 |
| Titel |
Melt inclusions suggest that small-scale heterogeneities in H2O and degrees of melting exist in the sources of monogenetic volcanoes on the Izu Peninsula, Japan |
| VerfasserIn |
A. R. L. Nichols, R. J. Wysoczanski, K. Tani, Y. Tamura, J. A. Baker, Y. Tatsumi |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024826
|
|
|
|
|
|
| Zusammenfassung |
| The Izu Peninsula, central Japan, is at the northern end of the Izu-Bonin-Mariana Arc system
(IBM) where it abuts the Japan Arc. The Peninsula consists of a basement of Miocene
submarine deposits upon which a number of polygenetic and monogenetic volcanic centres
have erupted. Along the eastern coast of the Izu Peninsula over 70 monogenetic volcanoes,
produced over the last 300 ka and covering 350 km2, form the Higashi-Izu Monogenetic
Volcano Group (HIMVG). The field also extends offshore into Sagami Bay where a further
50 monogenetic volcanoes form the Higashi-Izu Oki Submarine Volcano Group, and where
the most recent eruption (in 1989) occurred. The monogenetic activity is associated
with a change in the local stress field as the Izu block collided with Honshu. The
HIMVG are mostly basaltic, but towards the centre there are a number of dacitic
cones.
In order to better understand the sources of the HIMVG, and this area of the IBM,
inclusions hosted in crystals from scoria produced by two basaltic monogenetic
volcanoes have been analysed. The inclusions provide access to magmas prior to
the widespread crustal assimilation, differentiation and degassing processes that
have affected other rocks erupted in the IBM. The two monogenetic volcanoes,
Takatsukayama and Sukumoyama, from which the inclusions were collected are 5 km apart
and located in the north of the HIMVG. They erupted contemporaneously at the
initiation of HIMVG volcanism. The major (measured by EPMA), trace (LA-ICP-MS)
and volatile (FTIR spectroscopy) element compositions of the inclusions show
bimodal distributions. Inclusions from both volcanoes can be divided into a high
alumina (14.89 – 18.81 wt.% Al2O3) basalt type (HAB) and low alumina (11.02 –
13.73 wt.% Al2O3) basalt type (LAB), with some crystals containing both types of
inclusion. LAB inclusions from Sukumoyama, with higher MgO contents (11.52 –
12.38 wt.%), can be related to their HAB counterparts (8.00 – 10.24 wt.%) through
fractional crystallisation. However, the Takatsukayama LAB inclusions, with a similar
range of MgO contents (6.62 – 10.25 wt.%) to their HAB counterparts (6.38 – 9.16
wt.%), cannot be related so easily. Furthermore, whereas LAB inclusions from
Sukumoyama have similar incompatible major element (e.g., TiO2 and K2O) and trace
element signatures to HAB inclusions from both volcanoes, the Takatsukayama LAB
inclusions show significant enrichments in incompatible major elements and most trace
elements. Melts with the geochemical characteristics of the Takatsukayama LAB
inclusions have not been found elsewhere in the IBM to date. In terms of volatiles,
LAB inclusions are generally characterised by lower H2O contents. This could be a
result of the LAB melts being entrapped at shallower levels in the crust, but this is
difficult to reconcile with the occurrence of both types of inclusion in the same
crystal. Alternatively, combining all the available geochemical data, the Sukumoyama
HAB and LAB inclusions appear to be derived from lower degree melts and drier
sources than the HAB inclusions from Takatsukayama. The LAB inclusions from
Takatsukayama are also derived from dry sources, but even lower melt fractions,
producing melts enriched in incompatible elements. To be incorporated in the same
crystal these heterogeneities must occur on a very small scale. Finally, the lack of
whole rock LAB compositions elsewhere in the IBM suggest that LAB sources
are a minor component that usually become fully integrated within HAB melts. |
|
|
|
|
|
|