Características elementares e isotópicas (He e Ne) das lavas da
Terceira (Açores): implicações petrogenéticas
Elemental and isotopic (He and Ne) characteristics of the Terceira
(Azores) mafic lavas: petrogenetic implications
P. Madureira1, J. Mata2, M. Moreira3 & G. Queiroz4
1- Departamento de Geociências da Univ. de Évora/Centro de Geofísica de Évora;
2- Departamento de Geologia da Fac. Ciências Univ. Lisboa/Centro de Geologia da
Universidade de Lisboa; 3- Laboratoire de Géochimie et Cosmochimie - Institut de
Physique du Globe de Paris; 4- Departamento de Geociências da Universidade dos
Açores.
pedro@uevora.pt; jmata@fc.ul.pt; moreira@ipgp.jussieu.fr; gq@notes.uac.pt
SUMÁRIO
As razões isotópicas de He e de Ne permitem relacionar a ilha Terceira com uma pluma profundamente
enraizada. Ainda que os os elementos incompatíveis apontem para o início da fusão parcial na zona da granada,
as suas lavas apresentam, a maior parte das vezes, um carácter transicional e hiperstena normativa, que contrasta
com o referenciado para algumas das ilhas da Macaronésia (e.g. Canárias e Cabo Verde). Esta diferença será
discutida com base na variabilidade da espessura da litosfera subjacente aos arquipélagos em causa.
Palavras-chave: Açores, pluma mantélica, fusão parcial, litosfera, manto inferior
SUMMARY
He and Ne isotopic ratios obtained for Terceira lavas suggest the presence of a deep plume beneath the Azores.
Basalts from Terceira display incompatible elemental signatures in agreement with the onset of magma
generation in the garnet zone. However, they are mostly Hy normative and transitional between alkaline and
tholeitic. These magmatic affinities, clearly distinct from those reported for other Macaronesia archipelagos
(Canárias and Cape Verde) will be discussed considering lithosphere thicknesses underneath such archipelagos.
Key-words: Azores, mantle plume, partial melting, lithosphere, lower mantle
Terceira (30º 44’N, 27º 19’W) is one of the 9 islands
of the Azores Archipelago, located on the vicinity of
the Mid-Atlantic Ridge, at the Azores Triple
Junction.
Terceira lavas range in composition from basalts to
rhyolites (SiO2 from 47 to 70 wt%). Mafic lithotypes
plot, on a TAS diagram, above the compositional
divider between alkaline and sub-alkaline rocks.
However, the majority of samples are characterized
by normative hyperstene (up to 13%) conferring to
the Terceira mafic magmas a transitional character.
This type of lavas can be considered as belonging to
three volcanic systems (Fissural, Santa Bárbara and
Cinco Picos) usually considered younger than 1 Ma.
Lavas from these three volcanic systems present
distinct differentiation degrees. Fissural lavas are the
less evolved (Ni up to 285 pmm), while Santa
Bárbara lavas are the more differentiated (Ni < 35
ppm), and Cinco Picos rocks are placed midway in
terms of evolution indexes. However, based on Pb
and Sr isotopic signatures [1] and on the ratios of
highly incompatible elements, these 3 lava systems
cannot be considered cogenetic; therefore, it can be
1
concluded that they have evolved from primary
magmas
formed
from
mantle
sources
compositionally distinct.
Lavas from each of these three systems present a
significant compositional range, which is compatible
with the occurrence of intra-system crystal
fractionation processes, which were more important
inside the Fissural (F = 0.41) and Santa Bárbara (F=
0.47) systems. For the Fissural system the
compositional range can be successfully modeled
considering
the
crystal
fractionation
of
clinopyroxene, olivine and plagioclase in the
proportion 0.50:0.45:0.05. For Sta Bárbara,
clinopyroxene, plagioclase and Fe-Ti oxides
dominate the differentiation process (0.5:0.4:0.1).
The noticeably evolved character of the Sta Bárbara
lavas indicates the existence of magma chamber(s)
volumetrically significant. Geobarometric data
suggest the location of these chamber(s) at Moho
depths, which may have constituted a density
compensation level.
The fractionated rare earth patterns [(La/Yb)n = 711] and the low concentrations of Yb and Lu (1.92.7 and 0.25-0.37 respectively) points to the
presence of garnet as a residual phase. However, this
is at odds with the depth of magmatic segregation
inferred from major elements, which is estimated at
about 50-60 km, i.e. inside the spinel zone. This
apparent paradox is the result of a relatively young
(20 Ma) and thin lithosphere (probably less than 50
km) which allowed that melting of the ascending
plume, initiated at depths where garnet is the stable
aluminous phase, continued at the spinel zone.
Magmas issued from the garnet zone by smaller
melting degree, and hence highly rare-earth
fractionated, mixed with more voluminous (higher
F) and less enriched melts generated in equilibrium
with spinel lherzolite. This would also explain the
transitional affinities of Terceira Lavas as compared
with the, sometimes, highly undersaturated lavas
from other Macaronesia archipelagos where
lithosphere is around 100 km thick, thus restraining
plume melting to depths where garnet is stable.
During their translithospheric ascent magmas
interacted with the lithosphere as is strongly
suggested by the evidences for equilibrium with
amphibole, a phase not stable at asthenosphere or
plume temperatures [e.g. 2].
The noble gas data (He and Ne) enable us to
unequivocally demonstrate the contribution of an
unradiogenic and undegassed reservoir [see also 3].
This reservoir with relatively primitive signatures
[(21Ne/22Ne)corr = 0.0052 and 4He/3He > 9), is
usually assigned to the lower mantle [e.g. 4]. These
data are compatible with seismic tomagraphic
studies [5; 6]. and the experimental work developed
by [7].
Acknowledgements
This a contribution from the FCT project STAMINA
(PDCTM1999MAR15255). Subsidiary we were also
financed by Centro de Geofísica Évora and Centro
de Geologia da Universidade de Lisboa.
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