Volcanism at the Edge of the Hawaiian Plume: Petrogenesis of Submarine Alkalic Lavas from the North Arch Volcanic Field

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1 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 PAGES Volcanism at the Edge of the Hawaiian Plume: Petrogenesis of Submarine Alkalic Lavas from the North Arch Volcanic Field F. A. FREY 1, D. CLAGUE 2, J. J. MAHONEY 3 AND J. M. SINTON 3 1 DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA , USA 2 MONTEREY BAY AQUARIUM, RESEARCH INSTITUTE, MOSS LANDING, CA , USA 3 SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA RECEIVED AUGUST 15, 1998; REVISED TYPESCRIPT ACCEPTED OCTOBER 28, 1999 INTRODUCTION The age progression of the volcanic chain forming the Hawaiian Ridge is consistent with the mantle plume hypothesis whereby the Pacific plate moves to the north- west over a hotspot fixed in the mantle (e.g. Clague & Dalrymple, 1987). Moreover, as the site of an individual volcano approaches, overrides and moves away from the hotspot, a Hawaiian volcano evolves through a well- known sequence of stages (e.g. Clague & Dalrymple, 1987). The bulk of a Hawaiian volcano is tholeiitic basalt erupted during the shield-building stage when the volcano overlies the hotspot, but volcano growth begins and ends with alkalic volcanism; i.e. an early alkalic stage and a late alkalic rejuvenated (post-erosional) stage (Clague & Dalrymple, 1987). In addition to this well-established temporal change in lava composition, there is also a spatial control on lava composition (Wyllie, 1988; Clague et al., 2000). Specifically, alkalic submarine lavas are erupted on the flexural arch that surrounds the recently formed islands (Lipman et al., 1989; Clague et al., 1990). This arch and an associated trough closer to the islands (Fig. 1) result from lithosphere flexure caused by the volcanic loading of the Hawaiian volcanoes (Hamilton, 1957; Moore, 1987). Dredging in the vicinity of the arch has recovered alkalic lavas from water depths of 4 5 km in front of the plume track (the >14 20 ka lavas on the South Arch; Lipman et al., 1989; Clague et al., 2000) and along the north side of the plume track (the > Ma alkalic lavas covering km 2 north Submarine lavas erupted onto the Hawaiian arch km north of Oahu show that the areal extent of Hawaiian volcanism is much larger than previously recognized. The North Arch volcanic field comprises km 2 of > Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basalt to nephelinite). These lavas are similar in composition to rejuvenated-stage lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu). North Arch lavas that encompass the compositional extremes have similar Sr, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was the major post-melting process that affected the compositions of North Arch lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, factor of four, and relatively low extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting; in contrast to the Honolulu Volcanics, there is little evidence for residual hydrous phases, sulfides or Fe Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a mixed lithosphere plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, South Pacific lithosphere was contaminated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaiian plume. KEY WORDS: Hawaiian plume; North Arch; alkalic lavas; radiogenic isotopes; igneous geochemistry Corresponding author. fafrey@mit.edu Oxford University Press 2000

2 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 Fig. 1. Location map showing GLORIA imagery of the North Arch Volcanic Field and the dredge locations (Clague et al., 1990) for the samples analyzed in this study. The volcanic field has high backscatter, which is shown as bright in this image in contrast to dark, low- backscatter sediment-covered sea floor. of Oahu, known as the North Arch field; Clague et al., 1990; Dixon et al., 1997). Age estimates for South Arch lavas are based on palagonite thickness; for North Arch lavas they are based on palagonite thickness and thickness of the overlying sediments. Although not voluminous relative to the volcanoes, the total area of alkalic volcanism discovered on the submarine arch greatly exceeds that of alkalic lavas exposed on nearby islands. The estimated Ma age range for North Arch lavas overlaps with the younger part of the age range of rejuvenated-stage lavas on Kauai (> Ma, Clague & Dalrymple, 1988) and Niihau (> Ma, Clague & Dalrymple, 1987), and with the inferred age of rejuvenated-stage lavas erupted on the Koolau shield of Oahu (<1 Ma, Clague & Frey, 1982). The major element compositions of these submarine alkalic lavas are broadly similar to those of the lavas forming the rejuvenated stages at Kauai (Koloa Volcanics), Niihau (Kiekie Basalt), and Koolau (Honolulu Volcanics) volcanoes (Lipman et al., 1989; Clague et al., 1990, 2000). These rejuvenated-stage lavas, ranging from alkalic basalt to nephelinite and melilitite, are characterized by the paradoxical features of a depleted Sr and Nd isotopic signature (i.e. long-term evolution with relatively low Rb/Sr and high Sm/Nd), with high abundances of incompatible elements, including relatively high Rb/Sr and low Sm/Nd (Clague & Frey, 1982; Stille et al., 1983; Feigenson, 1984; Roden et al., 1984; Clague & Dalrymple, 1988; Maaloe et al., 1992; Reiners & Nelson, 1998). Chen & Frey (1983, 1985) proposed that: (1) the combination of incompatible element enrichment in lavas with a depleted isotopic signature is best explained by low extents of melting of MORB (mid-ocean ridge basalt) related mantle; (2) the transition from tholeiitic shield to alkalic rejuvenated-stage volcanism is explained by a mixing model whereby the ratio of depleted ( MORB source) to plume components increases as the volcano ages; i.e. the tholeiitic shield is largely derived from plume components, but important geochemical characteristics of rejuvenated-stage lavas are controlled by melts formed by low extents of melting of depleted ( MORB-related) mantle. Several types of mixing models have been evaluated for rejuvenated stage lavas. Roden et al. (1984) favored solid melt mixing, and they suggested that the sources for the Honolulu Volcanics formed by mixing melts from a depleted wallrock ( MORB-source) with the ascending plume. Specifically, they proposed that these mixed sources contain >2 5% of a melt derived from a MORB source by >0 3% melting; the limited isotopic variability of the Honolulu Volcanics indicates that the mixing process was efficient. Subsequently, this source was melted to varying extents to create alkalic lavas ranging from alkali olivine basalt to nephelinite and melilitite. Although the Koloa Volcanics are more heterogeneous in isotopic 668

3 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE ratios than the Honolulu Volcanics (Reiners & Nelson, petrogenesis of North Arch alkalic lavas erupted 1998), very similar petrogenetic models have been proposed km north of the Koolau shield with that of the Feigenson (1984) and Clague & Dalrymple (1988) alkalic lavas erupted during the rejuvenated stage on the favored models involving mixing of melts derived from islands. the plume by relatively large extents of melting with melts derived from a MORB source by much smaller extents of melting. In a recent study of the Koloa Volcanics, Reiners & Nelson (1998) suggested that variable SAMPLING AND ANALYTICAL amounts of melts formed by 0 1% melting of a MORB PROCEDURES source infiltrate the ascending plume and form a hetero- Samples studied geneous metasomatized source. The Koloa Volcanics were subsequently formed by variable extents of melting The samples analyzed are four glasses and 23 whole that correlated with the degree of metasomatism. rocks from 15 dredge locations in the North Arch field Mixing is a process common to all of these hypotheses. ( Fig. 1). These rocks are associated with the glasses A plausible physical setting for mixing is near the contact studied by Clague et al. (1990) and Dixon et al. (1997). of an ascending plume with entrained asthenosphere or the overlying oceanic lithosphere. Maaloe et al. (1992) independently evaluated mixing models for the Koloa Whole rocks Volcanics, and concluded that a mixing model similar to Major element analyses were obtained by X-ray fluorthat of Chen & Frey (1985) could explain the geochemical escence (XRF) at the US Geological Survey and were characteristics of the Koloa Volcanics. However, they reported in table 3 of Clague et al. (1990). Abundances also proposed an alternative model whereby the alkalic of V, Ni, Cu, Zn, Rb, Sr, Ba, Y, Zr and Nb (Table 1) Koloa lavas are representative of the Hawaiian plume. were determined by XRF at the University of Hawaii This alternative model for rejuvenated-stage lavas is not following the procedures of Norrish & Chappell (1967). consistent with the presence of geochemically similar Abundances of Sc, Cr, Co, Cs, La, Ce, Nd, Sm, Eu, Tb, alkalic lavas at the periphery of the plume. Yb, Lu, Hf, Ta and Th (Table 1) were determined by Although submarine lavas erupted on the Hawaiian instrumental neutron activation analysis at the Masarch share some important characteristics with subaerial sachusetts Institute of Technology following the prorejuvenated-stage lavas erupted on the Kauai and Koolau cedures of Ila & Frey (1984). shields, there are important differences. First, the margins of submarine lavas were quenched to form glassy rinds. All of these glasses are degassed, but some with relatively high volatile (H 2 O, CO 2, S, and Cl) contents were nearly Glasses closed systems with the degassed volatiles largely retained Sr, Nd and Pb isotopic ratios were determined and in the vesicles (Dixon et al., 1997). Second, the submarine isotope-dilution measurements of Nd, Sm, Sr, Rb and arch lavas have not been affected by post-magmatic Pb abundances were made at the University of Hawaii. subaerial alteration, which leads to mobility of K and The samples analyzed were mg of glass, hand- Rb in subaerial Hawaiian lavas (e.g. Frey et al., 1994). picked with the aid of a microscope for freshness and Therefore in the submarine lavas these elements can be absence of phenocrysts. Following ultrasonic cleaning of used to evaluate the role of residual amphibole and these glasses in 2 M HCl and ultrapure water, chemical phlogopite during melt segregation (e.g. Clague & Frey, preparation and mass spectrometric procedures were 1982; Francis & Ludden, 1995; Späth et al., 1996; Class similar to those described by Mahoney et al. (1991). The & Goldstein, 1997). results are presented in Table 2 with information on Dixon et al. (1997) concluded that lavas from the North analytical uncertainties, blanks, standard reference val- Arch field evolved from parental magmas formed by ues, and the fractionation corrections employed. variable extents of melting of a compositionally homogeneous source. During ascent olivine was fractionated (maximum of 34%) and partial degassing occurred upon eruption at 400 bar. Our goal in this paper is to use Minerals olivine and whole-rock compositions and isotopic ratios Minerals were analyzed on an SEMQ electron microprobe of Sr, Nd, Pb and He in the North Arch lavas to at the US Geological Survey in Menlo Park determine the mineralogy and composition of their source using natural and synthetic standards, 15 kv accelerating and to understand the partial melting process. To understand the processes leading to alkalic volcanism at the periphery of the Hawaiian hotspot, we compare the voltage, beam current of na, and a focused beam with a diameter of >1 μm. Representative analyses are presented in Table 3a c. 669

4 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 Table 1: Measured whole-rock abundances (ppm) of trace elements in North Arch lavas Sample Flow Sc V Cr Co Ni Cu Zn Rb Sr Cs Ba Y Zr Nb La Ce Nd Sm Eu Tb Yb Lu Hf Ta Th 9-1 9D D D D D-a D-b D-b D D-b D-c D-a D D D D-b D-a D-a D-a D-a D-a D D D-b See text for analytical procedures. Analytical errors for the X-ray fluorescence analyses are slightly greater than normal, owing to special procedures utilized in the analysis of small volumes of powder; estimated absolute errors, in ppm, are V (3), Ni (2), Cu (2), Zn (2), Rb (1), Sr (2), Y (1), Zr (2), and Nb (2). See table 1d of Frey et al. (1990) for estimates of precision and accuracy of instrumental neutron activation analyses. 670

5 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE Table 2: Isotope and isotope-dilution data for glasses Sample ε Nd 143 Nd/ 144 Nd 87 Sr/ 86 Sr 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb Nd (ppm) Sm Sr Rb Pb Isotopic fractionation corrections are 148 NdO/ 144 NdO = ( 148 Nd/ 144 Nd = ) and 86 Sr/ 88 Sr = Data are reported relative to 143 Nd/ 144 Nd = for La Jolla Nd and 87 Sr/ 86 Sr = for NBS 987 Sr. Total ranges measured for these standards in a 3 year period bracketing the analyses above were ± (0 2 ε units) for La Jolla Nd and ± for NBS 987 Sr. The total ranges measured for NBS 981 Pb were ±0 012 for 206 Pb/ 204 Pb, ±0 011 for 207 Pb/ 204 Pb, and ±0 038 for 208 Pb/ 204 Pb; Pb isotopic ratios are corrected for fractionation using the NBS 981 Pb standard values of Todt et al. (1996). Within-run errors on the isotopic data are less than or equal to the external uncertainties on these standards. Total procedural blanks were <20 pg for Nd, <100 pg for Sr, and <40 pg for Pb. Uncertainties on isotope-dilution abundances are estimated at <0 2% for Nd and Sm, <0 5% for Sr, 1% for Rb, and <1% for Pb. It should be noted that ε Nd = 0 corresponds to 143 Nd/ 144 Nd = RESULTS FOR NORTH ARCH LAVAS phenocrysts are independent of Fo content ( Fig. 2a). At a given Fo content the increase of CaO in olivine from Petrography and mineral compositions alkalic basalt to nephelinite reflects the concomitant Major element abundances were determined for 345 increase in the CaO content of the melts. olivine, 55 chromite and 22 augite crystals from 34, 26 The xenocryst cores form two compositional ranges: and four samples, respectively. Most of the samples Fo and Fo 76 79, with the rims on most kink-banded contain only skeletal to euhedral microphenocrysts of and low-cao crystals having compositions similar to olivine (from <1 to 15%) that usually enclose very small those of the phenocrysts in the same sample (Fig. 2b). euhedral chromite crystals. Minor amounts of kink-ban- The most forsteritic xenocrysts (>Fo 90 ) average 0 38 wt ded olivines, usually as angular crystal fragments, occur % NiO, similar to the average olivine for lherzolite in 13 of the 36 flow units. Olivine xenocrysts are identified xenoliths from Hawaii (e.g. Goto & Yokoyama, 1988) by these kink-bands or by low CaO contents [<0 20 wt and around the world (Carter, 1970). Most of these %; see discussion by Clague & Denlinger (1994)], al- xenocrysts have cores that range from 0 01 to 0 11 wt though not all kink-banded crystals have low CaO con- % CaO ( Fig. 2b). The low-forsterite xenocrysts average tents. The olivine microphenocrysts and xenocrysts are 0 14 wt % NiO and contain wt % CaO. The usually smaller than 0 5 mm, with the largest >1 mm. high-forsterite xenocrysts are almost certainly broken The olivine phenocrysts range from Fo 80 7 to Fo 87 8 (Fo crystal fragments from lherzolitic upper-mantle rocks is forsterite content). With increasing Fo, NiO increases whereas the low-forsterite xenocrysts are probably crystal from 0 13 to 0 39 wt % and CaO decreases from 0 69 fragments from recrystallized ocean crust layer 3 gabbro, to 0 17 wt %. At a given Fo content, especially in the as described by Clague et al. (1995) for a single small range Fo 80 85, the Ni abundances of North Arch olivine xenolith in the 1960 Kapoho lava from Kilauea Volcano, phenocrysts increase as the SiO 2 content of the host glass and by Clague & Chen (1986) for some two-pyroxene increases ( Fig. 2a). This trend is continued by the higher xenoliths from Hualalai Volcano. The presence of olivine Ni contents of olivine phenocrysts in the more SiO 2 -rich xenocrysts from mantle lherzolite and ocean crust gabbro Kilauea tholeiites ( Fig. 2a). This general trend for the indicates rapid magma migration from mantle depths to NiO content of olivine, at a given Fo content, to increase eruption on the sea floor. as the SiO 2 content of the melt increases is consistent Chromite has a wide range of compositions, with Cr 2 O 3 with a thermodynamic analysis of experimental data varying from 21 8 to 36 wt %, Al 2 O 3 from 11 8 to 23 3 for Ni partitioning between olivine and silicate melt wt %, MgO from 15 7 to 10 5 wt %, FeO (i.e. all iron (Hirschmann & Ghiorso, 1994). as Fe 2+ ) from 23 8 to 44 1 wt %, mg-number [100 The inverse CaO Fo trends for each lava type (i.e. atomic Mg/( Mg + Fe) using the inferred stoichometric alkalic basalt to nephelinite as indicated by the SiO 2 FeO contents from Table 3b] from 48 6 to 69 5, and groups in Fig. 2a) reflect the dependence of the olivine TiO 2 from 1 0 to 5 2 wt %, although 50 of 55 analyzed melt partition coefficient for CaO on the activity of iron chromite crystals have TiO 2 <3 5 wt %. There are broad [see fig. 4 of Jurewicz & Watson (1988)]. However, positive correlations of mg-number and Cr 2 O 3 and Al 2 O 3 in Kilauea tholeiitic basalts CaO contents of olivine content, and broad negative correlations of mg-number 671

6 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 Table 3: Representative microprobe analyses (a) Olivine Sample Type SiO 2 MgO CaO MnO FeO NiO Total Fo 9D-3 ol2 c-xk D-3 ol2 r-xk D-1 ol1 c-p D-1 ol1 r-p D-1 ol1 c-p D-1 ol1 r-p D-13 ol1 c-p D-13 ol1 r-p D-13 ol2 c-xk D-13 ol2 r-xk D-13 ol4 c-xk D-13 ol4 r-xk D-2 ol1 c-xk D-2 ol1 r-xk D-2 ol4 c-p D-2 ol4 r-p D-1 ol1 c-xk D-1 ol1 r-xk D-4 ol4 c-x D-4 ol4 r-x D-5 ol5 c-p D-5 ol5 r-p D-a ol1 c-xk D-a ol1 r-xk D-a ol4 c-p D-a ol4 r-p D-2 ol2 c-p D-2 ol2 r-p D-2 ol2 c-p D-2 ol2 r-p (b) Chromite Sample TiO 2 Al 2 O 3 Cr 2 O 3 MnO FeO Fe 2 O 3 MgO Total mg-no. 9D-3 S D-1 S D-13 S D-1 S D-2 S D-b S D-b S D-3 S D-3 S D-6 S

7 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE (c) Augite Sample SiO 2 TiO 2 Al 2 O 3 FeO Cr 2 O 3 MnO MgO CaO Na 2 O Total mg-no. Wo 15D-1 P3C D-1 P8C D-1 P8R D-4 P1C D-6 P G P G P Type: c, core; r, rim; p, phenocryst; x, xenocryst; k, kink-banded. mg-number is 100 atomic Mg/(Mg+Fe). and Fe 2 O 3 and TiO 2. Thus, with decreasing mg-number, 3a). Both samples are from the central portion of the the ulvöspinel component of the spinel increases. There North Arch field ( Fig. 1). are broad positive correlations of mg-number in spinel The whole rocks range in MgO from 6 3 to 14 0 and mg-number of the host glass and a better correlation wt %, with 17 of 23 samples having >9 5 wt % MgO of mg-number in spinel with Fo content of the host olivine. and only three samples having <7 2 wt % MgO. The spinels are cognate, and they were the first phase Clague et al. (1990) concluded that olivine was the to crystallize. only silicate phase to crystallize from North Arch Augite occurs in basanite and alkalic basalt samples samples with >6 4 wt % MgO. Addition of 6 26 wt from four flows (15D, 18D, 36D-a, and 37D) with glass % olivine (in 1% increments of equilibrium olivine MgO contents of wt %, but is absent in ne- until the liquidus olivine is Fo 91 ) to the observed wholephelinite to alkalic basalt glasses with >6 4 wt % MgO. rock compositions leads to inferred parental magma This indicates that augite joins olivine on the liquidus in compositions having >14 18 wt % MgO, which would these alkalic lavas at >6 4 wt % MgO. Augite varies in be in equilibrium with olivine of Fo 91 [Table 4; also mg-number from 84 to 74 5, with TiO 2 ( wt %), see Dixon et al. (1997), for a similar calculation based Al 2 O 3 ( wt %), Na 2 O ( wt %) and on glass compositions]. In MgO variation plots the the Wo component ( %) increasing, and SiO 2 whole-rock compositions define scattered but inverse ( wt %) decreasing with decreasing mg-number. trends of Na 2 O, CaO, Al 2 O 3 and TiO 2 abundances vs There is no systematic relation between Cr 2 O 3 and mg- MgO content (Fig. 3a). After addition of olivine to number, nor between mg-number in augite and that in achieve a lava composition in equilibrium with Fo 91 the host glass. Many of the augite crystals are slightly olivine, these trends, including those for K 2 O and P 2 O 5 too magnesian to be in equilibrium with their host glasses vs MgO, are better defined (Fig. 3a). The Al 2 O 3 MgO using an Fe/Mg K D = 0 25 [Grove & Bryan, 1983; trend is an exception; after olivine addition the Al 2 O 3 K D = (Fe/Mg)augite/(Fe/Mg)glass with all Fe as Fe 2+ ]. content ranges from 10 3 to 11 9 wt %, but it is not correlated with MgO content ( Fig. 3a). Although in MgO major oxide variation plots the North Arch lavas generally overlap with the fields for Whole rocks major element abundances the rejuvenated-stage lavas at Kauai (Koloa Volcanics) The major element abundances of glasses and associated and Koolau (Honolulu Volcanics), there are some imwhole rocks from the North Arch were reported by portant differences (Fig. 3b). Most of the lavas in these Clague et al. (1990). All of these lavas are alkalic; e.g. the two rejuvenated stages are characterized by relatively 143 North Arch glasses analyzed by Clague et al. (1990), high, >12 wt %, MgO contents. Relative to the North representing 36 distinct flow units, range in normative Arch lavas in the range of wt % MgO, the fields nepheline from 2 1 to 23%. Among the whole rocks defined by the Koloa and Honolulu Volcanics extend to analyzed, the extremes are sample 23-6 with 41 5 wt % much lower SiO 2 contents (i.e. melilite-bearing lavas) SiO 2 and 5 2 wt % (Na 2 O + K 2 O) and sample 22-2 than the North Arch lavas (Fig. 3b). Also, the Honolulu with 47 1 wt % SiO 2 and 3 0 wt % (Na 2 O + K 2 O); Volcanics field extends to higher P 2 O 5 and Na 2 O content, these two samples also have the highest (23-6) and lowest and the Honolulu Volcanics and especially the Koloa (22-2) abundances of Na 2 O, K 2 O, TiO 2 and P 2 O 5 (Fig. Volcanics extend to higher TiO 2 contents. In contrast to 673

8 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 Fig. 2. (a) NiO and CaO vs Fo content for olivine phenocrysts. At a given Fo content, the NiO content of olivine phenocrysts in North Arch lavas increases as the SiO 2 content of the glass increases (symbols in legend indicate SiO 2 range of glasses). The difference between the groups is largest at lower forsterite content, which causes the data arrays to fan, with the largest differences at the lowest forsterite content. The NiO increase is continuous from >40 to 48% SiO 2, but to illustrate the point, least-squares-fit lines are shown for only two North Arch groups (40 42% and 46 48% SiO 2 ). Submarine tholeiitic basalts from Kilauea (Clague et al., 1995) continue this trend (enclosed and labelled field). At a given Fo content the CaO content of olivine phenocrysts in North Arch lavas decreases as the SiO 2 content of the glass increases. As in the NiO panel, this continuous change is largest at lower forsterite content. The CaO contents of olivine in all the samples converge at higher forsterite. In this case, the Kilauea tholeiites (enclosed and labelled field) do not continue this trend. (b) NiO and CaO vs Fo content, for olivine xenocrysts identified by either the presence of kink-bands or unusually low CaO contents of cores. The xenocryst cores are mostly more magnesian than the phenocrysts, but the rims are similar to the phenocryst compositions. The high-fo xenocrysts are broken fragments of upper-mantle lherzolite. Several xenocrysts have unusually low forsterite contents; they are probably crystal fragments from ocean crust rocks. these differences, which reflect differences in magma composition, the extension of the fields for the Koloa and Honolulu Volcanics to lower CaO and Na 2 O contents ( Fig. 3b) reflects the effects of post-magmatic, subaerial alteration (e.g. Clague & Frey, 1982). Whole rock trace element abundances Abundances of Cr, Ni and Co are positively correlated with MgO content whereas abundances of Zn, Sc and V are inversely correlated ( Fig. 4). The Koloa and Honolulu Volcanics also define positive trends of Ni, Cr 674

9 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE Fig. 3. Abundances of the incompatible elements Th, Ba, La, Ce, Sr and P are positively correlated in the North Arch lavas, and very similar trends are defined by the North Arch lavas and the rejuvenated-stage Koloa and Ho- nolulu Volcanics ( Fig. 5). All North Arch lavas have steep negative trends in chondrite-normalized rare-earth element (REE) plots (Fig. 6). Abundances of La range from >44 to 140 chondrites whereas Yb and Lu abundances range only from >5 0 to 7 8 chondrites. The lowest abundances of light REE (LREE), and all and Co abundances vs MgO content, but their trends show much more scatter than the North Arch data. The increased scatter of these subaerial rejuvenated-stage lavas is even more apparent in the Zn, Sc and V vs MgO trends. We suspect that the scatter of the rejuvenated- stage lavas arises from three sources: (1) post-magmatic subaerial alteration; (2) imprecise data; (3) possibly vari- able source composition, especially for the Koloa lavas, which are isotopically heterogeneous (Reiners & Nelson, 1998). 675

10 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 Fig. 3. (a) Na 2 O, K 2 O, CaO, Al 2 O 3,TiO 2 and P 2 O 5 vs MgO content (all in wt %) in submarine North Arch whole-rock lavas [Χ, as measured; Β, calculated by incremental (1%) olivine addition to be in equilibrium with Fo 91 ]. It should be noted that in the panels for Na 2 O, K 2 O, CaO, TiO 2 and P 2 O 5 there is a group of eight lavas (only six in TiO 2 panel) with higher abundances of these oxides. These are lavas from Dredges 23, 24, 26 and 27. The measured Al 2 O 3 MgO trend extrapolates to MgO 45 6 at 0% Al 2 O 3. This corresponds to olivine of Fo Ε, Φ, measured and calculated data for submarine South Arch lavas, respectively (Clague et al., 2000). (b) Na 2 O, SiO 2,CaO,Al 2 O 3,TiO 2 and P 2 O 5 vs MgO content (all in wt %) showing measured North Arch lava data relative to three suites of rejuvenated-stage lavas: Hana Volcanics [data from Chen et al. (1990)], Honolulu Volcanics [HV; data from Clague & Frey (1982)] and Koloa Volcanics [KV; data from Clague & Dalrymple (1988)]. We do not plot the KV data of Reiners & Nelson (1998) because many of their samples have lost CaO, Na 2 O and K 2 O during postmagmatic alteration; 80% of their samples have weight loss on ignition exceeding 5%; however, their data further confirm that KV lavas extend to higher TiO 2 than HV lavas. 676

11 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE Table 4: Whole-rock compositions (wt %) for North Arch lavas adjusted to be in equilibrium with Fo 91 olivine Sample Flow SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2 O P 2 O 5 oliv. % MgO(gl) MgO(WR) added 9-1 9D D D D D-a D-b D-b D D-b D-c D-a D D D D-b D-a D-a D-a D-a D-a D D D-b Exp. melts 2 5% % Measured whole-rock data including FeO and Fe 2 O 3 are given in table 3 of Clague et al. (1990). Because the measured Fe 2 O 3 /FeO ratios of these unaltered lavas correlate with abundances of incompatible elements, the measured FeO and Fe 2 O 3 were used for the olivine addition calculation (1% increments of equilibrium olivine, K D = 0 3, were added until the lava composition is in equilibrium with Fo 91 ). Fo 91 was used by Dixon et al. (1997) in a similar normalization procedure for the associated glasses. The choice of Fo 91 is not important to the conclusions of this paper. gl, glass; WR, whole rock. Exp. melts are experimental melt compositions in equilibrium with olivine, clinopyroxene, orthopyroxene and garnet. The 2 5% melt results from melting of a MORB source peridotite at 28 kbar (Longhi, 1997, and personal communication, 1998). The low TiO 2,Na 2 O and K 2 O of this primary melt reflects the depleted nature of the MORB source. The 13% melt results from melting of a more fertile peridotite at 40 kbar (Walter, 1998). 677

12 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 Fig. 4. Abundances (ppm) of the compatible trace elements Ni, Cr and Co, and slightly incompatible elements Zn, Sc and V vs MgO content (wt %) in lavas from the North Arch. The North Arch MgO Ni trend extrapolates into the observed field for olivine phenocrysts. Shown for comparison are the fields for the rejuvenated-stage Honolulu and Koloa Volcanics [data from Clague & Frey (1982), Clague & Dalrymple (1988) and unpublished Sc and Co data of F. A.. Frey (1999) for the Koloa Volcanics]. Not plotted are data for 42 additional samples of the Koloa Volcanics (Reiners & Nelson, 1998). These data define inverse trends for Sc, Zn and V, which lie along extensions of the North Arch data to higher MgO. However, for Cr and Co these Koloa Volcanics data are offset to lower abundances; these data appear to be systematically too low. other highly incompatible elements, are in sample 22-2, in highly incompatible elements, including LREE, and but this sample has relatively high heavy REE (HREE) it has the lowest HREE content. These features lead to abundances. In contrast, sample 23-6 is the most enriched crossing REE patterns ( Fig. 6). 678

13 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE Fig. 5. Abundances (ppm) of the highly incompatible elements P, La, Sr, Ba, K and Rb vs Th content in lavas from the North arch [this paper and Clague et al. (2000)]. The extremes are defined by samples 22-2 and For comparison, data are shown for the rejuvenated-stage lavas of the Honolulu Volcanics and Koloa Volcanics [data from Clague & Frey (1982), Clague & Dalrymple (1988), Maaloe et al. (1992) and Reiners & Nelson (1998)]. The scatter to low K 2 O and Rb for these rejuvenated-stage lavas reflects alkali loss during post-magmatic alteration in a subaerial environment. Glasses isotopes Isotopic ratios of Nd, Pb, and Sr were analyzed for four North Arch glasses, including the compositionally extreme samples 22-2 and 23-6 (Figs 3a, 5 and 6). These two samples define the very small range of the four glasses in 143 Nd/ 144 Nd and 206 Pb/ 204 Pb (Table 2). The glasses display a very limited range in Sr and Nd isotopes, with 87 Sr/ 86 Sr = and 143 Nd/ 144 Nd = , corresponding to ε Nd =+8 2 to As shown in Fig. 7a, like the subaerial rejuvenatedstage lavas, the submarine lavas are offset from shield- stage lavas to much lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd; i.e. they occupy a region between the field for South Pacific MORB [note that the sea floor in the Hawaiian region was formed in the South Pacific during the middle Cretaceous; e.g. Waggoner (1993) and ref- 679

14 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 lie between the fields for modern Pacific MORB and Hawaiian shield volcanoes. An important observation is that the Hawaiian alkalic suites are typically more homogeneous in isotopic ratios than the nearby shields [e.g. compare the fields for the Honolulu Volcanics and Koolau shield ( Fig. 7)]. If a plume component contributes to the source of the alkalic lavas, it is more homogeneous than the sources contributing to the tholeiitic shield lavas. The 3 He/ 4 He ratios of vesicle gases (i.e. in crushed rather than heated samples) in nine North Arch glasses Fig. 6. Chondrite-normalized REE plots for North Arch lavas. The range for 23 lavas from the North Arch is essentially defined by samples 22-2 (11 3% MgO) and 23-6 (7 8% MgO), which have crossing patterns in both the measured data (shown) and when corrected to a common MgO content by olivine addition. erences therein] and fields for the various Hawaiian shields. This offset towards the MORB field has been used to argue that depleted mantle was a component in the source of rejuvenated-stage lavas (e.g. Chen & Frey, 1985; West et al., 1987). Significantly, the 87 Sr/ 86 Sr ratios of the North Arch and rejuvenated-stage lavas are shifted to values (3 5) 10 4 higher, at ε Nd of +8 to+9, than those of modern Pacific MORB (see Basu & Faggart, 1999). The North Arch glasses have only a small range in Pb isotopic ratios (Fig. 7c). For example, 206 Pb/ 204 Pb varies only by 0 098, from to The North Arch Pb isotopic field overlaps with that of the Honolulu, Hana and Lahaina Volcanics, and it is slightly above the Pacific MORB field in Fig. 7c. In the ε Nd (and 87 Sr/ 86 Sr) vs 206 Pb/ 204 Pb diagram (Fig. 7b), the North Arch data Fig. 7. (a) ε Nd vs 87 Sr/ 86 Sr; (b) ε Nd vs 206 Pb/ 204 Pb; (c) 208 Pb/ 204 Pb vs 206 Pb/ 204 Pb. Ε, North Arch data; Β, results for two South Arch lavas; Μ, results for two lavas dredged from the channel between Kauai and Oahu (Clague et al., 2000). Error bars in inset panels are for data in this paper. Fields for Ontong Java Plateau A-type lavas (OJP-A) and Manihiki Plateau Site 317 are for present-day isotopic ratios. Data sources for the various fields are as follows. Hana: Tatsumoto et al. (1987), Chen et al. (1990); Lahaina: Tatsumoto et al. (1987); Koloa: Sun (1980), Maaloe et al. (1992), Reiners & Nelson (1998), Lassiter et al. (1998); Honolulu: Sun (1980), Stille et al. (1983), Roden et al. (1984); Kilauea: Chen et al. (1996), J. Mahoney (unpublished data, 1996); Koolau: Roden et al. (1994); South Pacific MORB (covering 0 34 S): Macdougall & Lugmair (1986), White et al. (1987), Bach et al. (1994), Mahoney et al. (1994); Ontong Java and Manihiki: Mahoney et al. (1993), Tejada et al. (1996), and references therein. Data for all fields were adjusted to standard values in Table 2. Not shown is the field for altered 110 Ma MORBs from the upper 62 m of basaltic basement at Ocean Drilling Program Site 843 south of Kauai (King et al., 1993); these rocks are characterized by higher present-day 206 Pb/ 204 Pb ( ) at similar or higher ε Nd (+8 5 to +10 3) than the North Arch lavas; like the latter, however, the Site 843 samples tend to have slightly higher 208 Pb/ 204 Pb than modern Pacific MORB. These data cannot be used to infer the isotopic characteristics of 110 Ma oceanic mantle lithosphere. 680

15 FREY et al. VOLCANISM AT HAWAIIAN PLUME EDGE Fig He/ 4 He in crushed North and South Arch glasses vs K O 2 content (wt %) of the glass. North Arch data from Dixon et al. (1997) and South Arch data from Clague et al. (2000). A significant amount of the compositional variation of North Arch lavas can be explained by olivine fractionation and accumulation. Figure 3a shows that much of the scatter in MgO variation plots is reduced to coherent trends by correcting for the effects of olivine fractionation. An alternative approach is to evaluate the effects of olivine addition, i.e. variable mixing proportions of magma and olivine of constant composition. For example, extrapolation of the linear MgO Ni trend (Fig. 4) of the lavas intersects the center of the olivine phenocryst field at 45 6 wt % MgO (Fo 85 6 ). Addition of Fo 85 6 olivine is also consistent with extrapolation of the measured inverse trend between Al 2 O 3 and MgO abundances ( Fig. 3a). Compositional control by olivine with chromite inclusions is also consistent with the positive correlations among abundances of CaO, Na 2 O, K 2 O, TiO 2 and P 2 O 5 [see Fig. 3a, and fig. 4 of Clague et al. (1990)], the positive MgO Cr trend, and the modest increases (>20 30%) of Zn, Sc and V abundances with decreasing MgO content [Fig. 4; i.e. the partition coefficient between melt and olivine with chromite inclusions is >1 for Cr but between 0 1 and 1 for Zn, Sc and V (e.g. Beattie, 1994)]. Dixon et al. (1997) concluded that at eruption conditions some of the North Arch lavas were saturated with an immiscible Fe S O liquid. Zinc (Fig. 4) and Cu abundances, although widely scattered, increase with decreasing MgO, thereby providing no evidence for control of these elements by an immiscible sulfide melt during crustal evolution. Another post-melting process that can significantly affect lava compositions is post-magmatic alteration. It range from 1 3 to 8 5 times the atmospheric ratio (Dixon et al., 1997). The two lowest ratios, 1 3 and 2 2, are in the two high-k 2 O glasses with the lowest He content (Fig. 8); thereby suggesting radiogenic ingrowth of 4 He in the vesicle gases of these samples. Ratios for the other seven North Arch glasses range from 5 3 to 8 1 times the atmospheric 3 He/ 4 He ratio. The highest value overlaps with the range ( ) of three rejuvenated-stage lavas from Haleakala (Kurz et al., 1987), and is similar to the typical value for MORB (Kurz et al., 1998). is well known that exposure in a subaerial Hawaiian However, two South Arch lavas have much higher 3 He/ environment can lead to significant compositional 4 He (Fig. 8), thereby providing evidence for a plume- changes in <1 Ma lavas (e.g. Clague & Frey, 1982; Frey derived component. DISCUSSION North Arch lavas: post-melting processes The diversity of North Arch lava compositions, alkalic basalt to nephelinite, coupled with their near homo- geneity in Sr, Nd and Pb isotopic ratios, is consistent with the hypothesis that these lavas formed by varying extents of melting of a geochemically homogeneous source. Before assessing in detail the compositional effects of different extents of melting, it is necessary to determine the effects of post-melting processes on lava compositions. The compositions and petrography of North Arch lavas clearly show that accumulation and fractionation of olivine were important post-melting processes. Although the petrographic characteristics of olivine are complex, the correlation between glass and olivine compositions shows that the olivine phenocrysts were in equilibrium with the host melt (Fig. 2). et al., 1994). Earlier, we attributed the anomalously low CaO and Na 2 O contents of some rejuvenated-stage lavas to such alteration ( Fig. 3b). Another manifestation of subaerial alteration in rejuvenated-stage lavas is the broad tendency for K 2 O/P 2 O 5 to decrease with increasing volatile content, thereby showing the relative loss of K 2 O that typically occurs during subaerial alteration of Hawaiian lavas (Fig. 9). In contrast, K 2 O/P 2 O 5 is not correlated with H 2 O content in the submarine North Arch lavas (Fig. 9). North Arch lavas: constraints on the melting process Melting reaction and range in extent of melting Important aspects of the major oxide abundance vari- ations cannot be explained by olivine control. Notably, incremental addition of equilibrium olivine to achieve lava compositions in equilibrium with Fo 91 olivine does not result in a nearly uniform composition (Fig. 3a). The 681

16 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000 and garnet. The trend of decreasing CaO/Al 2 O 3 with increasing extent of melting ( Fig. 10a) is consistent with melt segregation in the garnet stability field [see Kinzler (1997), p. 873]. However, if a compositionally similar source is assumed for the Honolulu Volcanics and North Arch lavas, it is apparent that none of the Arch lavas were derived by the very low extent of melting that created the highly SiO 2 -undersaturated and Th-rich nepheline melilitites in the Honolulu Volcanics ( Fig. 10b). Clague & Frey (1982) summarized the early experimental evidence that highly SiO 2 -undersaturated lavas, such as the Honolulu Volcanics, are generated in the presence of CO 2. Subsequent experimental studies have confirmed an important role for CO 2 in generating the compositional characteristics of alkalic lavas (e.g. Adam, 1988). Most recently, Hirose (1997) using the diamond aggregate experimental approach showed that Fig. 9. K 2 O/P 2 O 5 vs volatile content as measured by total H 2 O + with increasing extent of melting of carbonated peridotite, CO 2 content. The general trend for K 2 O/P 2 O 5 to decrease with abundance of SiO2 increases, CaO decreases and Al 2 O 3 increasing volatile content in the Honolulu and Koloa Volcanics reflects is relatively constant; i.e. producing trends similar to loss of K 2 O during subaerial post-magmatic alteration. This inverse those of the North Arch and rejuvenated-stage lavas ( Fig. trend is not present in the North Arch lavas, which have K 2 O/P 2 O 5 >1 5. Data sources are as indicated for Fig a and b). Dixon et al. (1997) estimated that the vesicular vent samples from dredges 24 and 26 contained >5 wt % CO 2, and they suggested that the mantle source choice of Fo 91 is arbitrary; normalization to other Fo contained wt % CO 2. contents does not affect our subsequent conclusions. These adjusted compositions, however, show systematic compositional variations; specifically, they define inverse correlations between MgO content and abundances of Important residual minerals Na 2 O, K 2 O, TiO 2 and P 2 O 5, with the highest abundances The compositional data for North Arch lavas provide of these typically incompatible oxides in the eight lavas constraints on the residual minerals that controlled prim- from dredges 23, 24, 26 and 27 (Fig. 3a). These eight ary magma compositions. Most notably, there is compelling lavas also have the highest CaO/Al 2 O 3, the highest evidence for garnet as an important residual abundances of CaO and incompatible trace elements, phase. First, Al 2 O 3 was not an incompatible oxide during such as Rb, Nb and Th, and the lowest SiO 2 contents partial melting. This is evident from the limited variation (<43 wt %) (Figs 3a, 5 and 10). These samples were all in Al 2 O 3 content of the lava compositions calculated to recovered from small hills, interpreted as vents, rather be in equilibrium with Fo 91 olivine; that is, the variable than from the areally extensive horizontal flows. The measured Al 2 O 3 content of the erupted lavas resulted Koloa Volcanics (Clague & Dalrymple, 1988; Maaloe et from olivine accumulation or fractionation (Fig. 3a). The al., 1992), Honolulu Volcanics (Clague & Frey, 1982) compatible behavior of Al 2 O 3 is also manifested by the and North Arch lavas show the same compositional absence of a positive correlation between Al 2 O 3 and P 2 O 5 trends; samples with the highest content of incompatible abundances in the glasses [fig. 4 of Clague et al. (1990)] elements define the low SiO 2, high CaO and high CaO/ and adjusted whole-rock compositions ( Fig. 3a). Finally, Al 2 O 3 end of the compositional spectrum (Fig. 10a and the positive correlation between Th content and CaO/ b). Al 2 O 3 (Fig. 10a) is explained because Th and CaO Given the very limited isotopic variability of the North abundances decrease with increasing extent of melting Arch lavas (Fig. 7), we assume a homogeneous source whereas Al 2 O 3 content was buffered by residual garnet composition (relative to the scale of melting) for all North (e.g. Hirose, 1997; Kinzler, 1997; Walter, 1998). Residual Arch lavas. With this assumption, the factor of 3 4 range garnet is also implied by the crossing chondrite-norin abundance of highly incompatible elements, such as malized REE patterns ( Fig. 6) and the positive trend Rb, Nb and Th, in the North Arch lavas (adjusted to be between Tb/Yb and Th content ( Fig. 10c). This inference in equilibrium with Fo 91 ) indicates approximately a factor for residual garnet based on REE abundances is consistent of four range in extent of melting. Based on the trend in Fig. 10b, melts formed at the lowest extents of melting are largely derived from SiO 2 -poor phases, such as olivine with the CaO/Al 2 O 3 variation in North Arch lavas; i.e. sample 23-6 with relatively low HREE contents, high Tb/Yb, and high CaO/Al 2 O 3 formed in equilibrium 682