Asian Current Research On Fluids Inclusions
ACROFI 2010, Novosibirsk, Russia
MELT INCLUSIONS IN MINERALS FROM HOLOCENE TEPHRAS OF AVACHINSKIY
VOLCANO, KAMCHATKA
Krasheninnikov S.P.a, Portnyagin M.V.a,b
aV.I.Vernadsky
Institute of Geochemistry and Analytical Chemistry, ul. Kosygina 19, Moscow 119991, Russia,
e-mail:spkrasheninnikov@mail.ru
b
Leibniz Institute of Marine Research, IFM-GEOMAR, Wischhofstrasse. 1-3, D-24148 Kiel, Germany
Introduction
Melt inclusions presents melts trapped by
minerals during their growth in magma chambers
and at ascending to the surface. Several recent
studies of melt inclusions in island-arc rocks
revealed a strong bimodality of the compositions
and scarcity of intermediate melts with SiO2=59-66
wt% (e.g. Naumov et al., 1997; Reubi, Blundy,
2009). These observations were used to interpret
the origin of island-arc andesites by magma
mingling and crystal accumulation rather than by
fractional crystallization of basaltic magmas.
Whether or not all island-arc volcanoes are equal
with this respect remains not clear. On the other
hand, most investigations of melt inclusions in
island-arc rocks were focused on inclusions from
one or two minerals (e.g., Naumov et al., 1997;
Tolstykh, 2002). It cannot be excluded that such
data sets are biased and not representative for large
volume and heterogeneous magma systems.
In this work we report results of a systematic
study of melt inclusions in 6 minerals from 61
tephra samples from Avachinskiy volcano in
Kamchatka. These samples represent 40 Holocene
eruptions of this volcano including: 1) early phase
of rare and voluminous andesitic eruptions (7.253.5 ky BP) and 2) later phase of frequent eruptions
of basaltic andesites associated with the
construction of the Young Cone (3.5 ky BP to
present) (Braitseva, 1998). We use the data to
reconstruct the evolutional path of Avachinskiy
melts prior eruptions and the changes in the magma
feeding system beneath this volcano which
occurred during the last ~7,000 years.
Results
In the course of this study we have analyzed
~500 melt inclusions in Ol (60 an.), Cpx and Opx
(300 an.), Amph (60 an.), Pl (30 an.) and Mt (40
an.). All analyses were performed with the help of
JEOL JXA 8200 wave-length dispersive electron
microprobe at the IFM-GEOMAR (Kiel, Germany).
The results are shown in Figure 1.
The melt inclusions span a large range of
compositions from basalts to rhyolites. Both
melt inclusion and host rock compositions plot
predominantly along the line dividing low- and
middle-K island-arc series (Figure 1).
Figure 1. Composition of melt inclusions in different minerals
from tephras of Avachinskiy volcano. Field of whole rock
compositions is shown after (Bindeman et al., 2004;
Castellana, 1998). All concentrations are in wt%.
Asian Current Research On Fluids Inclusions
The trends of major elements are continuous,
and no apparent bimodality is observed in the data
set. Most of the major element variability can be
explained by fractional crystallization from parental
basaltic melts. The most primitive crystallizing
assemblage is represented by Ol and Cr-Sp.
Judging from decreasing CaO content in primitive
melts, Cpx also joinded Ol at very early stages of
crystallization. Plag appears on liquidus at ~52-53
wt% SiO2. Mt and Ilm started to crystallize at ~57
wt%. Significant change of crystallizing
assemblage occurred at ~60-62 wt% of SiO2, when
Opx replaced Ol and Amph and Ap become stable.
Paragenesis of OPx, CPx, Amph, Pl, Mt, Ilm and Ap
dominated the following evolution of melts toward
strongly acid compositions with 78-80 wt% SiO2.
Studied melt inclusions are rich in volatile
components. Judging from low totals of microprobe
analyses the amount of H2O in parental basaltic
melts was at least 2-3 wt% and increased up to 5-6
wt% in more silicic melts. SO3 content was as high
as 0.9 wt% in basaltic melts and decreased rapidly
with increasing SiO2. Cl concentrations in mafic
melts were ~0.07 wt% and increased to ~0.20-0.25
wt% at SiO2~70 wt% and then decreased in more
evolved melts, probably, due to separation of Clrich hydrous fluid from evolved magmas.
In comparison with host rocks, melt inclusions
have in general more silicic compositions, and this
difference increases with increasing SiO2 content in
the host rocks. For example, melt inclusions from
rare basalts of Avachinskiy volcano have SiO2
similar or slightly higher than host rocks. Melt
inclusions in basaltic andesites (SiO2=53-57 wt%)
of the later stage of volcano formation (<3500 ky
BP) have SiO2 which is typically ~6-8 wt% higher
than their host rocks. Melt inclusions in andesites
(SiO2=57-63 wt%) of the earlier stage (3500-7250
ky BP) are mostly rhyolitic (SiO2>70 wt%).
Because the composition of erupted magmas is
shifted to more mafic compared to melt inclusions,
accumulation of minerals was likely very important
process at the origin of all Avachinskiy rocks.
Magma mixing also played an important role
in the origin of Avachinskiy rocks as evident from
occurrence of contrasting matrix glasses in single
tephra samples and from inverse and rhythmic
zoning of minerals. In most cases, the contrasting
compositions can be related to each other by
fractional crystallization process. Thus, mixing of
less and more evolved magmas could plausibly
occur during periodic replenishment of magma
chamber with more primitive melts.
Some studied silicic melt inclusions in CPx
and Amph have relatively K-rich compositions and
cannot be related to parental basaltic melts by
simple crystallization process. These melts are
ACROFI 2010, Novosibirsk, Russia
considered to be exotic and can be formed by
localized melting of hydrothermally altered wall
rocks beneath volcano. The proportion of the exotic
melts is however relatively minor compared to the
predominant less K melts, and thus crustal melting
was likely not a major process to generate acid
melts of Avachinskiy volcano.
Conclusions
New data on composition of melt inclusions
from Avachinskiy volcano allowed us to
reconstruct the entire evolutional path of its
parental melts evolved from basalts to rhyolites.
Melt inclusions in different minerals form coherent
trends of major elements, which can be well
explained by fractional crystallization. Unlike other
island-arc volcanoes, Avachinskiy melts do not
display bimodality of SiO2 content. Melts of
intermediate compositions are relatively abundant
but they were found in less evolved rocks than
andesites. In general, our data demonstrate that melt
inclusions in typical arc rocks have systematically
more Si-rich compositions compared to their host
rocks, and this difference increases with increasing
SiO2 in rocks. This observation implies that evolved
rocks of island-arc volcanoes are indeed products of
magma mixing and accumulation of minerals,
which crystallized from more evolved melts. While
the majority of island-arc rocks are andesites, it is
therefore not surprising that large number of highly
evolved rhyolitic melt inclusions were analysed by
researchers in these rocks.
This work was supported by RFBR (#09-0501234 and 10-05-00147) and Russian-German
KALMAR project.
References
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