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| Titel |
The role of amphibole in Merapi arc magma petrogenesis: insights from petrology and geochemistry of lava hosted xenoliths and xenocrysts |
| VerfasserIn |
J. P. Chadwick, V. R. Troll, B. Schulz, L. Dallai, C. Freda, L. M. Schwarzkopf, H. Annersten, H. Skogby |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045149
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| Zusammenfassung |
| Recently, increasing attention has been paid to the role of amphibole in the differentiation of
arc magmas. The geochemical composition of these magmas suggests that deep to mid
crustal fractionation of amphibole has occurred. However, this phase is typically an
infrequent modal phenocryst phase in subduction zone eruptive deposits(1). Nevertheless,
erupted material only represents a portion of the magmatism produced in subduction zone
settings, with many opportunities for melts to stall on route to the surface. This
discrepancy between whole rock geochemistry and petrological interpretation of arc
magmas has lead many scientists to postulate that, at mid to deep crustal levels,
there may be significant volumes of amphibole bearing lithologies. Amphibole
instability at shallow levels can also contribute to its scarcity in eruptive deposits. This
argument is strengthened by field and petrological evidence, including the widespread
occurrence of amphibole-rich intrusive rocks in exhumed orogenicbelts formed during
subduction zone activity, e.g. the Adamello batholith (2),as well as the presence of
amphibole-rich xenoliths and xenocrysts preserved in arc lavas worldwide, e.g. in Indonesia,
Antilles, and Central America. Thus, amphibole appears to play an integral role in
subduction zone magmatism and identifying and constraining this role is central to
understanding arc magma petrogenisis. Amphibole-rich melts or bodies in the deep to
mid crust could be a significant hydrous reservoir for intra-crustal melts and fluids
(1).
In this preliminary study, we have carried out petrological and geochemical analyses of
recent basaltic andesite and amphibole bearing crystalline igneous inclusions and xenocrysts
from Merapi volcano in Java, Indonesia. The basaltic andesite geochemistry is consistent
with amphibole fractionation and the crystalline inclusions are cogenetic to the Merapi
magmatic system. These inclusions are likely to represent fractionation residues reflecting
deep- to mid-crustal processes given the stability field of amphibole. The individual
amphibole xenocrysts are also co-genetic to the Merapi magma system and indicative of
high-pressure crystallisation. Hydrogen isotope analyses of these large amphibole
megacrysts, record a broad range of dD ratios (permil deviation of D/H isotope ratio from
Standard Mean Ocean Water). The dD values of some of these crystals appear to be modified
significantly from expected primary compositions, particularly towards the rims of
amphiboles showing breakdown textures. The measured dD values possibly result from
H-isotope re-equilibration with surrounding volatile vapour during eruption or via
dehydration reactions. Mossbauer analysis of a selected pristine amphibole megacryst from
this suite records 67 % of iron as Fe3+ in the M-sites. Complementary IR spectroscopy
of this amphibole indicates no serious loss of OH groups. High H2O pressures at
formation depth for this crystal have stabilized full hydrous compositions at ~ 2% H2O
concentration in the amphibole. Such fully hydrated amphiboles could release their H2O on
depressurisation on ascent prior to eruption, a process that consistent with the dD
data.
Analysis of these samples is ongoing, however this initial data indicates that amphibole is
a key phase in Merapi magmatic evolution and is a likely source of volatiles through
dehydration on ascent. This is of particular significance given the fact that water content of
magma has a considerable impact on the explosive potential of subduction zone
volcanism.
(1) Davidson et al., 2007. Geology, 35: 787-790. (2) Tiepolo et al., 2002 Contrib. Min.
Pet., 144:1–15. |
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