G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Pulished y AGU and the Geochemical Society Article Volume 8, Numer 4 24 April 2007 Q04005, doi: /2006gc ISSN: Hafnium, neodymium, and strontium isotope and parentdaughter element systematics in asalts from the plume-ridge interaction system of the Salas y Gomez Seamount Chain and Easter Microplate Richard H. Kingsley Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA (kingsley@gso.uri.edu) Janne Blichert-Toft Ecole Normale Supérieure, CNRS UMR 5570, 46 Allée d Italie, F Lyon, France Denis Fontignie Départment de Minéralogie, Université de Genève, 1211 Geneva, Switzerland Jean-Guy Schilling Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA [1] We present a comprehensive data set with Hf, Nd, and Sr isotope ratios and parent-daughter trace element concentrations in 111 asalts and glasses from seamounts of the western Easter Salas y Gomez Seamount Chain (ESC), the Easter Microplate (EMP) spreading centers, and the East Pacific Rise (EPR). Sr and P radiogenic isotope ratios and related ratios of highly incompatile parent to lesser incompatile daughter elements grade from high values near the Salas y Gomez (SyG) hot spot location to low values 1000 km west. Here the west rift of the EMP is dominated y typical depleted mid-ocean ridge asalt (MORB). Hf and Nd radiogenic isotope ratios show the opposite gradients, which also reflect the longterm enriched nature of the hot spot source and mixing of the hot spot with the depleted upper mantle. Gradients of these parameters occur north and south along the EMP oundaries. These oservations confirm the plume-ridge interaction model proposed for this region y Schilling et al. and further characterize the SyG hot spot. The inary mixing relationship evident in the isotope variations of the asalts is somewhat compromised when the trace elements are considered. This complexity can e explained y modification of trace element aundances during the process of partial melting of the two end-memer components (enriched hot spot and depleted upper mantle). In addition, the melting variaility is evident in the asalt ulk compositions, which range from tholeiitic (EMP to 108 W) to alkali asalts (108 W to SyG). A single highly correlated data array in P-isotope space and a linear P-Sr isotope relation indicate that the SyG hot spot is homogeneous. The similarity of the P isotope ratios of the SyG hot spot to other long-term high-u/p mantle domains suggests an origin in suduction-modified altered oceanic crust. The SyG end-memer isotope composition appears to e pervasive in the south central Pacific mantle, evoking a widespread mantle contamination event y the SyG hot spot in the past. Components: 10,931 words, 13 figures, 5 tales. Keywords: strontium isotopes; plume-ridge interaction; Easter microplate; Salas y Gomez. Copyright 2007 y the American Geophysical Union 1 of 28

2 Index Terms: 1032 : Mid-oceanic ridge processes (3614, 8416); 1033 : Intra-plate processes (3615, 8415); 1038 : Mantle processes (3621). Received 27 June 2006; Revised 6 Decemer 2006; Accepted 15 January 2007; Pulished 24 April Kingsley, R. H., J. Blichert-Toft, D. Fontignie, and J.-G. Schilling (2007), Hafnium, neodymium, and strontium isotope and parent-daughter element systematics in asalts from the plume-ridge interaction system of the Salas y Gomez Seamount Chain and Easter Microplate, Geochem. Geophys. Geosyst., 8, Q04005, doi: /2006gc Introduction [2] Basalts recently erupted (less than 3 Ma) along the 1400 km-long western region of the Easter Salas y Gomez Seamount Chain (ESC) etween Salas y Gomez island (SyG) and the west rift of the Easter Microplate (EMP) provide a natural window for the study of the interaction of an intraplate hot spot with a migrating spreading center via lateral flow in the asthenosphere. The volcanism in the area was sampled on two cruises y dredging north and south along the EMP [Schilling et al., 1985] and y dredging the numerous seamounts in the intraplate region east of the EMP to SyG [Naar et al., 1993] (Figure 1). This unique collection of asalt glasses comprises a continuous spectrum from normal mid-ocean ridge tholeiitic asalts (N-MORB) erupted at the spreading centers to alkali ocean island-type asalts (OIB) erupted at an intraplate hot spot. The location of the SyG hot spot, and its relationship to the EMP, has een inferred from high La/Sm ratios and radiogenic P and Sr isotope compositions in the Easter and Salas y Gomez island lavas, in the asalt glasses of the seamounts located along the ESC, and in asalt glasses of the EMP and East Pacific Rise (EPR) rift zones [Schilling et al., 1985; Hanan and Schilling, 1989; Fontignie and Schilling, 1991; Poreda et al., 1993; Kingsley and Schilling, 1998; Cheng et al., 1999; Kingsley et al., 2002]. The ulk of these studies propose a geochemical inary mixing model comined with a physical plumeridge interaction model that explains the oserved asalt isotope and trace element chemistry in the region. The enriched end-memer of this interaction was identified as the SyG hot spot whose pure component somewhat resemles the high time-integrated U/P ratio (HIMU) mantle endmemer defined y Zindler and Hart [1986]. The exact nature of the SyG hot spot, taking into account new isotopic evidence presented here, is discussed elow. The other end-memer is defined as the depleted upper mantle with isotopic and trace element characteristics that resemle those of the most depleted asalts along the EPR. [3] The Hf, Nd, and additional Sr isotope data presented here use the same sample suite analyzed in the previous studies for P (some Sr), He, and D/H isotope ratios. We show that this new data reinforce the inary mixing hypothesis for the region. We also present parent-daughter trace element contents (i.e., Th, U, P, R, Sr, Sm, Nd, Lu, and Hf) for these asalt glasses, which emphasize the enrichment of incompatile elements in the SyG hot spot. [4] For the purposes of this study, we take the two end-memers of the radiogenic isotope arrays to signify long-term evolution of distinct parentdaughter trace element mantle domains. The characteristics of the variations in the radiogenic isotope ratio arrays, oth geographically and in Nd-Sr-Hf-P isotope space representation, reflect the mixing dynamics in the upper mantle. The difference etween the long-term time-integrated parent-daughter ratios determined from the radiogenic isotope ratios and the parent-daughter ratios actually measured in the erupted lavas likely represents mixing as well as recent petrogenetic processes, such as partial melting and fractional crystallization. We consider the entire region eneath the western ESC, the EMP, and the neary EPR ridge segments to e a system where a uoyant intraplate hot spot mantle source supplements the passive EMP-EPR spreading plate oundaries [e.g., Schilling, 1991] or, the material deficit at the EMP-EPR is drawing the SyG hot spot material to these spreading plate oundaries. In either case, mantle upwelling contriutes to decompression melting. The resulting magma erupts to form intraplate seamounts and to infuse the rift-zones of the microplate oundaries. Synchronized examination of isotopic and trace element data in light of these factors may help elucidate the origins and history of this particular hot spot and may help constrain models of upper-mantle dynamics and melting conditions. 2of28

3 Geosystems G 3 kingsley et al.: radiogenic isotopes /2006GC Figure 1. Dredge station location along the Easter Microplate (EMP) and Easter Salas y Gomez Seamount Chain (ESC) from which fresh asalt glasses were otained. See keys for meaning of color-coding used throughout this paper. EPR, East Pacific Rise. Bathymetry data compiled from Liu and Naar [1997a, 1997] and Smith and Sandwell [1994]. [5] For ackground information on the tectonics of the region, sample descriptions, locations, major element chemistry, and petrologic classifications of the asalt glasses reported on here, the reader is referred to works y Schilling et al. [1985], Sigurdsson et al. [1985] Kingsley and Schilling [1998], Pan and Batiza [1998], D. F. Naar and coworkers [Naar and Hey, 1986, 1991; Liu et al., 1993, 1994, 1995, 1996; Naar et al., 1993, 1995; Rappaport et al., 1994, 1995, 1997; Liu, 1996; Rappaport, 1996; Liu and Naar, 1997a, 1997], and Kingsley et al. [2002]. 2. Analytical Techniques [6] Hafnium isotope ratios were measured at the Ecole Normale Supérieure in Lyon on a VG Plasma 54 multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Strontium and Nd isotope ratio measurements were carried out at the Université de Genève on a Finnigan 262 multi-collector thermal ionization mass spectrometer (MC-TIMS). The chemical separations of Hf, Nd, and Sr from the ulk rocks were performed at the Graduate School of Oceanography, University of Rhode Island (GSO-URI) clean laoratory. Hf separation and mass spectrometric analysis followed the procedures descried y Blichert-Toft et al. [1997], while Sr and Nd separation and mass spectrometric analysis followed those reported previously y Schilling et al. [1992]. Tale 1 lists the results of the new radiogenic isotope ratio measurements. Standardization, normalization, and mass-fractionation corrections are 3of28

4 Tale 1. Sr, Nd, and Hf Isotope Ratios From the ESC and the EMP a Sample Latitude, S Longitude, W Median Depth of Recovery, m 87 Sr/ 86 Sr 143 Nd/ 144 Nd 176 Hf/ 177 Hf 2s.e Salas y Gomez Region GL-07 D30-1g GL-07 D31-2g GL-07 D32-4g GL-07 D34-1g GL-07 D34-3g GL-07 D GL-07 D35-7L GL-07 D36-1g GL-07 D36-3gL GL-07 D GL-07 D37-2g GL-07 D38-5g GL-07 D39-1g Y Y Y GL-07 D42-1g GL-07 D42-3g GL-07 D43-1g GL-07 D43-2g GL-07 D47-2g GL-07 D47-4g GL-07 D45-1g GL-07 D45-2g GL-07 D46-2g GL-07 D46-3g GL-07 D48-4g GL-07 D48-5g GL-07 D49-1g GL-07 D49-8g Easter Island Region GL-07 D57-2g GL-07 D57-3g GL-07 D58-2g GL-07 D59-2g GL-07 D59-3g GL-07 D60-1g GL-07 D60-3g GL-07 D61-2g EA P3BA EA P Ahu Volcanic Region GL-07 D50-1ag GL-07 D50-1g GL-07 D51-1g GL-07 D51-3g GL-07 D55-1g GL-07 D56-2g GL-07 D56-3g GL-07 D52-1g GL-07 D52-5g GL-07 D53-2g GL-07 D53-5g Easter Microplate East Rift EN113 10D-1g EN113 11D-1Ag of28

5 Tale 1. (continued) Sample Latitude, S Longitude, W Median Depth of Recovery, m 87 Sr/ 86 Sr 143 Nd/ 144 Nd 176 Hf/ 177 Hf 2s.e EN113 11D-1Bg EN113 12D-1g EN113 13D-1g EN113 13D EN113 14D-1g EN113 2D-2g EN113 3D-1g EN113 41D-1g EN113 42D-1g EN113 43D-2g EN113 44D-5g EN113 4D EN113 4D-1Ag EN113 5D-1Ag EN113 5D-1Cg EN113 6D-1Ag EN113 6D-1Bg EN113 7D-1g EN113 8D-1g EN113 9D-1Ag PA04-11-CA PA04-11-CB PA04-12-A EN113 15D-1g EN113 16D-1g EN113 17D-1g EN113 39D-2g EN113 40D-1g Easter Microplate West Rift EN113 20D-1g EN113 21D-1g EN113 22D-1g EN113 23D-1g EN113 24D-1Ag EN113 24D-1Bg EN113 25D-1g EN113 26D-1g EN113 27D-1g EN113 28D-1Ag EN113 29D-1g EN113 30D-1g EN113 30D-2g EN113 31D-1g EN113 32D-1g EN113 33D East Pacific Rise North and South of the EMP EN113 34D-1g EN113 35D-1g EN113 36D-1g EN113 37D-1Ag EN113 38D-11g EN113 1D-2Ag EN113 1D-2Bg EN113 45D-1Ag EN113 46D-2g of28

6 Tale 1. (continued) Sample Latitude, S Longitude, W Median Depth of Recovery, m 87 Sr/ 86 Sr 143 Nd/ 144 Nd 176 Hf/ 177 Hf 2s.e EN113 47D-2Ag EN113 47D-2Bg EN113 48D-1g a Sr, Nd, and Hf chemical separations carried out at URI. Hf isotopic compositions were measured y MC-ICP-MS (the Lyon VG Plasma-54). 176 Hf/ 177 Hf was normalized for mass fractionation relative to 179 Hf/ 177 Hf = Hf/ 177 Hf of the JMC-475 Hf standard = ± Hf standard was run every second or third sample to monitor machine performance. Uncertainties reported on Hf measured isotope ratios are in-run 2s/square root of n analytical errors in last decimal place, where n is the numer of measured isotopic ratios. The Sr isotope ratios measured in Geneva in static mode are mass fractionation corrected to an 88 Sr/ 86 Sr of and normalized to the Eimer and Amend (E&A) standard with 87 Sr/ 86 Sr = During the two periods of measurement (respectively of aout 13 and 8 months), the E&A standards were ± (2s from the mean, n = 74) and ± (2s from the mean, n = 21). The 143 Nd/ 144 Nd ratios were measured in static mode and are mass fractionation corrected to 146 Nd/ 144 Nd = and normalized to the La Jolla Nd standard = During the two periods of measurement (of aout 10 and 8 months, respectively), the Nd La Jolla standard analyses were very constant, and the general mean was ± (2s from the mean, n = 100). The 87 Sr/ 86 Sr values from the EMP are given y Fontignie and Schilling [1991]. given in the footnotes to Tale 1. Tale 2 lists the parent-daughter trace element concentrations determined y isotope dilution and internal and external standardization using solution-ased analyses on an inductively coupled plasma mass spectrometer (HR-ICP-MS, Finnigan ELEMENT) at GSO-URI. Details of the method and its precision can e found in the footnotes to Tale 2 and are given y Kingsley [2002]. 3. Results and Spatial Variations [7] The Hf, Nd, Sr, and P radiogenic isotopes show longitudinal gradients from the SyG region to the west rift of the EMP (Figure 2). Values of the P isotope ratios and 87 Sr/ 86 Sr decrease gradually from the SyG hot spot to the EMP west rift, while values of 176 Hf/ 177 Hf and 143 Nd/ 144 Nd increase. The parent-daughter trace element ratio trends from SyG to the EMP west rift are similar to the isotope ratio gradients (Figures 3a and 3) ut show greater relative variations along the gradients. In fact, any ratio of a trace element of higher incompatiility to one of lower incompatiility measured in these asalts shows decreasing values from SyG to the EMP west rift. In contrast to the smooth eastwest geochemical and isotopic trends of the ESC, the variations along the north-south trending axis of the EMP east rift show minima in 176 Hf/ 177 Hf and 143 Nd/ 144 Nd and a maximum in 87 Sr/ 86 Sr (Sr isotope data previously reported y Fontignie and Schilling [1991]) occurring etween 26 S and 27.5 S, which coincides with the closest point on the EMP to the SyG hot spot (Figure 4). [8] Isotopic and trace element data otained y other laoratories on asalts from the ESC and EMP [Haase and Devey, 1996; Haase et al., 1996; Hékinian et al., 1996; Cheng et al., 1999; Haase, 2002] fall mostly within the range of data otained from our laoratories (Figures 3a and 3). Exceptions are one location on the west rift of the EMP analyzed y Haase [2002], which exhiits significantly higher R/Sr and U/P ratios, and a few offridge samples near Easter Island with high Lu/Hf ratios (Figures 3a and 3). [9] The first-order spatial isotopic and trace element gradients were to e expected, ased on the previously cited geochemical models for the region. In our discussion we will use the new isotopic data to reinforce the inary mixing hypothesis. We then use this hypothesis to infer the extent of partial melting that produced the asalts, and will also explore how the SyG hot spot fits with the regional mantle heterogeneities in the SE Pacific. 4. Discussion 4.1. Geochemical Gradients [10] The directions of the isotopic gradients are consistent with mixing of a progressively smaller amount of the enriched end-memer with distance across the region from SyG to the EMP. This likely reflects the higher time-integrated parent-daughter elemental ratios of R/Sr, U/P, and Th/P, and lower ratios of Lu/Hf and Sm/Nd in the hot spot source (SyG) than in the depleted asthenospheric source (e.g., eneath the EMP west rift). The variations of these geochemical indicators as a function of distance (gradients) are fully consistent with the down-channel mixing hypothesis descried in the mantle plume source-migrating ridge sink (MPS-MRS) model of J.-G. Schilling and coworkers 6of28

7 Tale 2. MgO and Parent-Daughter Trace Element Data for Basalts From the ESC and EMP a Sample Rock Type MgO, wt% R, Sr, Nd, Sm, Lu, Hf, P, Th, U, Salas y Gomez Region GL-07 D30-1g Alkali B GL-07 D31-2g Alkali B GL-07 D32-2g Alkali B GL-07 D32-4g Alkali B GL-07 D34-1g Alkali B GL-07 D34-3g Alkali B GL-07 D35-7 Alkali B GL-07 D36-1g Alkali B GL-07 D37-2 Alkali B GL-07 D37-2g Alkali B GL-07 D38-5g Alkali B GL-07 D39-1g Alkali B Y Alkali B Y Alkali B Y Alkali B GL-07 D42-1g Alkali B GL-07 D42-3g Alkali B GL-07 D43-2g Alkali B GL-07 D45-1g Alkali B GL-07 D45-2g Alkali B GL-07 D46-2g Alkali B GL-07 D46-3g Alkali B GL-07 D48-2g Alkali B GL-07 D48-4g Alkali B GL-07 D48-5g Alkali B GL-07 D43-1g Tholeiite GL-07 D43-4g Tholeiite GL-07 D47-2g Tholeiite GL-07 D47-4g Tholeiite GL-07 D49-1g Tholeiite GL-07 D49-8g Tholeiite Easter Island Region GL-07 D57-2g Tholeiite GL-07 D57-3g Tholeiite GL-07 D58-2g Tholeiite GL-07 D59-2g Tholeiite GL-07 D59-3g Tholeiite GL-07 D60-1g Tholeiite GL-07 D60-3g Tholeiite GL-07 D61-2g Tholeiite EA P3BA Tholeiite EA P4 Tholeiite EA P7A Tholeiite Ahu Volcanic Region GL-07 D50-1ag Tholeiite GL-07 D51-1g Tholeiite GL-07 D51-3g Tholeiite GL-07 D51-5g Tholeiite GL-07 D55-1g Tholeiite GL-07 D56-2g Tholeiite GL-07 D56-3g Tholeiite GL-07 D52-1g Tholeiite GL-07 D52-4g Tholeiite GL-07 D52-5g Tholeiite GL-07 D53-1g Tholeiite GL-07 D53-2g Tholeiite GL-07 D53-5g Tholeiite of28

8 Tale 2. (continued) Sample Rock Type MgO, wt% R, Sr, Nd, Sm, Lu, Hf, P, Th, U, Easter Microplate East Rift EN113 10D-1g Tholeiite EN113 11D-10g Tholeiite EN113 11D-1Ag Tholeiite EN113 12D-1g Tholeiite EN113 13D-10g Tholeiite EN113 13D-1g Tholeiite EN113 13D-4 Tholeiite EN113 14D-1g Tholeiite EN113 14D-10g Tholeiite EN113 1D-2Ag Tholeiite EN113 2D-2g Tholeiite EN113 3D-10g Tholeiite EN113 3D-1g Tholeiite EN113 41D-1g Tholeiite EN113 42D-1g Tholeiite EN113 43D-1g Tholeiite EN113 43D-2g Tholeiite EN113 44D-5g Tholeiite EN113 45D-1Ag Tholeiite EN113 4D-1Ag Tholeiite EN113 5D-10g Tholeiite EN113 5D-1Ag Tholeiite EN113 6D-1Ag Tholeiite EN113 7D-1g Tholeiite EN113 8D-13g Tholeiite EN113 8D-1g Tholeiite EN113 9D-1Ag Tholeiite PA04-11-CA Tholeiite PA04-12-A Tholeiite EN113 15D-1g Tholeiite EN113 16D-1g Tholeiite EN113 17D-10g Tholeiite EN113 17D-1g Tholeiite EN113 39D-2g Tholeiite EN113 40D-1g Tholeiite Easter Microplate West Rift EN113 20D-1g Tholeiite EN113 21D-1g Tholeiite EN113 22D-10g Tholeiite EN113 22D-1g Tholeiite EN113 23D-1g Tholeiite EN113 24D-1Ag Tholeiite EN113 25D-1g Tholeiite EN113 26D-10g Tholeiite EN113 26D-1g Tholeiite EN113 27D-1g Tholeiite EN113 28D-1Ag Tholeiite EN113 29D-1g Tholeiite EN113 30D-10g Tholeiite EN113 30D-1g Tholeiite EN113 30D-2g Tholeiite EN113 31D-10g Tholeiite EN113 31D-1g Tholeiite EN113 32D-1g Tholeiite EN113 33D-1 Tholeiite East Pacific Rise North and South of the EMP EN113 34D-1g Tholeiite EN113 35D-10g Tholeiite of28

9 Tale 2. (continued) Sample Rock Type MgO, wt% R, Sr, Nd, Sm, Lu, Hf, P, Th, U, EN113 35D-1g Tholeiite EN113 36D-1g Tholeiite EN113 37D-1Ag Tholeiite EN113 38D-11g Tholeiite EN113 46D-2g Tholeiite EN113 47D-2Ag Tholeiite EN113 48D-1g Tholeiite a MgO from Pan and Batiza [1998] for the ESC and from Sigurdsson et al. [1985] for the EMP. R, Hf, and Lu y internal and external standardization ICP-MS. Sr, Nd, Sm, P, Th, and U analyzed y isotope dilution ICP-MS. Precisions of the trace element analyses are estimated from the percent relative standard deviation of repeated (n = 20) measurements of an in-house MORB standard (EN026 10D-3) and are 2.3%, 0.9%, 1.2%, 1.5%, 1.7%, 2.5%, 2.3%, 1.2%, and 2.7% for R, Sr, Nd, Sm, Lu, Hf, P, Th, and U, respectively. for this area [Schilling et al., 1985; Hanan and Schilling, 1989; Fontignie and Schilling, 1991; Schilling, 1991; Kingsley and Schilling, 1998; Kingsley et al., 2002]. Furthermore, due to the long half-lives of the 176 Lu- 176 Hf and 147 Sm- 143 Nd systems (37 Gy and 106 Gy, respectively), the two end-memer sources must have remained isolated from each other for at least 100 My, given reasonale parent-daughter source ratios, for the distinct isotope ratio differences to have had enough time to develop. [11] For the EMP east rift, north-south isotopic composition and parent-daughter ratio gradients (not shown) confirm that either mixing occurs along the strike of the EMP east rift, as proposed y earlier investigations [Schilling et al., 1985; Hanan and Schilling, 1989; Fontignie and Schilling, 1991; Kingsley and Schilling, 1998; Pan and Batiza, 1998] or the width of the anomaly represents the width of the channel containing the incoming hot spot material Tests of the Dynamic Binary Mixing Model [12] In this section we address whether the isotope systematics of Nd, Hf, and Sr confirm the inary mixing model suggested y P isotopes Graphic Representations in Isotope Ratio Space [13] Isotope ratio versus isotope ratio diagrams for the various systems (Figures 5 to 5g) reveal quasilinear arrays that exhiit consideraly more scatter than the P-P diagram (Figure 5a), eyond that which can e accounted for y analytical errors. Binary mixing models [e.g., Vollmer, 1976; Langmuir et al., 1978] were used to model the isotopic variations and to constrain the possile hyperolic curvature values of r, where r = n y /n x, and n y is the ratio of the concentration of the denominator element (of the y-axis variale) in the two components of the mixture (e.g., n Sr = Sr component1 /Sr component2 for the 87 Sr/ 86 Sr isotope ratio), and n x is the ratio for the x-axis variale. Thus r gives an indication of the relative concentrations of the daughter elements in the mixing end-memers, whether they e solids or melts. Assuming that the last mixing event occurred in the recent past (i.e., <100 Ma), the isotope ratios in the two modeled end-memers can e considered invariant. The end-memer isotope ratios in the model were chosen to represent values slightly eyond the data range. [14] Comparison of the mixing models and the data arrays lead to the following oservations: [15] 1. A tight linear data array is present in 208 P/ 204 P- 206 P/ 204 P space (Figure 5a), which is consistent with a inary mixing process ecause P appears in oth denominators with a resulting value of r = 1. The existence of a single linear array and the very low level of scatter in the data indicate that the P isotope ratios in the two end-memers are constant for any of the mixtures that contriuted to the formation of the sampled asalts (i.e., the two sources are homogeneous). The data array of the 207 P/ 204 P versus 206 P/ 204 P diagram (not shown) also exhiits an overall linear trend, ut with more data scatter due primarily to a greater relative analytical error on 207 P/ 204 P. However, detailed scrutiny of the P-P isotope arrays indicates that a third component, with a minor influence on the inary mixing array, may e present. This would skew the arrays to slightly lower 208 P/ 204 P and 207 P/ 204 P at a given 206 P/ 204 P in the center portions [Kingsley and Schilling, 1998]. This component would e different than any of the recognized P isotopic mantle domains such as HIMU, Enriched Mantle 1 (EM1), Enriched Mantle 2 (EM2), or LoMU (low time-integrated 9of28

10 Figure 2. Longitudinal gradients of radiogenic isotope ratios measured in asalts from the Easter Salas y Gomez Seamount Chain, the east and west rifts of the Easter Microplate, and the East Pacific Rise north and south of the microplate. Symols are color-coded on the asis of the 206 P/ 204 P values. High values (red) indicate a greater proportion of a hot spot related component relative to a depleted upper mantle component in the mixtures that produced the asalts. See Figure 1 for sample location map. Black dots are data from Cheng et al. [1999] and Haase [2002]. U/P ratio) [Zindler et al., 1982; Douglass et al., 1996] ecause the presence of these end-memers would skew data in the middle of the arrays to higher 208 P/ 204 P and 207 P/ 204 P. The radiogenic P end-memer of the mixing array is located at a higher 208 P/ 204 P and a lower 206 P/ 204 P value than HIMU (as defined y the rocks of Tuuaii and Mangaia). Thus the SyG mantle source is characterized y a higher timeintegrated Th/U ratio than the HIMU component. The ESC-EMP array passes through and eyond the C and FOZO domains, and as currently defined, are ruled out as suitale end-memers (Figure 5a). [16] 2. The data array for the P-Sr system (Figure 5) is linear, ut shows more scatter than the P-P system. The single linear array is in contrast to many multi-component OIB mixing 10 of 28

11 Figure 3a. Longitudinal gradients of parent-daughter ratios of the U-Th-P and R-Sr isotope systems measured in asalts from the Easter Salas y Gomez Seamount Chain, the east and west rifts of the Easter Microplate, and the East Pacific Rise north and south of the microplate. See Figure 1 for sample location map. Black dots are data from Haase and Devey [1996] and Haase [2002]. Ratios normalized to primitive mantle values of McDonough and Sun [1995]. systems that are easily affected y the highly variale 87 Sr/ 86 Sr values of the various mantle domains (HIMU with 87 Sr/ 86 Sr = 0.703, depleted mantle (DM) = , EM-1 = , EM-2 = 0.709). Thus mixing etween just two end-memers is suggested for the ESC-EMP. The range of r i = n P / Sr that encompasses more than 95% of the samples extends from 0.5 to 3.0, where r i is the value otained y solving the inary mixing equation for r using constant isotope ratios in the end-memers. [17] 3. The data arrays for the Nd-Sr (Figure 5f) and Hf-Nd (Figure 5e) systems are essentially linear. The scatter in the data requires r i = n Sr /n Nd and r i = n Nd /n Hf values from 0.34 to 1.9 and 0.5 to 1.9, respectively. [18] 4. The data arrays for the Hf-P (Figure 5d) and Hf-Sr (Figure 5g) systems also show linear trends encompassing r i = n P /n Hf and r i = n Sr /n Hf values from 0.45 to 1.8 and 0.25 to 1.5, respectively. 11 of 28

12 Figure 3. Longitudinal gradients of parent-daughter ratios of the Sm-Nd and Lu-Hf isotope systems measured in asalts from the ESC, EMP, and EPR north and south of the microplate. See Figure 1 for sample location map. Black dots are data from Hékinian et al. [1996], Haase and Devey [1996], and Haase [2002]. Ratios normalized to primitive mantle values of McDonough and Sun [1995]. [19] Assuming that the isotope ratios in mixing end-memers are unaffected during upper mantle magmatic processes, and that concentrations of end-memer daughter elements entering mixtures remain constant, one would expect tight curvilinear arrays to appear on isotope ratio-isotope ratio diagrams much like that oserved for the P-P diagram (Figure 5a). This is not the case, however (Figures 5 to 5g). The asolute concentrations of trace elements could e affected to varying extents y partial melting, wall-rock reactions, fractional crystallization, and hydrothermal alteration. We rule out wall-rock interaction and hydrothermal alteration on the asis of the regular regional isotopic and parent-daughter ratio gradients oserved (Figures 2, 3a and 3), the direct positive relationship etween the D/H ratio and the P isotope ratios (no significant hydrothermal alteration [Kingsley et al., 2002]), the relatively thin lithosphere [Naar et al., 1993; Mammerickx, 1981; Kingsley and Schilling, 1998], and the good correlation etween U and Th concentrations (Figure 6). Fractional crystallization will affect trace element concentrations, especially those of Sr and P, which are sequestered in plagioclase, ut this effect is mitigated in our models y normalizing to a fixed MgO level (see elow). This leaves only partial melting for consideration as a modifier in a inary mixing process. If thorough mixing occurred during or after melting, the effects of partial melting would lead to variale concentrations of daughter elements in the melts entering into a mixed melt pool, giving rise to a variety of r values and associated mixing lines. These melt packets need not e limited to two melts from separate coarsely mixed solid lithologies, ut could e numerous melts of distinct compositions, depending on the timing of their evolution from a continuous 12 of 28

13 Figure 4. Latitudinal gradients of radiogenic isotope ratios measured in asalts from the east and west rifts of the Easter Microplate (EMP), and the East Pacific Rise (EPR) north and south of the microplate. P isotope data from Kingsley and Schilling [1998]; Sr isotope data from Fontignie and Schilling [1991]. Black dots and circles are data from Haase [2002]. fractional melting regime in a two-component solid mantle. [20] If we assume partial melting is the last process to occur prior to or during the mixing event evident in the present asalt suite, then we can test whether there is enough variation in the trace elements and their ratios to justify the scatter in the isotope-ratio - isotope-ratio diagrams y calculating maximum and minimum r value ratios (e.g., n Sr /n P ) ased on first-order estimates of trace element concentrations in parental melts (i.e., concentrations corrected for effects of fractional crystallization, see elow). We used the range of trace element compositions in the samples with 206 P/ 204 P < 18.5 to represent the depleted end-memer range and that in the samples with 206 P/ 204 P > 19.6 to represent the enriched end-memer range. This allowed for the calculation of maximum and minimum elemental r values (i.e., curves laeled r e in Figure 5). These are compared with r i values that enclose >95% of the isotope ratios, which are calculated using only the isotopic ratio estimations for the end-memer components and the inary mixing equation. We note that in every case, the elemental range exceeds the isotopic range of r values. Also, the r i and r e value ranges among the various isotope systems appear well correlated (Figure 7). Therefore, to a first order, there appears to e enough variation in elemental concentrations to account for the diversity in isotope ratios, and the extent of variation oserved in the isotope ratios is related to the estimated extent of variation in the trace element concentrations Principal Component Analyses [21] The isotope ratios were processed y principal component analysis (PCA) to determine statistically if more than two mixing memers are present. For the three P isotope ratios, the three principal components were 98.5%, 1.4%, and 0.1% of the variaility, respectively. The first principal component is closely related to the data variation and direction in P-isotope space, and is due to the inary mixing of the enriched and depleted mixing memers. The scattering of the data along this axis traces perfectly the isotopic gradient and 13 of 28

14 Figure 5. Radiogenic isotope ratios measured in asalts from the Easter Salas y Gomez Seamount Chain (ESC), the east and west rifts of the Easter Microplate (EMP), and the East Pacific Rise (EPR) north and south of the microplate. (a) Note extremely tight mixing array in P isotope space is consistent with inary mixing. ( g) Overlain curves are inary mixing models with variale r values, where r is ratio of concentration ratios of denominator elements in the two sources (e.g., r = n P /n Sr, where n P = P (component2) /P (component1) ). r e (dashed gray lines) represent maximum and minimum of r values ased on estimates of elemental concentrations in primary melt endmemers. r i (solid gray lines) are calculated y the inary mixing equation and using assumed isotope ratio compositions of the end-memers. 14 of 28

15 Figure 6. The Th/U ratio is constant across the entire ESC-EMP region. See previous figures for meanings of symol types and colors. the geographical distance to the SyG hot spot. PCA, using all six isotope ratios, results in a similar distriution where the first component carries the main part of the total variaility (mixing of two end-memers) and the other components are less significant. Numerically, for 80 analyses, the proportion of the total variaility is 95.2%, 1.9%, 1.6%, 0.8%, and 0.5%, respectively. Only the primary component displays clear direction. The great weight of the first six-isotope principal component is remarkale considering the oserved scatter, which is evidently derived from variale daughter element concentrations in the mixing end-memers (see aove discussion, Figure 5, and compare these results to PCA using isotope ratios from the North Mid-Atlantic Ridge and Iceland of Blichert-Toft et al. [2005]). We find these results to e strong evidence for the dominance of only two mantle components in the mixtures oserved Tests Involving Trace Elements [22] Another test of linear inary mixing compares radiogenic isotope ratios to trace element ratios of when the denominator of the trace element ratio is the daughter element. This is the so-called pseudo-isochron diagram (radiogenic isotope ratio versus parent-daughter ratio). These diagrams confirm the inary mixing hypothesis (Figure 8), as they all show generally linear trends consistent with inary mixing. However, significant scatter occurs eyond that expected from analytical uncertainties. [23] Partial melting and fractional crystallization effects are more apparent on diagrams of trace element ratios versus trace element ratios (Figure 9). For U/P and Th/P ratios (Figure 9a), where the numerators are oth highly incompatile elements, there will e little relative change of the ratios during partial melting and fractional crystallization, thus preserving the oserved tight linear array that results from inary mixing. The other trace element ratio variations exhiit varying degrees of scatter due to the variale effects of partial melting and fractional crystallization. Vectors plotted on these diagrams readily show the scattering effects of 2% to 25% partial melting and 0% to 70% fractional crystallization. For example, the R/Sr ratio is strongly affected y fractional crystallization of plagioclase and this is manifested as high ratios in asalts resulting from magmas that have undergone a high degree of crystallization (Figure 9). [24] In summary, the inary mixing hypothesis proposed to explain the P isotope variations is confirmed y the single linear arrays oserved for the cominations of the Sr, Nd, and Hf isotope systems, y the results of the six-isotope ratio PCA, and y the single linear arrays in incompat- Figure 7. Comparison of the range of r values etween estimates ased on the isotopic ratios (r i ) and the elemental ratios (r e ) for inary mixtures represented y ESC-EMP asalts. 15 of 28

16 Figure 8. Pseudo-isochron diagrams for asalts from the ESC, EMP, and EPR north and south of the EMP. Highly fractionated samples with MgO less than 4% are not plotted. Straight line arrays on these diagrams indicate age significance or inary mixing. 16 of 28

17 Figure 9. Trace element ratio versus trace element ratio diagrams for asalts from the ESC, EMP, and EPR north and south of the EMP. Ratios normalized to primitive mantle values. Excess scatter eyond the smooth tight arrays expected for ideal inary mixing likely due to variale effects of partial melting and fractional crystallization. Vectors indicate trace element ratio ehavior during fractional partial melting ( pm using average ulk partition coefficients for melting a garnet peridotite from 2% to 25%), and fractional crystallization ( fx using average ulk partition coefficients during 0% to 70% crystallization). Ratios normalized to primitive mantle values of McDonough and Sun [1995]. ile trace element ratio space. Furthermore, the tight linear array of 208 P/ 204 P versus 206 P/ 204 P implies that the SyG hot spot solid mantle source is itself extremely homogeneous with respect to its P isotopic composition, thus favoring a homogeneous mantle plume model. It follows that the SyG source is also homogeneous with respect to Sr, Nd, and Hf isotope ratios. For this not to e the case, one would have to invoke some ad-hoc process 17 of 28

18 that would cause spatial heterogeneity in the Hf, Nd, and Sr isotope systems while retaining homogeneity in the P isotope system. The scatter in the data arrays eyond that oserved in the P-P system can e explained y the modification of the mixing system through variale amounts of the daughter elements extracted from the two mantle sources. This presumaly occurs y a magmatic process such as partial melting, with superposition of some susequent additional variation due to variale extents of fractional crystallization (especially for the low-mgo asalts) Modeling of Magma Formation and Evolution [25] Although mixing exerts the dominant control over the variation of the isotope ratios and the ratios of incompatile elements in the region, the concentrations of the incompatile elements and ratios of incompatile to slightly incompatile elements (e.g., Nd/Lu) can e significantly affected y partial melting and, to a lesser extent, fractional crystallization. The next step in this analysis is to assume that the variation in the trace element concentrations is primarily due to inary mixing and that examination of additional variation will reveal details of the melting and fractional crystallization involved in the genesis of these asalts. First, filtering the elemental concentrations to reduce the effects of variale degrees of fractional crystallization needs to e addressed Filtering of Fractional Crystallization Effects [26] To estimate the trace element contents of the parental magmas from the compositions of the erupted magmas, we used the modeling algorithm MELTS [Ghiorso and Sack, 1995] to reconstruct the fractional crystallization pathways. This algorithm was first used y Pan and Batiza [1998] to define liquid lines of descent in separate high-, intermediate-, and low-pressure regimes y fitting trends in SiO 2,Al 2 O 3, FeO, CaO, and MgO in the same ESC-EMP asalt sample suite studied here. Similar pressure regimes were used in the present model and are as follows: a high-pressure (0.45 GPa) trend for the alkali asalts; a 0.25 GPatrend for the intraplate tholeiites; and a lowpressure (10 MPa) trend for the spreading center tholeiites. These pressure regimes conform to expected lithosphere thickness, where greater pressures of fractionation would e expected in the progressively thicker oceanic lithosphere from the microplate to SyG. In the Pan and Batiza [1998] model, and in the present model, a single parental magma composition was used as the starting point at MgO = 8.5%. This starting point may not e reasonale considering that the alkali asalts were likely derived from a higher pressure melt regime than were the ridge tholeiites, thus affecting the phase proportions as they enter the melt and resulting in variale primary melt compositions. In our work, the use of the MELTS algorithm has een extended eyond the work of Pan and Batiza [1998] to estimate trace element compositions in near-parental magmas. Correction of trace element concentrations for the effects of fractional crystallization depends on two important variales: the total amount of solids removed and the types of the crystallizing phases. Where plagioclase is present as a crystallizing phase, the correction is particularly relevant for P and Sr ecause of the relatively high partition coefficients of these elements in this phase. For all three pressure trends, the MELTS calculations indicate that f (fraction of crystals removed) is a smooth function of MgO content. This function allows for the use of the MgO oserved in the glasses as an index of fractionation, which has also een used y many others [e.g., Klein and Langmuir, 1987, 1989]. In addition, the MELTS algorithm gives the amount of each phase removed and its relative proportion for any given MgO content (or f) in the oserved lavas, which provides the ulk partition coefficient at any step in the modeled crystallization sequence. [27] The trace element contents of the lavas were normalized to their expected concentrations at an MgO value of 8.5%. For asalts with MgO less than 8.5%, the actual MELTS phases and proportions were used in the calculation. For asalts with MgO greater than 8.5%, fractionation of pure olivine (Fo 90 ) was assumed. In oth cases, the correction was achieved y applying the Rayleigh fractionation equation, C 0 = C L /(1 f) (D 1), where C 0 is the concentration of the trace element at MgO = 8.5%, C L is the oserved concentration of the trace element in the asalt, f is the fraction of crystals removed (or added in the case of MgO greater than 8.5%) and is proportional to the percentage of MgO. D is the ulk partition coefficient defined as Sx i K i, where x i is the proportion of phase i and K i is the crystal/melt distriution coefficient of phase i and coexisting melt. Tale 3 lists the K values used throughout this study and their literature sources. Trace element variation diagrams (not shown) that include data prior to and after the correction show that the data scatter is 18 of 28

19 significantly reduced when the fractional crystallization correction is made. This oservation provides some confidence in this approach. Also, for the model in the next section, the few samples with MgO less than 4% have een ignored ecause this method appears to overcorrect some of these highly fractionated samples resulting in unrealistically low corrected trace element concentrations. For a fuller explanation of this method, see Kingsley [2002] Modeling Partial Melting [28] Non-linearity is evident in data arrays on diagrams of isotope ratio versus the inverse of the elemental concentration that have een corrected for fractional crystallization (Figure 10) in all four of the radiogenic systems (P, Hf, Nd, and Sr). The scatter and curvature of the data in Figure 10 cannot e explained y melting a source with a constant degree of melting and a constant solid mineral composition. The effect of partial melting under variale conditions is implicated in the alteration of the concentration in the parental magmas of these moderately incompatile daughter elements. [29] To model partial melting to explain the variation in trace element concentrations, we used a two-component mantle as a mixed solid mantle containing two materials characterized y unique isotope ratios and trace element concentrations. The scale of the heterogeneity needs to e less than that of the melt generation zone (i.e., on the order of kilometers). An accumulation of melts from this coarsely mixed solid mantle then produces the particular magmas or asalts oserved at the surface. The geochemical data from any particular asalt may e the result of mixing melts from the two separate lithologies, each with distinct ulk partition coefficients that melt to different extents (e.g., peridotite and pyroxenite). However, for simplicity, we modeled the solid mixed mantle as a single entity, and thus the melting parameters for the model should e considered as weighted averages of any distinct lithologies present in a coarsely mixed solid mantle. This simplification is done to make use of linear mixing arrays on diagrams of isotope ratio versus the inverse of the daughter element concentration that would e present if various volumes of solid mixed mantle could e sampled as a whole efore any partial melting occurs. The accumulative nonmodal fractional melting model (AFM) of Shaw [1970] was used to forward model the partial melting process. The pertinent equation is C L /C 0 = (1/F)(1 (1 PF/D 0 ) (1/P) ), where C L is the average concentration of the trace element in the mixture of melt fractions, C 0 is the original concentration of the trace element in the mantle source, F is the fraction of the solid melted, D 0 is the initial ulk solid crystal/melt partition coefficient, and P is the ulk partition coefficient for the trace element in the phases entering the melt. For simplicity, melt retention (porosity) in the melting region, melt reactions, and melt zone geometry variations are not included in the model. [30] One approach to using the forward model is to estimate F. In other words, what degree of partial melting is required for the production of any given parental magma composition that satisfies linearity in the mixed solid source (i.e., linear trends on a diagram of isotope ratio versus the inverse of daughter element concentration (Figure 11a)). A inary mixture is first simulated in the solid mantle source (as aove) and the degree of melting F is then varied to match the oserved fractional crystallization-corrected concentrations in the parental magmas of each sample. The model assumes a constant mantle mineral assemlage and D 0 for a garnet peridotite (i.e., we do not consider the possiility of separate lithologies affecting the D 0 ). The partition coefficients used and source references are given in Tale 3. Assumptions were made aout the Hf, Nd, Sr, and P isotopic and trace element compositions of two mantle solid source end-memers. For the depleted upper mantle endmemer, the Hf, Nd, Sr, and P trace element concentrations were assumed to e those recommended y the GERM dataase [Jones and Drake, 1986; Rehkamper and Hofmann, 1997; Salters and Stracke, 2004], while the isotope ratios were aritrarily assigned so that they lie just eyond the sample least radiogenic in Sr and P and most radiogenic in Nd and Hf of the entire data set (EN113 26D-1g, Figures 5a 5g). Hf, Nd, Sr, and P concentrations in the enriched SyG endmemer were set at enrichment factors (hot spot source/depleted source) of one, two, and three times the depleted upper mantle source. The isotope ratios for the SyG end-memer were set aritrarily, slightly eyond the linear trends in isotope-isotope space, as in the mixing models discussed previously. The proportions of the two end-memer components in the solid mantle were determined on the asis of the oserved Hf, Nd, Sr, and P isotope ratios. The Hf, Nd, Sr, and P concentrations in the solid source of each asalt were calculated from the linear relationship 19 of 28

20 Tale 3. Partition Coefficients and Mineral Modes Used in This Study a Element ol opx cpx (Low-Ca) cpx (High-Ca) plag gar (Low-Ca) Partition Coefficients P Nd Hf Sr Reference Matrix for Partition Coefficients P ,11 6 1,3,10 6 Nd , 5, 6, 7, 8, 15 2, 3, 12, 15 6, 7, 13, 16 Hf , 4, 5, 6, 7, 16 2, 4 6, 7, 16 Sr 9 9 2, 5, 6, 7 1, 2, 3, 17 6, 7, 16 Mineral Modes for the Accumulation Fractional Melting Model Solid Garnet Peridotite Liquid Basalt a Average values were taken where multiple references are present. References: (1) Bindeman et al. [1998]; (2) Blundy et al. [1998]; (3) Dunn and Sen [1994]; (4) Fujimaki et al. [1984]; (5) Hart and Dunn [1993]; (6) Hauri et al. [1994]; (7) Johnson [1998]; (8) Kelemen et al. [1992]; (9) Kennedy et al. [1993]; (10) Leeman [1979]; (11) Lundstrom et al. [1996]; (12) Matsui et al. [1977]; (13) Philpotts et al. [1972]; (14) Salters et al. [2002]; (15) Schnetzler and Philpotts [1970]; (16) van Westrenen et al. [2001]; (17) Villemant et al. [1981]. Calculated y applying the log relationship of Sr to Sm and Zr in high-ca cpx to Sm and Zr in low-ca cpx. predicted y inary mixing in the solid source (Figure 11a). These estimates were used as C 0 in the AFM equation, with C L as the oserved value in the asalt corrected for fractional crystallization. The extent of melting F for each sample was determined y iterating the AFM model. The results are summarized in Tales 4a and 4. Some representative trends along the ESC-EMP are shown in Figure 11. If the results of all four systems for each sample are averaged, we find that smaller F values are required for the alkali asalts at the eastern end of the ESC (averaging aout 7% near Salas y Gomez) than are required to match the more tholeiitic asalts at the western end of the intraplate region (averaging aout 9% in the Easter Island and Ahu field region). F values along the microplate oundaries were at a maximum (averaging aout 11% for the EMP east rift and aout 13% for the EMP west rift), ut were more variale than those calculated from the intraplate asalts (Figure 11). On closer inspection of the individual systems, the F values resulting from calculations on the Nd and Sr isotope systems are intermediate to the extremes of the Hf (with a few instances of unrealistically low F values) and P isotope systems (with a high value of 39% for generation of the picritic asalt EN113 26D-1). These extremes in P and Hf may reflect the possiility that either the partition coefficients are not well known for these elements, or they are more sensitive to variale values of D 0 (i.e., variale amounts of separate modal lithologies) in the solid mix. Nevertheless, for all four systems, the calculations result in trends of low degree of melting for the alkali asalts near SyG to higher degree of melting for the intraplate and rift zone tholeiites, and to still higher degree of melting for the picritic and highly depleted asalts (EN113 30D, 4D, and 26D) at aout 19%. Also, the F values ased on one system co-vary with those of any of the other systems (Figure 11f). The F variations along the ESC-EMP ased on this model appear reasonale relative to conventional models for the generation of tholeiites and alkali asalts, in that OIB alkali asalts are produced y smaller mean F at higher mean pressures than tholeiitic OIB or MORB. The regional trend of modeled F ased on the Hf, Nd, Sr, and P isotope data is the opposite of that modeled y Pan and Batiza [1998], ut reasonaly consistent with that determined y a Na (8) method [e.g., Klein and Langmuir, 1987]. However, there is little correlation in the F values among the three methods when they are compared on a sample y sample asis Nd-Hf Isotope Systematics [31] The Hf and Nd data for the ESC-EMP form a linear trend that is su-parallel to (i.e., similar 20 of 28

21 Figure 10. Isotopic ratios versus inverse of daughter element concentration corrected for effects of fractional crystallization for asalts from the ESC and EMP. On these diagrams, pure inary mixing will e reflected in straight line arrays [e.g., Langmuir et al., 1978; Vollmer, 1976]. The oserved curvilinear and dispersed data distriutions are due to processes other than inary mixing (i.e., partial melting). 21 of 28