Other
Scientific paper
Sep 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.2349v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 9, pp.2349-2363
Other
5
Scientific paper
Selected major and trace elements, rare earth element (REE) and 87 Sr / 86 Sr data are presented for arc basalts from Gunung Slamet volcano, Java, Indonesia. On the basis of stratigraphy, trace element content, Zr / Nb , and 87 Sr / 86 Sr ratios, Slamet basalts can be broadly categorized into high abundance magma (HAM) and low abundance magma (LAM) types. Provided the quantities of `immobile' trace elements (in aqueous systems) such as Nb, Hf and Zr in the mantle wedge and ensuing magmas are unaffected by additions from subducted lithosphere or overlying arc crust, a model may be developed whereby LAM are generated by higher degrees of melting in the mantle wedge (13%) compared to HAM (7%). Hf / Nb or Zr / Nb ratio systematics indicate that prior to metasomatism by the underlying lithosphere, the Slamet mantle wedge was similar in chemical character to transitional-MORB source mantle. Conversely, examination of immobile/mobile incompatible trace element ratios (IMITER) provide clues to the nature of the metasomatizing agent, most likely derived from the subducted slab (basalts and sediments). HAM have constant IMITER ( e.g. Nb / U , Zr / K ), whereas LAM show a negative correlation between IMITER and 87 Sr / 86 Sr . Metasomatism of the mantle wedge was modelled by interaction with either a slab-derived-melt or -aqueous fluid. Yb / Sr and 87 Sr / 86 Sr ratios from Slamet basalts and oceanic sediments suggest that `bulk' mixing of the latter into the mantle wedge is unlikely. Instead, sediments probably interact with overlying mantle in the same way that subducted basalts do-either as melts or fluids. In the case of slab-derived melts mixing with `pristine' mantle, good agreement with back-calculated values for HAM and LAM sources can be achieved only if a residual phase such as rutile persists in the subducting lithosphere. In the case of fluids, excellent agreement with back-calculated values is obtained for all elements except heavy REE. It is tentatively suggested that aqueous slab-derived fluids, relatively rich in mobile incompatible elements, are the probable metasomatizing agent responsible for the chemical characteristics, particularly low IMITER, of Slamet and other island arc basalts (IAB). Because the mobilities/solubilities of Sr in high pressure and temperature fluids are poorly known, the modelled subduction fluids are not necessarily efficient at raising 87 Sr / 86 Sr in the overlying mantle wedge. As a result, positive correlations between e.g. Ba / La vs. 87 Sr / 86 Sr need not be observed in arc suites, especially if the relative mobilities of Sr, Ba, and La are dependent upon intensive parameters during metasomatism. Assimilation of arc crust by uprising magmas (up to ~14% of crustal Sr) can account for the range of 87 Sr / 86 Sr in HAM. However, calculating the amounts of arc crustal assimilation by uprising magmas is poorly constrained since such modelling is highly dependent upon previous estimates of the degree of metasomatism undergone by the mantle wedge.
Nicholls Ian A.
Vukadinovic Danilo
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