in the Lake Natron - Engaruka area in northern Tanzania
(East African Rift) were sampled. The main goal of the project is to: (i) characterize the geochemistry of the monogenetic
cones, and (ii) to see if there are any spatial and/or temporal changes in the erupted compositions. Here we present the results
of 60 analyzed cones.
All investigated rocks are silica-undersaturated, ranging in composition from primitive olivine melilitites, via nephelinites
to basanitic/basaltic compositions. Mg-numbers ranges from 77 to 20 (averaging 55), and roughly 10% of the 60 cones classify
as being peralkaline ((Na+K)/Al). Although there is some minor scatter in Mg# vs. SiO 2 , clear trends are present in major element
variation diagrams going from the olivine melilitites to the basanitic compositions. The scatter, however, suggests that
fractional crystallization is not the dominant process involved in the forming of the magmas. Mantle xenoliths in volcanoclastic
deposits give a clear indication that most of the magmas rose directly to the surface without ponding and evolving
in crustal magma chambers. Therefore these clear trends are suggested to originate out of the melting process. This is especially
interesting considering that different compositions represent different degrees of melting as conclude out of REE
pattern. Melilititic rock suites, which are the most common, are considered of being produced at depth by partial melting
of a carbonated mantle source. Indication for such a mantle source possibly being present beneath the Lake Natron -
Engaruka area is found in the metasomatic veins that crosscut lherzolitic - dunitic xenoliths (found in 13 of the investigated
cones). These veins consist of amphibole, phlogopite, minor clinopyroxene and a carbonate phase. However, the exact origin
of this carbonate phase is unclear and requires further work.