Trace element modeling of aqueous пuid peridotite ... - Springer Link

Contrib Mineral Petrol (1998) 132: 390±404

Ó Springer-Verlag 1998

John Ayers

Trace element modeling of aqueous ¯uid ± peridotite interaction in the mantle wedge of subduction zones

Received: 6 October 1997 / Accepted: 8 May 1998

Abstract Recently measured partition coecients for Rb, Th, U, Nb, La (Ce), Pb, Sr, Sm, Zr, and Y between lherzolite assemblage minerals and H2O-rich ¯uid (Ayers et al. 1997; Brenan et al. 1995a,b) are used in a two-component local equilibrium model to assess the e€ects of interaction between slab-derived aqueous ¯uids and wedge lherzolite on the trace element and isotopic composition of island arc basalts (IAB). The model includes four steps representing chemical processes, with each process represented by one equation with one adjustable parameter, in which aqueous ¯uid: (1) separates from eclogite in the subducted slab (Rayleigh distillation, mass fraction of ¯uid released F ¯uid); (2) ascends through the mantle wedge in isolated packets, exchanging elements and isotopes with depleted lherzolite (zone re®ning, the rock/¯uid mass ratio n); (3) mixes with depleted lherzolite (physical mixing, the mass fraction of ¯uid in the mixture X ¯uid); (4) induces melting to form primitive IAB (batch melting, mass fraction of melt F melt). The amount of mantle lherzolite processed by the ¯uid in step (2) determines its isotopic and trace element signature and the relative contributions of slab and wedge to primitive IAB. Assuming an average depleted lherzolite composition and mineralogy (70% olivine, 26% orthopyroxene, 3% clinopyroxene and 1% ilmenite) and using nonlinear regression to adjust parameter values to obtain an optimal ®t to the average composition of IAB (McCulloch and Gamble 1991) yields values of F ¯uid ˆ 0.20, n ˆ 26, X ¯uid ˆ 0.17, and F melt ˆ 0.15, with r 2 ˆ 0.995 and the average relative error in trace element concentration ˆ 6%. The average composition of IAB can also e€ectively be modeled with no contribution from the slab other than H2O (i.e., skip model step 1): n ˆ 27, X ¯uid ˆ 0.21, J. Ayers Department of Geology, P.O. Box 105B, Vanderbilt University, Nashville, TN 37235, USA; E-mail: [email protected]; Fax: 615-322-2138; Tel.: 615-322-2158 Editorial responsibility: T.L. Grove

F melt ˆ 0.17, with r 2 ˆ 0.992. By the time the ¯uid reaches the IAB source, exchange with depleted wedge lherzolite reduces the 87Sr/86Sr ratio isotopic composition to near-mantle values and the slab contribution to 5 wt%) must be added to lherzolite in the IAB source. Decreasing X ¯uid below 0.05 causes model results to have unacceptably high levels of error and petrologically unreasonable values of F melt. That H2O contents of IAB are generally 5 wt%) be added to lherzolite in the IAB source to obtain the observed composition for average IAB, no matter what assumptions are made in the trace element models. Although the trace element patterns of ¯uids in equilibrium with wedge lherzolite have appropriate trace element signatures for metasomatizing the IAB source, their low solute contents require high mass fractions of ¯uid to a€ect signi®cantly the composition of the source. Either all of the mineral/¯uid partition coecients measured by Ayers et al. (1997), Brenan et al. (1995) and Stalder et al. (1998) are systematically high (which seems unlikely since di€erent experimental and analytical methods were used in each study), or some complexing agent that uniformly increases D wedge/ ¯uid values has not been accounted for, or most of the ¯uid escapes from the source region while leaving behind its dissolved load. Clearly, more work must be done to reconcile the results of trace element modeling with those from studies of H2O contents of IAB (e.g., Sisson and Layne 1993) and melting of hydrous peridotite to form IAB (Kushiro 1990).

Acknowledgements Thanks to James Brenan and Terry Plank for organizing the ``Fluxes in Subduction Zones'' symposium at the Seventh Annual V.M. Goldschmidt Conference that prompted me to start this study. Funding was provided by NSF grant #EAR9317105. Ocial reviews by Je€ Ryan and an anonymous reviewer, informal reviews by Jim Meen, James Brenan, and Roland Stalder, and comments from Bill Leeman and Frank Dorendorf all helped to improve the manuscript.

After Nabalek (1987) Derivation of equation for concentration of trace element in ¯uid Cf during the zone re®ning process as function of n, the rock/ ¯uid mass ratio, with input of trace element-containing ¯uid Cfi at the base of the column (symbols same as used by Nabalek, 1987): Closed system: Co ˆ X f Cff ‡ …1 ÿ X f †Crf ˆ X f Cfi ‡ …1 ÿ X f †Cri Dˆ

Crf Cff

Co ˆ X f Cff ‡ …1 ÿ X f †DCff ˆ X f Cff ‡ DCff ÿ X f D Cff ˆ Cff …X f ‡ D ÿ X f D† Cff ˆ

Co (batch equilibrium) X f ‡ D…1 ÿ X f † Let N ˆ Cff ˆ

Cff ÿ Cfi ˆ

Cfi

‡

Mr 1 ÿ X f ˆ ; substitute for Co : Mf Xf i i Cf ‡ NCr ND ‡ 1

NCri Cfi …ND

ND ‡ 1 Cri ÿ Cfi D dCf ˆ dN … DN ‡ 1†2

‡ 1†

ND ‡ 1

ˆ

In the limit that N ˆ Mr/Mf ˆ 0,   dCf ˆ Cri ÿ Cfi D dN f

ZCf



Cff

dCf Cri ÿ Cfi D

ˆN

" # Cri ÿ Cfi D 1 ln D Cri ÿ Cff D   Cri ÿ Cff D exp N D ˆ Cri ÿ Cfi D

Nˆ

NCri ÿ Cfi ND ND ‡ 1

403 Since exp ()1) * exp (1) ˆ 1, multiply both sides by exp ()N D)  i  Cri Cr ÿ Cff ˆ ÿ Cfi exp…ÿND† D D  i  Cri Cr f Cf ˆ ÿ ÿ Cfi exp…ÿND† D D Compare to zone re®ning equation of Harris (1974):   Cff 1 1 ÿ ÿ 1 exp…ÿN D† ˆ D D Co If Co ˆ Cri as we assumed,  i  Ci Cr Cff ˆ r ÿ ÿ Cri exp…ÿND† D D In Eq. 3 )Cif replaces the term )Cir in the parentheses. This term represents the initial concentration in the material at the bottom of the column. In normal zone re®ning, this is a zone where the solid is completely molten, so that the initial concentration of the trace elements in this zone must equal that in the unmodi®ed bulk rock Cir . In our model, it is a ¯uid with initial trace element concentration Cif that enters the base of the column.

References Ayers JC, Eggler DH (1995) Partitioning of elements between silicate melt and H2O-NaCl ¯uids at 1.5 and 2.0 GPa pressure: implications for mantle metasomatism. Geochim Cosmochim Acta 59: 4237±4246 Ayers JC, Dittmer SK, Layne GD (1997) Partitioning of elements between peridotite and H2O at 2.0±3.0 GPa and 900±1100 °C, and application to models of subduction zone processes. Earth Planet Sci Lett 150: 381±398 Bebout GE (1996) Volatile transfer and recycling at convergent margins: mass-balance and insights from high-P/T metamorphic rocks. In Bebout GE, Scholl DW, Kirby SH, Platt JP (eds), Subduction top to bottom. Am Geophys Union Geophys Monograph 96, pp 179±194 Brenan JM, Watson EB (1988) Fluids in the lithosphere. 2. Experimental constraints on CO2 transport in dunite and quartzite at elevated P-T conditions with implications for mantle and crustal decarbonation processes. Earth Planet Sci Lett 91: 141± 158 Brenan JM, Shaw HF, Phinney DL, Ryerson FJ (1994) Rutileaqueous ¯uid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high ®eld strength element depletions in island-arc basalts. Earth Planet Sci Lett 128: 327±339 Brenan JM, Shaw HF, Ryerson FJ, Phinney DL (1995a) Mineralaqueous ¯uid partitioning of trace elements at 900 °C and 2.0 GPa: constraints on the trace element chemistry of mantle and deep crystal ¯uids. Geochim Cosmochim Acta 59: 3331±3350 Brenan JM, Shaw HF, Ryerson FJ (1995b) Experimental evidence for the origin of lead enrichment in convergent margin magmas. Nature 378: 54±56 Davidson JP (1996) Deciphering mantle and crustal signatures in subduction zone magmatism (over-view). In: Bebout GE, Scholl DW, Kirby SH, Platt JP, (eds) Subduction Top to Bottom. Am Geophys Union Geophys Monogr 96, pp 251±262 Davies JH, Stevenson DJ (1992) Physical model of source region of subduction zone volcanics. J Geophys Res 97: 2037±2070 Ellam RM, Hawkesworth CJ (1988) Elemental and isotopic variations in subduction related basalts: evidence for a three component model. Contrib Mineral Petrol 98: 72±80 Elliott T, Plank T, Zindler A, White W, Bourdon B (1997) Element transport from slab to volcanic front at the Mariana arc. J Geophys Res 102 B7: 14991±15019 Ewart A, Hawkesworth CJ (1987) The Pleistocene-Recent TongaKermadec arc lavas: interpretation of new isotopic and rare

earth data in terms of a depleted mantle source model J Petrol 28: 495±530 Frey FA, Prinz M (1978) Ultrama®c inclusions from San Carlos, Arizona: petrologic and geochemical data bearing on their petrogenesis. Earth Planet Sci Lett 38: 129±176 Giaramita MJ, Sorenson SS (1994) Primary ¯uids in low-temperature eclogites: evidence from two subduction complexes (Dominican Republic, and California, USA). Contrib Mineral Petrol 117: 279±292 Green TH, Pearson NJ (1987) An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature. Geochim Cosmochim Acta 51: 55±62 Haggerty SE (1991) Oxide mineralogy of the upper mantle. In: Lindsley DH (ed) Oxide minerals: petrologic and megnetic signi®cance. (Reviews in Mineralogy, vol. 25) Mineral Soc Am, Washington, DC, pp 355±416 Harris PG (1974) Origin of alkaline magmas as a result of anatexis: anatexis and other processes within the mantle. In: Sorenson H (ed). The alkaline rocks, J Wiley and Sons, pp 427±435 Hauri EH, Wagner TP, Grove TL (1994) Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene, and basaltic melts. Chem Geol 117: 149±166 Hawkesworth CJ, Ellam R (1989) Chemical ¯uxes and wedge replenishment rates along recent de-structive plate margins. Geology 17: 46±49 Hawkesworth CJ, Gallagher K, Hergt JM, McDermott F (1993a) Mantle and slab contributions in arc magmas. Annu Rev Earth Planet Sci 21: 175±204 Hawkesworth CJ, Gallagher K, Hergt JM, McDermott F (1993b) Trace element fractionation processes in the generation of island arc basalts. Phil Trans R Soc London A 342: 179±191 Holloway JR (1997) Generation of subduction zone magmas: experimental constraints In: 7th Annu VM Goldschmidt Conf, LPI Contrib 921, Lunar Planet Inst Houston, pp 96±97 Hoogewer€ JA, et al (1997) U-series, Sr-Nd-Pb isotope and rareelement systematics across an active island arc-continent collision zone: implications for element transfer at the slab-wedge interface. Geochim Cosmochim Acta 61: 1057±1072 Kempton PD (1987) Mineralogic and geochemical evidence for di€ering styles of metasomatism in spinel lherzolite xenoliths: enriched mantle source regions of basalts? In: MA Menzies MA, Hawkesworth CJ Mantle metasomatism (eds) Academic Press, London, pp 45±90 Keppler H (1996) Constraints from partitioning experiments on the composition of subduction-zone ¯uids. Nature 380: 237±240 Kogiso T, Tatsumi Y, Nakano S (1997) Trace element transport during dehydration processes in the subducted oceanic crust. 1. Experiments and implications for the origin of ocean island basalts. Earth Planet Sci Lett 148: 193±205 Kushiro I (1990) Partial melting of mantle wedge and evolution of island arc crust. J Geophys Res 95: 15929±15939 McCulloch MT, Gamble JA (1991) Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet Sci Lett 102: 358±74 Miller DM, Goldstein SL, Langmuir CH (1994) Cerium/lead and lead isotope ratios in arc magmas and the enrichment of lead in the continents. Nature 368: 514±520 Mysen BO, Kushiro I, Fujii T (1978) Preliminary experimental data bearing on the mobility of H2O in crystalline upper mantle. Casnegie Inst Washington Yearb. 77: 793±7 Nabalek PI (1987) General equations for modeling ¯uid/rock interaction using trace elements and isotopes. Geochim Cosmochim Acta 51: 1765±1769 Navon O, Stolper E (1986) Geochemical consequences of melt percolation: the upper mantle as a chromatographic column. J Geol 95: 285±307. Peacock SM (1990) Fluid processes in subduction zones. Science 248: 329±337 Poli S, Schmidt MW (1995) H2O transport and release in subduction zones: experimental constraints on basaltic and andesitic systems. J Geophys Res 100: 22299±22314

404 Press WH, Flannery BP, Teokolsky SA, Vetterling WT (1989) Numerical recipes: the art of scienti®c computing. Cambridge Univ Press, Cambridge, UK Rogers G, Hawkesworth CJ (1989) A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge. Earth Planet Sci Lett 91: 271±285 Ryan J, Morris J, Bebout G, Leeman B (1996) Describing chemical ¯uxes in subduction zones: insights from ``depth-pro®ling'' studies of arc and forearc rocks. In: Bebout GE, Scholl DW, Kirby SH, Platt JP, (eds) Subduction top to bottom Am Geophy Union Geophys Monogr 96, pp 263±268 Selverstone J, Franz G, Thomas S, Getty S (1992) Fluid variability in 2 GPa eclogites as an indicator of ¯uid behavior during subduction. Contrib Mineral Petrol 112: 341±357 Shibata T, Nakamura E (1997) Across-arc variations of isotope and trace element compositions from Quaternary basaltic volcanic rocks in northeastern Japan: implications for interaction between sub-ducted slab and mantle wedge. J Geophys Res 102B4: 8051±8064 Sisson TW, Layne GD (1993) H2O in basalt and basaltic andesite glass inclusions from four subduction-related volcanoes. Earth Planet Sci Lett 117: 619±635 Spear FS (1993) Metamorphic phase equilibria and pressure-temperature-time paths. Mineral Soc Am Spera FJ (1987) Dynamics of translithospheric migration of metasomatic ¯uid and alkaline magma. In: Menzies MA, Hawkesworth CJ (eds) Mantle metasomatism. Academic Press, London, pp. 1±20 Stalder R, Foley SF, Brey GP, Horn I (1998) Mineral-aqueous ¯uid partitioning of trace elements at 900 °C±1200 °C and 3.0 GPa to 5.7 GPa: new experimental data set for garnet, clinopyroxene and rutile and implications for mantle metasomatism. Geochim Cosmichim Acta (in press)

Stevenson DJ (1986) On the role of surface tension in the migration of ¯uids and melts. Geophys Res Lett 13: 1149±1152 Sun SS, McDonough WF (1989) Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Saunders A.D. and Norry M.J., eds., Magmatism in the Ocean Basins, Geol. Soc. Lond. Spec. Pub. 42, pp 313± 345 Tatsumi Y, Eggins S (1995) Subduction zone magmatism. Blackwell Science, Cambridge, USA Tatsumi Y, Kogiso T (1997) Trace element transport during dehydration processes in the subducted oceanic crust: 2. Origin of chemical and physical characteristics in arc magmatism. Earth Planet Sci Lett 148: 207±221 Tatsumi Y, Hamilton DL, Nesbitt RW (1986) Chemical characteristics of ¯uid phase released from a subducted lithosphere and origin of arc magmas: evidence from high-pressure experiments and natural rocks. J Volcanol Geothermal Res 29: 293± 309 Watson EB, Brenan JM, Baker DR (1990) Distribution of ¯uids in the continental mantle. In:Menzies MA (ed) Continental Mantle. Clarendon Press, Oxford, pp. 111±125 Wendt JI, Regelous M, Collerson KD, Ewart A (1997) Evidence for a contribution from two mantle plumes to island-arc lavas from northern Tonga. Geology 25: 611±614 Wood BJ, Bryndzia LT, Johnson KE (1990) Mantle oxidation state and its relationship to tectonic environment and ¯uid speciation. Science 248: 337±345 Woodhead J, Eggins S, Gamble J (1993) High ®eld strength and transition element systematics in island arc and back-arc basin basalts: evidence for multi-phase melt extraction and a depleted mantle wedge, Earth Planet Sci Lett 114: 491±504