GEMOC ARC National Key Centre

T.H. Green, GEMOC, Macquarie

The overall geochemical characteristics of island arc magmas are well established, and combined with experimental phase equilibrium studies, are used to constrain petrogenetic models. Partition coefficient data for key trace elements distributed between derived melts and proposed residual minerals are essential ingredients in evaluating the models. Important residual minerals such as garnet and clinopyroxene are noteworthy for their compositional variation with pressure, temperature and bulk composition. Available partition coefficient (D) data do not always encompass the compositions of these minerals at the solidus, and values available for use in testing petrogenetic models may not be appropriate (cf. Blundy et al, 1998). Also, in many cases D data have been obtained for anhydrous conditions, rather than the hydrous conditions almost universally considered appropriate to subduction zone melting processes. In addition, an accessory mineral rutile has been widely proposed as a key residual mineral, possibly essential to explaining depletion of high-field-strength elements (HFSE) in arc magmas. However available data on rutile D values for the HFSE are ambiguous, especially in relation to possible Nb/Ta and Zr/Hf fractionation, and the possible decoupling of Nb and Ta from Zr and Hf, pointed to by some geochemical data from island arc volcanics. Rutile may also be an essential mineral needed to explain the Nb and Ta depletion in continental crust relative to chondritic mantle, by its occurrence in eclogitic reservoirs in the mantle (derived from oceanic crust residual to melting or dehydration processes occurring during subduction) (McDonough, 1991). Thus it is important to determine D behaviour for the HFSE hosted by rutile.

In his report on the first "State of  the Arc" workshop, Turner (1997) recorded that further data on experimentally-determined partition coefficients were needed, including clarification of the role of residual Ti-rich phases in the down-going slab. The new data presented here to address the above two aspects involved in constraining petrogenetic models should also go some way towards filling the gaps noted by Turner.

D values for garnet, clinopyroxene, orthopyroxene and melts from hydrous basaltic compositions.
Hydrous (5-27% weight added water) basaltic compositions were crystallized from 2-7.5 GPa at 1080-1200 C, producing garnet, clinopyroxene, orthopyroxene and mica large enough for SIMS analysis for 13 REE, HFSE, Y and Sr, and determination of their D values (Green et al, 1999). The results for garnet and clinopyroxene are relevant to derivation of  hydrous silica-rich melts or aqueous fluids from subducted oceanic crust (at high pressure silicate solute-rich aqueous fluids approach strongly hydrous silicic melts in composition), because these minerals (and rutile) will control the trace element signature of these melts or fluids. Such melts or fluids may subsequently contribute to island arc volcanism, either directly (adakites), or by interacting with and metasomatizing the mantle wedge source for island arc basalts.

Fig. 1 compares the Ds for garnets crystallized under hydrous or anhydrous conditions. DREE for hydrous conditions show a steeper pattern (and a tightening of the partitioning parabola, Van Westrenen et al, 1999) indicating stronger fractionation of the REE. Also for hydrous conditions, DHFSE are slightly lower and there is an apparent decrease in DZr/DHf and increase in DHf/DSm with increase in pressure from 3-7.5 GPa, such that the potential for garnet to fractionate these elements decreases with increasing depth until at >200 km little fractionation of Zr-Hf-Sm may be evident. Thus melt products with no modification of source Zr-Hf-Sm systematics may indicate garnet-dominated fractionation processes and depth of origin >200km.

For clinopyroxene at 2 GPa DHREE are >1, and DSr and DZr show negative spikes between DREE  on a mantle compatibility diagram (Fig.2). In contrast, at 3-4 GPa, clinopyroxene shows DHREE <1 and no negative spike for DSr. These results are consistent with predictions for changes in Ds with changes in clinopyroxene composition (e.g. Blundy et al, 1998). The Sr behaviour provides a possible indicator of clinopyroxene fractionation at great depth (no Sr anomaly) or at relatively shallower levels (positive Sr anomaly), so long as subsequent plagioclase fractionation does not occur. Orthopyroxene shows a similar negative DSr spike and correlation of DREE with Al content as the results for clinopyroxene at 2 GPa (Fig.3). However orthopyroxene  has relatively higher DHFSE compared with neighbouring DREE, giving support to the possible role for orthopyroxene in explaining relative depletion of HFSE in arc volcanics as proposed in the mantle reaction model of Kelemen et al (1993).

Acknowledgements
This research has been supported by grants from the Australian Research Council and Macquarie University. The hospitality of Dr. J Blundy, University of Bristol and his collaboration in part of this work is gratefully acknowledged, as is the assistance of Dr. J. Adam, Macquarie University.

References
Barragan, R., Geist, D., Hall, M., Larson, P. and Kurz, M., 1998. Subduction controls on the compositions of lavas from the Ecuadorian Andes. Earth and Planetary Science Letters 154, 153-166.

Blundy, J.D., Robinson, J.A.C. and Wood, B.J., 1998. Heavy REE are compatible in clinopyroxene on the spinel lherzolite solidus. Earth and Planetary Science Letters 160, 493-504.

Brenan, J.M., Shaw, H.F., Phinney, D.L. and Ryerson, F.J., 1994. Rutile-aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high field strength element depletions in island-arc basalts. Earth and Planetary Science Letters 128, 327-339.

Eggins, S.M., Woodhead, J.D., Kinsley, L., Sylvester, P., McCulloch, M.T., Hergt, J., and Handler, M., 1997. A simple method for the precise determination of 40 or more trace elements in geological samples by ICP-MS using enriched isotope internal standardization. Chemical Geology 134, 311-326.

Elliott, T., Plank, T., Zindler, A., White, W. and Bourdon, B., 1997. Element transport from slab to volcanic front at the Mariana arc.
Journal of Geophysical Research 102, 14991-15019.

Green, T.H., Blundy, J.D., Adam, J. and Yaxley, G.M., 1999. SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2-7.5 GPa and 1080-1200 C. Lithos (submitted).

Jenner, G.A., Foley, S.F., Jackson, S.E., Green, T.H., Fryer,B.J. and Longerich, H.P., 1993. Determination of partition coefficients for trace elements in high pressure-temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Geochimica et Cosmochimica Acta 58, 5099-5103.

Johnson, K.T.M., 1998. Experimental determination of partition coefficients for rare earth elements between clinopyroxene, garnet, and basaltic melt at high pressures. Contributions to Mineralogy and Petrology 133, 60-68.

Kelemen, P.B., Shimizu, N. and Dunn, T., 1993. Relative depletion of niobium in some arc magmas and the continental crust: partitioning of K, Nb, La and Ce during melt/rock reaction in the upper mantle. Earth and Planetary Science Letters 120, 111-134.

Linnen, R.L. and Keppler, H. 1997. Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth's crust. Contributions to Mineralogy and Petrology 128, 213-227.

McDonough, W.F., 1991. Partial melting of subducted oceanic crust and isolation of its residual eclogitic mineralogy. Philosophical Transactions of the Royal Society, London, Series A 335, 407-418.

Rudnick, R.L., Barth, M., McDonough, W.F. and Horn, I., 1999. Evidence for a mafic, rutile-bearing reservoir in the earth. Ninth Annual V.M. Goldschmidt Conference, Boston, Abstract Volume p.252.

Stalder, R., Foley, S.F., Brey, G.P. and Horn, I., 1998. Mineral-aqueous fluid partitioning of trace elements at 900-1200 C and 3.0-5.7 GPa: New experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism. Geochimica et Cosmochimica Acta 62, 1781-1801.

Turner, S., 1997. Scientists share knowledge about island arc magmatism. EOS  78, 333.

Van Westrenen, W., Blundy, J.D. and Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. American Mineralogist  84, 838-847.