Fjölrit RALA - 15.06.2004, Síða 157

Weathering and allophane neoformation in soils on volcanic ash from the
Azores.
M.Gerard \ S.Caquineau \ J.Pinheiro 2 and G.Stoops 3
'UR Geotrope, Institut de Recherche pour le Developpement, Bondy, France; 2Departamento de Ciencas
Agrarias, Universitade dos Acores do Heroismo, Azores, Portugal; 3Laboratorium voor Mineralogie, Petrologie
en Micropedologie. Universiteit Gent, Belgium
Two profíles of the Azores, respectively on the islands of Faial and Pico are studied from
the point of view of petrography, micropedology and mineralogy. They consist of 150 cm
thick sequences of paleosols developed on dominantly basaltic pyroclastic material,
essentially of explosive origin, submitted to mesic-udic moisture regime. Attention is focused
on the weathering of lapilli and ashes, and the neoformation of allophane. Combination of
optical studies and in situ microprobe chemical analyses on thin section, X-ray diffraction
analyses and selective extraction techniques on bulk samples revealed that allophane is
present in the micromass of the groundmass, in the alteromorphs after lapilli or pumice and in
clay coatings. As alteromorph after lapilli or pumice, allophane is the end product of the glass
alteration in those samples. It is highly hydrated and related to in situ alteration. In the case of
pseudo-alteromorph after feldspar, a dissolution-precipitation process creates large allophane
coatings less hydrated.
A grading alteration is observed trough three steps: 10% hydration of the glass associated to
strong cations and Si leaching, allophane coatings, allophane alteromorphs with development
of intra-grains bridges. Al/Si ratio of allophanes ranges between 1.3 and 2.5. Their chemical
signature as determined by microprobe, varies from a pure alumino-silicate pole to an
alumino-silicate Fe-Ti enriched pole (Fe/Al around 1). These ratios partly correlate with the
ratios obtained during repetitive oxalate extractions. The Fe enriched allophane phase is
proposed as a mixture of allophane and Fe oxyhydroxides at nanometric scale. Within the
allophane bridges, micrometric zonations with Fe and Mn oxyhydroxides were also observed.
In the Pico profíle, an A1 rich allophane is rather homogeneously distributed associated to
gibbsite formation, especially important at the base of the pedon. In the base of the Faial
profile, a Si rich allophane is associated to halloysite suggesting pedogenic environmental
changes. This paragenesis Si rich allophane - halloysite - gibbsite characterizes a paleosol on
olivine basalt.
The two profiles are also controlled by a strong iron segregation visible at different scales
with various secondary phases (ferrihydrite, hematite, iddingsite) that suggest also variations
of the paleo-environment.
A complex weathering pattern characterizes the Faial paleosols (weathering and
hydrothermalism) within the horizons developed on 0 to 5500 years old pyroclastic deposits.
The weathering pattem of the older Pico paleosols (over 10000 years old), is more simple but
affected in the topsoil by huge humus content. The age of the soils seems to be the major
factor for weathering differentiation but a climatic change is suspected around 10000 years
ago.