COMPOSITIONAL CHANGES IN FLUIDS ACROSS THE KAMCHATKA ARC
T. Churikova1, 2, G. Wörner2, N. Mironov3, A. Kronz2, P. Pletchov3, M. Portnyagin4, 5
1
Institute of Volcanology and Seismology FED RAS, Piip Avenue 9, PetropavlovskKamchatsky, 683006, Russia. E-mail churikova@pisem.net
2
Geowissenschaftliches Zentrum Göttingen, Universität Göttingen, Germany
3
Moscow State University, Moscow, Russia
4
Vernadsky Institute of Geochemistry, Moscow, Russia
5
GEOMAR Research Center, Wischhofstr. 1-3, Buildg. 4, D-24148 Kiel, Germany
Compositional changes in volatile elements across the Kamchatka arc were studied by
electron microprobe and SIMS analysis of major, trace, and volatile elements (S, Cl, F) in
homogenized melt inclusions from high-Mg olivines from arc basalts, which have been collected
along the E-W transect of the Kamchatka arc, from Eastern Volcanic Front to back arc region
(Churikova et al., 2001).
Highest sulfur concentrations (2500 ppm) occur in the melt inclusions from Central
Kamchatka Depression (CKD) lavas. Chlorine concentrations in melts from Eastern Volcanic
Front (EVF) and CKD are similar (750 to 1100 ppm) and higher than for Sredinniy Ridge back
arc (SR, 500 ppm). In contrast, the EVF and CKD melt inclusions are depleted in fluorine (300
to 450 ppm), while the SR melts are enriched (800 ppm), increasing the F/Cl ratio from arc front
to back arc by a factor of five. Our data show that the melts were partially (Cl, S) or undegassed
(F) for most melts before entrapment.
Volatile element - trace element ratios in general correlate with fluid-mobile elements (B,
Li) and suggest successive changes in fluid compositions in these arc magmas with increasing
slab depth. In detail, we identify three distinct fluid compositions. In the eastern volcanic arc
front (EVF), fluids are highly enriched in B, Cl and chalkophile elements. This fluid carries
additionally LILE (U, Th, Ba, Pb), F, S and LREE (La, Ce), having relatively low U/Th. This
fluid is dominating EVF sources and is reduced in the central Kamchatka volcanic zone (CKD)
and is not documented in the back arc region.
The second CKD-fluid affects the Kluchevskaya Group of volcanoes and is enriched in S
and U, which results in the highest S/K2O and U/Th and low Cl/S ratios. Additionally this fluid
unusually enriched in 87Sr, 18O and 11B.
The third fluid observed for back arc volcanoes of the Sredinny Ridge (SR) is highly
enriched in F, Li and Be, carrying also high amounts of LILE, LREE and probably some Nb,
which results in correlations of F/Ce with Li/Yb, Li/Dy, Sr/Y, Ba/Y, and Nb/Yb. This fluid starts
to be released below CKD and becomes the dominant fluid in the back arc region.
We argue that the dehydration of different water-rich minerals at different depths from
the arc front (low P-T conditions) to back arc (high P-T conditions) explains the difference in
fluid composition and the decoupling of B and Li across the Kamchatka arc. In the arc front
fluids derived from the amphibole and serpentine dehydration would be water-rich, low in silica
and high in B, LILE, sulfur and chlorine. It was shown (Bureau and Keppler, 1999) that such
fluids, derived from the first stage of slab dehydration, could be very mobile and carry large
amounts of water, producing high degrees of melting, like in the EVF and CKD.
Stalder and Ulmer (2001) showed that dense hydrous magnesian silicates (DHMS) like
clinohumite (H2O = 2-3%) and phase A (H2O = 13%) could carry about 10% of the initially
subducted water to the depth up to 400 km. The released fluids at such pressure would be highly
enriched in silica and magnesium and would be more viscous and less mobile, but still distinct
from hydrous melts and could metasomatise upper mantle below SR, producing melts highly
enriched in Li, F, LILE, LREE and HFSE. The subduction trace element patters, however, would
still be maintained.
Work was supported DFG-RFBR 00-0504000_GSRU_а and Ministry of research grants ¹
43.700.11.0005 and ¹ 43.043.11.1606.
Reference
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fluids in the upper mantle; experimental evidence and geochemical implications. Earth
Planet. Sci. Lett. 165, 187-196.
3. Wörner, G., Churikova, T., Leeman, W., Liebetrau, V., Tonarini, S., Heuser, A., 2001. FluidMobile Trace Element and U-series Isotope Variations Across Kamchatka: Timing and
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GPa; significance of clinohumite and phase E as water carriers into the transition zone.
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