Earthand Planetary Science Letters 210 (2003) 481^497 www.elsevier.com/locate/epsl Back-arc basin basalt systematics Brian Taylor a;Ã, Fernando Martinez b a Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA b Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA Received 4 December 2002; received in revised form 14 March2003; accepted 19 March2003 Abstract The Mariana, east Scotia, Lau, and Manus back-arc basins (BABs) have spreading rates that vary from slow ( 6 50 mm/yr) to fast ( s 100 mm/yr) and extension axes located from 10 to 400 km behind their island arcs. Axial lava compositions from these BABs indicate melting of mid-ocean ridge basalt (MORB)-like sources in proportion to the amount added of previously depleted, water-rich, arc-like components. The arc-like end-members are characterized by low Na, Ti and Fe, and by high H2O and Ba/La; the MORB-like end-members have the opposite traits. Comparisons between basins show that the least hydrous compositions follow global MORB systematics and an inverse correlation between Na8 and Fe8. This is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures (Lau/Manus hotter than Mariana/Scotia). This interpretation accords with numerical model predictions that faster subduction-induced advection will maintain a hotter mantle wedge. The primary compositional trends within eachBAB (a positive correlation between Fe8, Na8 and Ti8, and their inverse correlation with H2O(8) and Ba/La) are controlled by variations in water content, melt extraction, and enrichments imposed by slab and mantle wedge processes. Systematic axial depth (as a proxy for crustal production) variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate. Hydrous fluxing enhances decompression melting, allowing depleted mantle sources just behind the island arc to melt extensively, producing shallow spreading axes. Flow of enriched mantle components around the ends of slabs may augment this process in transform-bounded back-arcs such as the east Scotia Basin. The re-circulation (by mantle wedge corner flow) to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the East Lau Spreading Center, Manus Southern Rifts, and Mariana Trough and the very depleted lavas of east Scotia segments E8/E9. The crust becomes mid-ocean ridge (MOR)-like where the spreading axes, further away from the island arc and subducted slab, entrain dominantly fertile mantle. ß 2003 Elsevier Science B.V. All rights reserved. Keywords: Mariana Trough; Lau Basin; Manus Basin; east Scotia Basin; basalt; back-arc basin; mantle wedge; hydrous melt 1. Introduction * Corresponding author. Tel.: +1-808-956-6649; Fax: +1-808-956-3723. Basalt geochemical data from the world’s E-mail address: [email protected] (B. Taylor). spreading centers reveal systematic correlations 0012-821X / 03 / $ ^ see front matter ß 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0012-821X(03)00167-5 EPSL 6632 5-5-03 482 B. Taylor, F. Martinez / Earth and Planetary Science Letters 210 (2003) 481^497 at bothglobal and local scales related to theex- data, and our corrections for crystal fractiona- tent of mantle partial melting (e.g. [1]). At the tion, for lavas from the Lau, Manus, Mariana global scale, the negative correlation of Na2O and east Scotia BABs (Fig. 1). We then cross- and FeO contents of mid-ocean ridge basalts plot key chemical indices and discuss what their (MORBs), fractionation-corrected to a common trends reveal about the addition of water from the 8% MgO (Na8, Fe8), re£ects a positive correla- subducting slab and its e¡ects on extents of man- tion between the average degree and pressure of tle melting. Finally, we evaluate a physical model melting ^ as do Ca8/Al8 versus Si8/Fe8 ratios ^ of mantle corner £ow and melting [26,27] in light that correlates with regional increases in mantle of the observed geochemical trends. potential temperatures [1^5]. At slow-spreading ridges, segment-scale compositional variations de- ¢ne local trends that are orthogonal to the global 2. Data and fractionation corrections one [3,4,6]. Several possible causes for the local trends have been proposed [3,4,6^8], of which Comprehensive geochemical analyses of closely compositional heterogeneity and variable dynam- spaced axial samples have been published for the ics of melt supply are perhaps the most likely [9]. Lau [28^36], Manus [37], Mariana [38^42] and Although analyses of some basalts from back- east Scotia [43,44] basins. Our data base, com- arc basin (BAB) spreading centers were included piled from these publications, primarily uses anal- in the initial global compilations (e.g. Lau Basin, yses of freshglasses and is available 1. Additional Mariana Troughand east Scotia Basin averages unpublished analyses were provided by P. Fryer, in [1,2]), the global compositional variation of J. Pearce and R. Stern for Mariana Troughsam- BAB lavas has not been investigated previously. ples southof 15‡N and by E. Stolper and S. New- Basalts above subduction zones typically have man for water analyses of segment E2 samples higher oxygen fugacity (fO2) and water contents from the east Scotia Basin. We consulted original (e.g. [10^12]), and hence signi¢cant di¡erences swath bathymetry and/or sidescan surveys of the from MORB trends may be expected. Although spreading axes to limit analyses to samples from MORB magmas are known to contain small the neovolcanic zones and to exclude fractionated amounts of water (e.g. [13^15]), muchinterpreta- samples from propagation tips. Overviews of the tion of the global and local MORB trends has tectonic setting and evolution of eachof theBABs been based on the results of laboratory experi- (Fig. 1) are provided in the following papers: Lau ments that melted uniform peridotite sources [45], Manus [46], Mariana [47], and east Scotia under anhydrous conditions (e.g. [16^18]). The [48]; as well as in [27]. presence of water lowers the mantle mineral sol- Laboratory experiments [16^22] indicate that idus temperatures and promotes greater degrees higher pressure melts have higher FeO and that of melting (e.g. [19^21]). Higher water contents melting to greater extents lowers the contents of increase Si8 and Ca8, decrease Fe8 and Na8, incompatible elements suchas Na 2O. The global when compared to anhydrous melts on a vola- MORB trend of decreasing Na8 withincreasing tile-free basis [20]. Furthermore, the presence of Fe8 is therefore correlated with increasing mantle water delays the onset, and changes the composi- potential temperatures [1^3]. To investigate such tion, of plagioclase fractionation [22^25]. Sup- systematics in the BAB basalt (BABB) data we pressing plagioclase crystallization relative to ol- corrected the published geochemical analyses for ivine produces higher Al8 and lower Fe8. The net the e¡ects of varying crystal fractionation by cal- result is that, compared to MORB, BAB lavas culating compositions at a common 8% MgO. To show greater compositional variations and exag- limit possible errors introduced by this correction, gerated major element trends, providing a fruitful in most cases we used only analyses with6^8.5% environment in which to study crustal accretion processes. In this paper we ¢rst discuss the geochemical 1 See tables 1^4 in the online version of this article. EPSL 6632 5-5-03 B. Taylor, F. Martinez / Earth and Planetary Science Letters 210 (2003) 481^497 483 Fig. 1. Back-arc basin location diagrams. Thick lines are spreading segments, dashed lines are rifts, thin lines are transform faults, triangles locate volcanoes. Spreading segment abbreviations include: (A) CLSC, Central Lau Spreading Center; ILSC, In- termediate Lau Spreading Center; ELSC, East Lau Spreading Center; VFR, Valu Fa Ridge; (B) METZ, Manus Extensional Transform Zone; MSC, Manus Spreading Center; SR, Southern Rifts; SER, SE Ridges; (C) CG, Central Graben; VTZ, Volca- no Tectonic Zone. Transform abbreviations include: (A) PR, Peggy Ridge; (B) DT, Djaul Transform; WT, Weitin Transform; WIT, Willaumez Transform. MgO. In some instances, especially for shallow minerals, as a function of melt composition, ridges dominated by fractionated lavas, we in- Plank and Langmuir [49] calculated: cluded samples withas little as 5% MgO. Na8 ¼½Na O þ 0:115ð83MgOÞ=½1 þ 0:133ð83MgO: To account for varying liquid-lines-of-descent 2 and the varying compositions of fractionating We, like Leat et al. [43], used this equation for EPSL 6632 5-5-03 484 B. Taylor, F. Martinez / Earth and Planetary Science Letters 210 (2003) 481^497 all samples, which facilitates comparisons of our 3. Compositional systematics results between basins and withMORB. From an inspection of the BABB data, we de- 3.1. Lau Basin rived the similar equation: Along the spreading axes of the Lau Basin, Fe8 ¼½FeO þ83MgO=½1 þ 0:25ð83MgOÞ there is a ¢rst-order progression from arc-like to where FeO* expresses all iron as Fe2þ. This func- MORB-like lava compositions withincreasing tion lowers the fractionation slopes as Fe8 de- distance away from the Tofua arc (e.g. [34]). Sam- creases, in accordance withpredicted trends and ples from the arc-proximal and shallow Valu Fa other data [15,20,24,25]. It does not account for Ridge (VFR) and southern East Lau Spreading the reversal in slope at the onset of plagioclase Center (sELSC) have the lowest Na8 and Ti8, crystallization, which occurs at lower MgO values and the highest Ba/La and H2O(8) (Figs.