Contributions to the Petrography and Geochronology of Volcanic Rocks from the Leeward Hawaiian Islands

Home , French Frigate Shoals, Necker Island (Hawaii), Nihoa

G. BRENT DALRYMPLE MARVIN A. LANPHERE J- U.S. Geological Survey, 345 Middlefteld Road, Menlo Park, California 94025 EVERETT D. JACKSON }

ABSTRACT the Pacific lithospheric plate moves over a fixed melting spot in the mantle, now centered near the island of Hawaii, from which magma is Petrographic and chemical analyses of basalt from Nihoa Island, supplied. Most of these hypotheses predict that the rate of propaga- Necker Island, French Frigate Shoals, and Midway Atoll, all in the tion of volcanism along the chain will be a direct function of the leeward part of the Hawaiian chain, confirm that these islands are relative motion between the Pacific plate and the asthenosphere. This subaerial remnants of tholeiitic shield volcanoes similar to those that melting-spot hypothesis recently was extended to include the Em- form the principal Hawaiian Islands. Chemistry suggests that Gard- peror Seamount chain, which on the basis of bathymetry (Chase and ner Pinnacles may be part of the alkalic cap on a tholeiitic shield. others, 1970; Jackson and others, 1972) appears to be a continuation Weighted mean potassium-argon ages of 7.0 ± 0.3 m.y. for Nihoa, of the Hawaiian chain of shield volcanoes, possibly bent northward 10.0 ± 0.4 m.y. for Necker, 11.7 ± 0.4 m.y. for French Frigate, and by a change in the direction of motion of the Pacific plate during 17.9 ± 0.6 m.y. for Midway demonstrate that the ages of these vol- Tertiary time (Morgan, 1972a, 1972b). Morgan proposed that the canoes increase northwestward, continuing the trend of increasing Hawaiian-Emperor, the Austral-Marshall, and the Line-Tuamoto age away from the active volcano of Kilauea shown by the main chains were all formed by motion of the Pacific plate over three fixed islands. The increase in age with distance along the chain, however, "plumes" in the mantle that convect upward at rates of ~2 m per appears to be nonlinear. The results support the general hypothesis yr and that supply heat to the volcanoes and provide the force that that the volcanoes of the Hawaiian chain have a common origin and drives the Pacific plate (Morgan, 1971). were formed as the Pacific plate moved northwestward over a melting McDougall (1971) suggested that the melting spot results from spot in the mantle. Key words: Hawaiian Islands, geochronology, diapiric upwelling of mantle material into a propagating fracture. potassium-argon, volcanic chain, melting spot, Pacific plate, petrog- According to his model, the melting spot is not fixed but moves at raphy, chemical analyses, basalt. about the same velocity as the Pacific plate but the opposite direction. Recent paleomagnetic data, however, indicate that Midway Atoll INTRODUCTION was formed at lat. 15° ± 4° N., which is significantly south of both its The Hawaiian Islands lie at the southeast end of a quasi-linear chain present latitude and the latitude predicted by McDougall's moving- of more than 50 shield volcanoes that trends west-northwest ~4,000 spot hypothesis, but is not significantly different from the present km across the Pacific Ocean (Fig. 1 A). Except for a few small islands latitude of Hawaii (Gromme and Vine, 1972). and atolls, most of the volcanoes west of the island of Kauai are now Shaw (1973) proposed that the Hawaiian melting spot is caused by completely below sea level. Jackson and orhers (1972) showed that viscous shear between the lithosphere and asthenosphere. His model the alignment of volcanoes in the chain is r..ot exactly linear but that includes a thermal feedback mechanism that results in episodes of individual volcanoes lie on short, sigmoidal loci that may reflect ex- melting and eruption, and cyclic irregularities in the volcanic tensional strain (Fig. IB). propagation rates that do not directly reflect the over-all rate of On the basis of the relative degrees of erosion, early workers pre- plate motion. More recently, Shaw and Jackson (1973) proposed dicted that the shield volcanoes of the Hawaiian chain increase in age that the Hawaiian melting spot is geographically stabilized by a northwestward from the active volcano of Kilauea on the island of gravitational anchor — a dense, downwelling residium that is Hawaii (Dana, 1849,1890; Stearns, 1946,1966). This prediction has formed as a result of the shear melting process. been quantitatively confirmed by K-Ar age measurements One corollary of the general melting-spot hypothesis is that the (McDougall, 1964; Funkhouser and others, 1968; Dalrymple, 1971; ages of the volcanoes in the Hawaiian-Emperor chain should in- McDougall and Swanson, 1972; Doell and Dalrymple, 1973). crease to the northwest and north. A second corollary of Wilson's Recent hypotheses for the origin of the Hawaiian chain are based (1963), Morgan's (1971), and McDougall's (1971) hypotheses is on the suggestion by Wilson (1963) that the volcanoes are formed as that all of the volcanoes in the chain should be of similar (or re-

Geological Society of America Bulletin, v. 85, p. 727-738,4 figs., May 1974

727

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Figure 1. (A) Index map of the north Pacific showing the location of the Hawaiian and Emperor chains. (B) Loci (curved lines) of shield volcanoes in the Hawaiian and southernmost Emperor chains. Crosses mark major topographic highs that are presumably coincident with major volcanic centers (after Jackson and others, 1972). Bathymetry is schematic after Chase and others (1970). (C) K-Ar age of tholeiitic volcanism relative to distance from Kilauea for the Hawaiian-Emperor chain. Circles are based on ages published by McDougall (1964) for Haleakala, West Maui, East Molokai, West Molokai, Koolau, and Kauai volcanoes; Doell and Dalrymple (1973) for Koolau and Waianae volcanoes; McDougall and Swanson (1972) for Kohala Mountain; Clague and Dalrymple (1973) for Koko Seamount; and Jackson and others (1972) for Niihau. Dots are data from Table 1.

lated) type and chemistry; the lava must also be derived from the two samples from Nihoa and Necker Islands, respectively. K-Ar ages mantle. This second condition is true for the principal Hawaiian of 15.7 ± 0.9 m.y. and 16.6 ± 0.9 m.y. on two Midway samples, Islands, and although the other islands and seamounts in the chain obtained in a prelimina ry stage of this study, were: quoted by Ladd and appear to be shield volcanoes of the Hawaiian type, petrologic and others (1967), who discounted the data because of an apparent chemical data are too few to be certain. The models of Shaw (1973) conflict with fossil evidence; this conflict is discussed below. Using a and Shaw and Jackson (1973) also include a mantle source for the variety of rock types, Clague and Dalrymple (15'73) obtained an age lava but provide more latitude for petrologic and chemical diversity of 46.4 ± 1.1 m.y. tor Koko Seamount, 300 km north of the if the mantle is laterally inhomogeneous. Thus, the geochronology Hawaiian-Emperor bend. Ozima and others (1970) reported K-Ar and petrology of individual volcanoes in the chain are a critical test ages of 41.8, 40.4, and 21.2 m.y. on three volcanic rocks from Suiko of the general hypothesis that the Hawaiian-Emperor chain is Seamount, 800 km north of the bend. According to their description, caused by movement of the Pacific plate over a melting spot in the however, the samples are badly altered and the ages probably are at mantle and also are important in deciding between several pro- best minimum values for the age of the seamount. posed mechanisms. Petrologic data on the islands and seamounts leeward of Kauai are Jackson and others (1972) haveshown that the westward increase equally scarce. Palmer (1927) collected and desc :ibed flows and dikes in age of volcanoes of the main Hawaiian Islands is not linear. The rate of basalt from Nihoa and Necker Islands and La Perouse Pinnacle on of tholeiitic volcanism along the two most recent loci, which include French Frigate Shoals and concluded that these islands were rem- all of the volcanoes from Kauai to Kilauea, has been accelerating. nants of larger basaltic volcanoes. Chemical analyses (Washington Beyond Kauai, few age data are available. Funkhouser and others and Keyes, 1926; Palmer, 1927) and petrographic examination (1968) reported K-Ar ages of 7.5 ± 0.4 m.y. and 11.3 ± 0.6 m.y. on (Palmer, 1927; Macdonald, 1949) indicate that the samples col-

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y Tanager Peak (- Millers Peak 23° ' Ail 3d 1 Z^' /1 Shark Boy \ ! (C Annexation Peal^^^y—S \ \L — ' \ 23° / 34' 30' „ Summit Hill /

\ \ ^0.5 KM \ 0 0.5 KM 23? N 1 1 03) - J OCf B I 1

I66°30 I66°20' 166° 10 I66°00 177°25 177° 20 1 1 1 i FRENCH FRIGATE MIDWAY ATOLL s SHOALS V>s s f Tern lsland -^~~^ V !,'V'P" < ' ' 1 ! -.y—x < v \ S 23° - i / \ 50 1 i \ / i \ \ \ I i \ 28° \ ' \ ^ vJ" \\ 15' i! ; v ->,/\East Island V, \ ¡\ La Perouse » I'Vi \\ ¡ I ] Pinnacle \ O \ \ \ \ \ Vx ) 1 S / j 1 w\ \ o / \ \ / \ V ?<> / \ \ •è'7 \ \ 1 / <\? r\ 23' ¿i*-J L ' 11 40 — ? # 1 // , 28° 0 5 10 KM 10 1 i i jyy

C 1 1 , ^ 1

Figure 2. Index maps of (A) Nihoa Island, (B) Necker Island, (C) French Frigate Shoals, and (D) Midway Atoll. Triangles show locations of analyzed samples. Detailed locality descriptions are given in the Appendix. Elevation contours (solid lines) are in feet; bathymétrie contours (dashed lines) are in fathoms.

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lected by Palmer are mainly tholeiitic. Macdonald (1969) studied 12 to 15 km at its top. Volcanic rocks are not exposed on Midway, but the petrology of basalt cores, from Midway Atoll and concluded two holes, one on Sand Island and one near the northe rn reef (Fig. 2D), that the Midway lavas are i:holeiitic basalt similar to Hawaiian were drilled through the coral cap into basalt in 1965 (Ladd and subaerial flows. others, 1967,1970). Basalt was found from 157 to 173 mintheSand This paper reports new K-Ar and petrochemical data on basalt Island hole and from 384 to 504 m in the reef hole , both holes bot- samples from Nihoa Island, Meeker Island, French Frigate Shoals, tomed in basalt. Samples from both drill holes were examined for this Gardner Pinnacles, and Midway Atoll and relates these data to the study, but the basalt from the Sand Island hole is badly altered and recent hypotheses on the origin of the Hawaiian-Emperor chain. analytical data were obtained only on samples from the reef hole. Sample numbers in the Appendix indicate the depth [in feet) of recov- DESCRIPTION OF THE ISL ANDS AND SAMPLE LOCALITIES ery and can be directly correlated with core descript. ons by Ladd and Nihoa Island is located 750 km west-northwest of the island of others (1967,1970), fossil studies by Cole (1969) ar d Todd and Low Hawaii (Fig. IB). It is the largest of the leeward islands, with an (1970), petrologic work by Macdonald (1969), and paleomagnetic area <62 hectares (155 acres) and a maximum elevation of 273 m investigations by Gramme and Vine (1972). at Millers Peak (Fig. 2A). Nihoa is the subaerial remnant of a large seamount that is ~90 km wide at its base and has a flat top ~25 PETROGRAPHY km in diameter and is 50 to 60 m below sea level. The island is The purpose of the petrographic and chemical studies was to entirely volcanic and is bounded by steep cliffs on all but the south confirm that these volcanoes are tholeiitic shields of the Hawaiian side. The lavas of Nihoa dip 5° to 10° southwest, which led Palmer type and to be certain that potassium-argon ages were determined on (1927) to conclude that Nihoa is part of the southwest quadrant of the freshest possible basalt of the shield-building stage. the original subaerial cone. Numerous dikes on the island trend Seventy-nine samples oi: basalt were available for study, including northeast-southwest and have nearly vertical dips; their abundance 17 from Nihoa, 10 from Necker, 10 from French Frigate, 12 from suggests that the present remnant is not far from a rift zone of the Gardner, and 30 from Midway volcanoes. Fifty-eight of the less original volcano. In 1968, samples for paleomagnetic study (Doell, weathered samples were sectioned and examined to determine their 1972), age dating, and chemistry were collected from 16 flows and general suitability for K-Ar dating and chemical analysis. Of these, 20 one dike, all exposed in the small canyon that runs from Adams were selected for more detailed petrographic examination (Table 1). Bay to the top of Millers Peak (for Iocs, see Fig. 2A; for description Thirteen of the fresher rocks were chemically analyzed (Table 2). see App.). The sampled flows and dike on Nihoa Island an; tholeiitic basalt Necker Island, which lies ~ 1,000 km from the island of Hawaii, is which is petrographically similar to that of the well-studied islands in the highest point on a large, roughly elliptical seamount ~90 by 110 the southeastpart of the chain (Macdonald, 1949,1 968; Wright and km at its base. The seamount has a flat top 30 by 75 km, ~ 30 m below Fiske, 1971; Wright, 1971). The rocks range from porphyritic to sea level. The island has a total area of 17 hectares (41 acres), a aphyric. Phenocrysts and microphenocrysts are chromite-bearing maximum elevation of 82 m at Summit Hill, and a peculiar elongate olivine, although small glomeroporphyritic clots of olivine and of shape (Fig. 2B). It is volcanic and almost completely bounded by cliffs. olivine and clinopyroxene are widespread but not abundant. The lava flows dip 5° to 10° to the north-northwest and are cut by Groundmass textures are intergranular to diabasic, although a few dikes that trend west-northwest, which suggests that the island is near rocks have intersertal textures. All the flows studied are vesicular. As a a northwest-southeast rift zone. In 1968,10 la>a-flows were sampled group, the rocks are quite fresh. The olivine phenocrysts are uni- from the west and east ends of the island. (Fig. 2B; App.). The results formly rimmed by very thin zones of iddingsite, which may partly of paleomagnetic investigations on the Necker samples were reported represent alteration during cooling of the flows (Macdonald, 1949). by Doell (1972). Intersertal glass in some rocks is perfectly fresh, but in others it is French Frigate Shoals, 1,100 km west of the island of Hawaii, is a locally altered to an opaque, isotropic substance, particularly around crescent-shaped coral atoll that occupies the eastern half of a sea- vesicles. Vesicles in a few samples are partially filled with zeolites; in mount ~40 km in diameter at its top and 75 km wide at its base (Fig. one sample, brown clay m nerals were observed on a few vesicle walls. 2C).The atoll includes 15 to 16 small sand islets, the largest of which, Six of the least altered rocks of the suite were selected for chemical and Tern Island, is only slightly over 4 hectares (11 acres) in area. The only geochronologic analysis; petrographic features of these rocks are exposures of volcanic rock on French Frigate Shoals are La Perouse summarized in Table 1. With only minor exceptions, the six flows on Pinnacle, a rock roughly 150 rn by 25 m by 37 m high, and an unnamed which age data were obtained meet the usual criteria for acceptability nearby rock 30 m by 12 m by 3 m high. La Perouse Pinnacle consists of for K-Ar dating (Mankinen and Dalrymple, 1972). The amount of a stack of lava flows that dip 1 ° to 2° to the northwest. Ten samples for glass and the degree of alteration in these flows is so minor that any this study were obtained from the east and the west ends of La Perouse adverse effects on the calculated ages are probably negligible. Pinnacle in 1971 by David L. Olsen of the U.S. Bureau of Sport Two Nihoa samples were chemically analyzed (Table 2). Both Fisheries and Wildlife (for loc. see App.). rocks contain amounts of Fe203 in excess of that in historic Mauna Gardner Pinnacles are two adjacent rocks 1,300 km northwest of Loa and Kilauea lavas (Wright, 1971), but not ir. excess of that in the island of Hawaii. Their total area is only ~3 hectares (8 acres), and analyzed tholeiites from other shield volcanoes ir. the southeastern they reach a maximum height of 52 m above the sea. These rocks are part of the chain. Part of the Fe203 is no doubt formed by oxidation the westernmost subaerial exposures of volcanic rocks in the during cooling, and part is in the iddingsite that rims olivine crystals. + Hawaiian chain and project upward from a huge elliptical seamount The combined water (H2Q ) in both samples is high, considering the with a basal diameter of ~ 100 by 200 km and a fl at top 90 by 30 km small amount of hydrous alteration that can be observed in thin sec- that lies ~35 m below sea level. According to Palmer (1927), the lavas tions of these samples. Tie analyses are recalculated to 100 percent of Gardner Pinnacles dip 15" to the west and are cut by several east- dry weight, and "FeO" is total iron expressed as the sum of FeO and west dikes that dip 60° to 80° northward. Eugene Kridler (U.S. Bureau 0.9 Fe20:l, calculated before normalization of the analyses (see of Sport Fisheries and Wildlife) collected 12 samples for this study in Table 2).' 1971 (for loc. see App.). The 10 flows sampled on Necker Island are petrographically simi- Midway Atoll, 2,400 km from the island of Hawaii, is the western- lar to tholeiitic basalt from the southeastern islands, but are distinct most land in the Hawaiian chain with the exception of Kure, which is from the Nihoa flows. Two of the Necker flows contain abundant 95 km farther west. Midway consists of two principal islands and a porphyritic olivine, but phenocrysts of augite also are present. The fringing reef that lie atop a seamount ~50 to 60 km wide at its base by remainder of the flows are aphyric or contain only a small percentage

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/727/3418286/i0016-7606-85-5-727.pdf by guest on 01 October 2021 TABLE 1. PETROGRAPHIC DATA FOR ANALYZED SAMPLES

Sample Location Phenocrysts Vesicles* Groundmass* Alteration Other no. (and microphenocrysts)* work* Mi neral Percent Average Percent Average Percent Average Dominant size size texture (mn) (ram)

8G101 Nihoa Olivine 15.5 0.5 22.0 1.0 62.5 0.06 Inter- 0.03 ran iddingslte rims on olivine K-Ar Contains a few glomeropor- granular phenocrysts. A very few vesicles C.A. phyrltlc clots of olivine and have partial zeolite fillings cllnopyroxene 8G103 Nihoa Olivine 26.1 1.0 5.6 1.5 68.3 0.01 Inter- 0.02 mm Iddingslte rims on olivine K-Ar 8G103 1s fron the same granular phenocrysts (8G104) locality and flow unit as 8G104 8G126 Nihoa Olivine 1.7 0.5 0.5 0.4 97.8 0.10 Inter- Rock 1s fresh; a very small amount K-Ar Contains a few glomeropor- granular of unaltered intersertal glass 1s C.A. phyritic clots of cllnopy- present roxene Olivine 20.2 1.3 26.2 1.5 53.6 0.05 Inter- 0.03 mm 1dd1ngsite rims on olivine granular phenocrysts; a very small amount of intersertal dark-brown glass is present, perhaps slightly altered 8G140 Nihoa Olivine 27.0 0.1-3.0 23.6 2.0 49.4 0.05 Inter- Same as 8G133 K-Ar (seriate) granular 8G211 Nihoa Olivine 20.4 0.1-2.0 19.4 2.0 60.2 0.10 Diabasic 0.03 mm 1dd1ngsite rims on olivine K-Ar (seriate) phenocrysts ÔG332 Necker Olivine 27.9 2.0 12.8 2.0 57.1 0.05 Inter- 0.05 mm iddinsite rims on olivine C.A. Augi te 2.2 0.8 sertal phenocrysts; a very small amount of unaltered intersertal glass 1s present Olivine 18.6 2.0 9.3 2.0 69.4 0.02 Inter- Same as 8G332 except glass is Augite 2.7 2.5 serta! somewhat more abundant and partially devltrified 1.0 78.9 0.02 Vitrophy- Groundmass olivine completely con- C.A. ric verted to iddlngsite. Rather abundant groundmass glass partially devltrified Olivine 1.3 0.7 13.9 0.8 73.4 0.03 Inter- Olivines partially altered to 4.0% of sample consists of Pigeonite 3.1 0.5 granular brown fibrous material glomeroporphyritlc clots of Plagioclase 4.3 0.4 olivine, pyroxene, and plagioclase 8G355 Olivine 2.3 2.5 71.1 0.03 Inter- 0.05 mm Iddinsite rims on olivine 11.0% of sample consists of granular phenocrysts. Olivines partially clots like those of 8G347 altered internally to brown fibrous material. Small amounts of intersertal glass 1s devltrified LPP-E-15 La Perouse Olivine 30.0 1.5 61.6 0.07 Inter- 0.05 rims of iddlngsite on olivine K-Ar 3.0% of glomeroporphyritlc Pinnacle granular crystals C.A. clots M.O nm in diameter composed of cllnopyroxene aggregates LPP-W-20 La Perouse Olivine 22.4 1.0 13.8 3.0 62.3 0.07 Inter- Same as LPP-E-15 K-Ar 1.5% of 0.5 nm clinopyroxene Pinnacle granular C.A. aggregates LPP-W-30 La Perouse Olivine 34.9 1.0 6.1 2.0 58.5 0.09 Inter- 1.0 mm rims of iddingslte on K-Ar 0.5% of 0.5 mm cllnopyroxene Pinnacle granular olivine phenocrysts. Centers of C.A. aggregates some olivines partially altered brown fibrous material LPP-W-35 La Perouse Olivine 30.3 1.5 5.8 2.0 60.9 0.08 Inter- 0.7 mm rims of iddingslte on K-Ar 3.0% of 0.5 mm cllnopyroxene Pinnacle granular olivine phenocryst. Center of C.A. aggregates one phenocryst partially converted to a clay mineral GP-1B Gardner (Pyroxene) 3.1 0.7 Hone 78.7 0.03 Inter- Pyroxene microphenocrysts com- Modal nepheline looked for Pinnacles Plagioclase 17.5 1.5 granular pletely converted to chlorite; but not found Spinel 0.7 0.5 groundmass pyroxene appears unaltered. One chlorite vein in rock. Calcite present as patches and small veins, mostly in plagioclase R1261 Midway (Olivine) 15.9 0.3 12.6 1.0 71.5 0.05 Inter- 0.03 mm rims of Iddlngsite on K-Ar granular olivine phenocrysts. Interior of C.A. crystals completely altered to E.P. montmorillonite. Vesicles have 0.1 mm walls of alternating bands of green and brown montmorillonite. A very small amount of glass in groundmass is devitrified. Ground- mass plagioclase is slightly altered to clay mineral R1277 Midway (Olivine) 29.2 0.3 18.1 3.5 52.8 0.08 Inter- Slightly less altered than R1261, K-Ar granular but similar C.A. E.P. R1594 Midway 11.4 1.5 .6 0.03 Inter- A little olivine in groundmass K-Ar Groundmass texture shows granular completely altered to Iddingslte. E.P. local poik111t1c plagioclase, Vesicles have 0.01 mm banded mont- similar to Kllauean lava morillonite fillings. Except for lake rocks olivine, groundmass appears fresh R1600 Midway 01 ivine tr. 0.1 15.4 1.5 81.7 0.04 Inter- Same as above. Auglte micropheno- K-Ar R1602 1s same flow unit as Augite 2.9 0.8 granular crysts completely unaltered (1600, 1602) R1600 C.A. (1602) E.P. (1600, 1602)

* Average points counted = 213, maximum = 327, minirtum = 130. t K-Ar = potassium-argon age, Table 3; C.A. = chemical analysis, Table 2; E.P. = electron probe examination, see text.

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Nihoa Necker La Perouse Pinnacles Gardner Pinnacles Midway Sample 8G101* 8GI01+ 8G126* 8G126+ 8G332* 8G332+ 8G333* 8G333+ 8G334* 8G334Î LPP-E-15* LPP-E-15+ LPP-W-20* LPP-W-20+ LPP-W-30* LPP-U-30+ LPP-W-35* LPP-H-35+ GP-1B* GP-1B+ R1261* R1261+ R1277* R12771" R1602* R1602*

Sitt, 46.57 47.92 49.44 50.77 45.19 46.41 45.46 47.19 45.66 47.76 46.12 46.72 46.21 46.72 45.95 46.69 45.23 46.21 44.12 45.38 44.57 47.73 46.30 50.10 47.29 49.44

ALJOJ 12.57 12.93 14.11 14.49 9.24 9.49 11 .31 11.74 13.98 14.62 10.27 10.40 10.13 10.24 10.40 10.57 10.28 10.51 16.24 16.70 13.07 14.00 13.06 14.13 14.61 15.28

...... F»!°3 6.02 3.55 4.81 6.36 7.19 3.96 3.90 4.26 5.13 6.40 9.07 ... 9.33 ... 5.51 ...

FeO 6.34 12.10 7.65 11.13 7.87 12.53 6.93 13.13 5.50 12.52 8.19 11.90 8.37 12.01 8.10 12.12 7.02 11.89 6.75 12.87 4.16 13.19 4.34 13.78 6.21 11.68

Mgfl 9.09 9.35 6.17 6.34 17.97 18.46 12.43 12.90 7.02 7.34 16.59 16.81 17.03 17.22 16.41 16.67 15.88 16.22 5.28 5.43 8.95 9.59 7.16 7.75 6.90 7.21

CaO 10.27 10.57 10.19 10.47 8.71 8.95 9.34 9.69 10.75 11.24 9.38 9.50 9.16 9.26 9.17 9.32 9.78 10.00 11.24 11.55 7.84 8.40 7.78 8.42 9.74 10.19

Na20 1.93 1.99 2.39 2.45 1.32 1 .36 1.60 1.66 1 .99 2.08 1 .59 1 .61 1.56 1.58 1.60 1 .63 1.61 1.64 2.74 2.82 2.44 2.61 2.44 2.64 2.48 2.53

K20 .28 .29 .43 .44 .35 .36 .36 .37 .38 .40 .35 .35 .34 .34 .34 .35 .34 .35 .55 .57 .31 .33 .40 .43 .29 .30

...... H2O* 1.09 1.06 1.04 1.46 1 .76 .39 .32 .55 ... .63 1.93 ... 2.75 ... 2.61 ... 1.36 ...... H2O" 1.07 1 .12 .97 1 .39 1 .84 .28 .21 ... .44 ... .60 ... .19 ... 2.74 ... 3.74 — 2.33 ...

Ti02 2.83 2.91 2.84 2.92 1 .99 2.04 2.56 2.66 3.18 3.33 2.15 2.18 2.13 2.15 2.13 2.16 2.14 2.18 2.71 2.79 3.27 3.50 2.12 2.29 2.68 2.80

P205 .76 .78 .46 .47 .23 .24 .42 .44 .45 .47 .31 .31 .28 .28 .30 .30 .70 .71 .36 .37 .41 .44 .23 .25 .26 .27

MnO .16 .16 .15 .15 .15 .15 .16 .17 .16 .17 .17 .17 .19 .19 .17 .17 .17 .17 .24 .25 .12 .13 .16 .17 .15 .16 ...... C02 .00 .01 .01 .01 .01 .01 .01 .01 .01 .02 .02 .02 .02 .00 .03 .03 1.22 1.25 .07 .07 .02 .02 .08 .08 ...... S03 .98 1.01 .35 .36 ...... 00 ...

S — — — — ,03 .03 .08 .08 .07 .07 .03 .03 .01 .01 .04 ¡04 JO JO .02 .02 .02 ¡02 .03 ¡03 — .

Subtotal — ...... 99.88 ... 99.87 ... 99.94 ... 99.80 ... 99.86 ... 99.86 ... 99.64 ... 99.99 ... 99.79 ... 99.72 — 99.89 ...

Less zero ...... 02 ... .04 ... .04 ... .02 ... .01 ... .02 ... .05 ... .02 ... .01 ... .02 — .10 ...

Total 99.96 100.01 99.92 100.00 99.86 100.03 99.83 100.04 99.00 100.01 99.78 100.00 99.85 100.02 99.84 100.02 99.59 100.01 99.98 100.00 99.78 100.01 99.70 100.01 99.89 100.00

...... O 8.18 1.29 6.05 2.06 .40 5.32 — - .38 3.04 ... 7.12 ... 4.37

or 1.66 1.71 2.60 2.60 2.07 2.13 2.19 2.19 2.30 2.36 2.07 2.07 2.01 2.01 2.01 2.07 2.01 2.07 3.25 3.37 1.89 1.95 2.48 2.54 1.77 1 .77

ab 10.02 10.22 20.48 20.73 11 .26 11 .50 13.78 14.04 17.17 17.60 13.54 13.62 13.28 13.37 13.62 13.79 13.79 13.89 23.27 23.25 21.31 22.03 21.49 22.34 21.49 21.93

an 28.55 29.00 26.81 27.24 18.48 18.72 22.93 23.48 28.63 29.37 19.94 20.12 19.69 19.84 20.31 20.49 20.00 20.27 30.43 31.21 24.51 25.51 24.49 25.43 28.59 29.17

ne — ...... 34 ...... — ......

th 1.76 1.79 — ...... ac ::: :::

•0 7 ». 7.66 8.86 9.01 9.88 10.04 8.87 9.03 9.48 9.71 10.31 10.38 10.01 10.08 9.82 9.93 10.10 10.23 6.42 6.59 5.16 5.36 5.86 6.09 7.79 7.97

en 22.90 23.28 15.55 15.79 24.55 13.98 31.44 16.26 17.82 13.34 21.38 13.04 2i. ot 13.33 21.05 15 77 23.91 12.46 13.18 2.72 22.97 12.79 18.58 14.80 17.61 14.01

fs 2.32 17.71 6.07 15. 16 4.07 6.04 3.43 10.06 ... 12.89 4.38 5.77 4.54 5.85 4.34 5.82 3.16 5.66 3.03 3.91 ... 9.98 ... 16.69 2.// 1 j. 34

fo — ...... 14.51 22.41 ... 11.11 ... 3.46 14.11 20.20 14.75 20.71 13.43 20.14 11.23 19.58 ... 7.57 ... 7.77 ... 3.16 ... 2.77

fa — ...... 2.65 10.67 ... 7.57 ... 3.67 3.19 9.84 3.43 10.02 2.92 10.12 1.64 9.79 ... 11 .99 ... 6.69 ... 3.93 ... 2.91

itit 8.83 ... 5.21 ... 7.05 ... 9.36 ... 8.95 ... 5.77 ... 5.68 ... 6.22 ... 7.51 — 9.30 ... 4.38 ... 8.61 ... 8.19 ...

hm — ...... 1.15 ...... 6.33 ... 3.78 ......

11 5.43 5.53 5.45 5.55 3.82 3.87 4.94 5.05 6.15 6.32 4.10 4.14 4.06 4.08 4.06 4.10 4.10 4.14 5.17 5.30 6.40 6.65 4.20 4.35 5.22 5.32

ap 1.82 1.85 1.11 1.11 .55 .57 1.02 1 .04 1 .09 1.11 .73 .73 .66 .66 .71 .71 1.68 1.68 .85 .88 .99 1.04 .57 .59 .64 .64

pr — ... .£5 .67 .06 .06 .15 .15 .13 .13 .06 .06 .02 .02 .08 .08 .19 .19 .04 .04 .04 .04 .06 .06 ......

cc — — .02 .02 .02 .02 .02 .02 .02 .02 .05. .05 .05 .05 — — ¡07 .07 7.7R 2.84 .16 .16 ¡05 ¡05 J8 J8

Total 98.96 100.04 98.87 99.94 98.97 100.01 98.53 100.00 98.21 99.96 99.63 100.02 99.70 100.02 99.47 100.02 99.39 100.03 98.10 100.01 97.18 100.02 97.29 100.03 98.62 100.01

* Analyses as received; analyst George E. Riddle, project leader Lee C. Peck, U.S. Geological Survey,

t Analyses dry reduced normalized.

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- ^ — 12 "FeO" 2 0 TiO, 1.0 —

0 1 I fffe^'' I il L •o 1.0 T k2o 1 1 32 v x sk. v \ x x x»I J I r- v 5.0 — 28

— 24 4.0

20 3.0 ] No.O o <9 16 AI2O3 2.0 12 1.0

i i i i 24 1 T T Axxrwv i i i i L 20 — 56 1 16 — .W. I r i 52 Co 0 12 1 48 SiO, 8 44 4 — 40 wvKww I J L J L 50 40 30 20 10 0 50 40 30 20 10 MgO MgO

Figure 3. MgO variation diagrams of Hawaiian basalts. Basic data from Wright (1971). OL = Kilauea and Mauna Loa olivine compositions; OPX = orthopyroxene compositions; CPX = clinopyroxene compositions; and PL = plagioclase compositions. Solid lines are historic Mauna Loa olivine-controlled lavas; long dashed lines are prehistoric Kilauea lavas; and short dashed lines are 1959 Kilauea lavas. Dry reduced analyses from Table 2 are plotted as points. Circles represent lavas from Nihoa, squares Necker, point-up triangles from La Perouse Pinnacle, point-down triangles from Gardner Pinnacles, stars from Midway. Oxide contents are in weight percent.

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TABLE 3. ANALYTICAL DATA AN'I POTASSIUM-ARGON AGES ON VOLCANIC ROC CS FROM of phenocrysts; in these, olivine is accompanied by either augite, NIHOA ISLAND, NECKER ISLAND, FRENCH FRIGATE SHOALS, AND MIDWAY ATOLL pigeonite, or plagioclase. In the aphyric rocks, glomeroporphyritic clots composed of olivine, clinopyroxene, and, dominantly, plagio- We i g'lt Ar j Sample no. ft rad Calculated age* clase are abundant. These clots appear similar to those in tholeiitic (gm;) rad («t. í) Ar1*0 (10« yr) lava erupted at Kilauea in 1967—1968 (Kinoshita and others, 1969) (no I/gm) tota) and 1823, 1868, and 1919-1920 (Wright, 1971) and are probably SIBOA ISLAMD _1: autolithic. Groundmass textures range from vitrophyric to inter- 8GI0I 0.311, 0.321 2.1(82 * 10 (5.3 i 0.3)

granular and, as a group, the rocks contain more glass than do 2.031 8GIOA 0.183, 0.186 1.986 7.5 t 0.3 those from Nihoa, although in all samples the glass constitutes less 2.129

than a few percent of the rock. Olivine phenocrysts are uniforrrly ri2 20 3.917 rimmed by iddingsite, and the interiors of many olivine crystals are 8GI26 0.403, 0.408 9 83 4.07» 6.7 ± 0.2 [10.1(2 4.154 partly replaced by an unidentified brown fibrous material. Some in- Í20. II 0.615 tersertal glass is fresh, but most is devitrified to exceedingly fine '8GI33 0.170, 0.175 4.9 ± 5.0 crystalline aggregates with low birefringence. The alteration of 8GI35 O.232, 0.249 21.25 3-222 (9.0 1 I.I)

3.325 groundmass glass to a dark isotropic substance in the vicinity of 8G140 O.322. 0.314 7.1 1 0.6 3.369 vesicles is somewhat more marked than in Nihoa rocks. Five of the "8G210 0.346, O.345 2.169 (4.2 1 0.1) least altered rocks were selected for analysis (Table 1). Of the two samples dated, only 8G355 contains interstitial glass, but the 8G21 0.405, 0.408 2.073 (3-5 i 0.1) .8G212 O.338, O.334 amount is so small (< 0.5 percent) that the age should not be ad- 2.292 (».5 t 0.1) versely affected; both rocks are very fresh except for partial altera- Best weighted meal 7.0 t 0.3 tion of the olivine phenocrysts. Three additional samples from the ÎŒCXEB ISLAND

Necker collection were chemically analyzed (Table 2). 8.82 0.596, 0.561 8.68 10.3 ± 0.5 The 10 samples from La Perouse Pinnacle contain abundant chromite-bearing olivine phenocrysts, and some are technically pic- 8G355 0.590, 0.623 9.8 ± 0.4

rites (or oceanites). Most of the rocks also contain glomero- Weighted mean - 10.0 t 0.4 porphyritic clots of clinopyroxene, which are especially common in samples from the west side of the pinnacle. Groundmass textures are u mnuss prnucix ífrsks trigíts skoals) 8.35 4.616 > LPP-E-15 0.320, O.322 11.1 i 1.0 intergranular in samples from the west side but generally intersertal to 7.60 5.93

vitrophyric on the east. All the rocks are vesicular, a few highly so. All 11.11 538 LPP-W-20 0.321. 0.334 11.1 ± 0.4 olivine phenocrysts are rimmed by iddingsite. Most olivine, in addi- < 8.96 5.36 32.2j 10.22 6.44 LPP-W-30 0.333. 0.334 12.9 ± 0.3 tion, contains internal veins of a fibrous brown material; in a few 9.38 6.33

samples, internal parts of olivine grains are altered to clay minerals. In 4.508 13.6 10.1 i 0.4 the more altered samples, some vesicles are partly filled with banded LPP-V-35 0.303. 0.308 4.626 16.6 (

montmorillonite. Zeolite and aragonite fill vesicles in some rocks. Weighted « Again, only the freshest rocks in the collection were choser for MTDWAÏ ATOLL analysis (Table 1). Except for some alteration of olivine, the four Í 7.45 8.09 40.0 R-1261 0.284, 0.293 18.9 i 0.6 dated samples are fresh and devoid of any petrographic features that 1 7.512 7.98 47.2 ' 7.48 6.09 12.2 might invalidate their K-A rage. These same four samples were chemi- R-I277 0.385, 0.386 10.8 ± 0.6 7.58 6.24 16.1 cally analyzed (Table 2). 6.66 5.72 41.3 'R-1594-A 0.244, 0.248 16.7 t 1.7 ] 9.45 6.47 37.7 The 12 samples collected from Gardner Pinnacles are, unfortu- > 18.: 10.17 6.65 28.6 nately, badly altered. All but two of the samples are vesicular; samples R-I594-B O.25O, 0.255 18. 16.; 12.81 6.77 I9.6 rocks by microphenocrysts of pyroxene and, less commonly, olivine. 9.94 6.38 34.9 R-1602 0.274, O.278 15.9 ± 1.0 Plagioclase phenocrysts in most rocks are partly replaced by cdcire; 8.86 6.66 7-5

pyroxene microphenocrysts in most rocks are completely replaced by Best weighted mean • 17.9 ± 0.6 chlorite. Alteration of groundmass minerals is variable. In one rock, they appear quite fresh; in others, the groundmass plagioclase and «Brackets enclose samples from same lava Letters A and B indie separate samples fron the pyroxene are almost completely replaced by calcite and chlorite. same depth in the Midway core. 10 1 10 1 U0 1 None of the rocks was considered suitable for potassium-argon dat- tA - 0.585 * 10* yr" , - 4.72 m !0* yr" , K /Ktotal - 1-19 » lo" * mol/mol. The ± figures are est I-rates of the analytical precision at the 68 percent confidence level (Cox and Dalrymple, ing. The freshest rock (see Tables 1 and 2) was selected for chemical 1967). Parentheses enclose data considered unreliable. See text foe discussion of weighted means analysis. and best weighted means. Rocks from two intervals from the reef drill hole (Ladd and others, 1967; Macdonald, 1969) on Midway were examined for possible chemical and geochronologic analysis. Basalts from the first interval, and 389 m (1,261 and 1,277 ft) are given in Table 1. In thin section, all between 384 and 389 m (1,261 and 1,277 ft), were characterized for of the rocks from this interval are vesicular tholeiites with abundant the most part as "partly altered" by Macdonald; basalts from the microphenocrysts of olivine in a moderately fine grained, inter- second interval, 485 and 490 m (1,591 and 1,606 ft), were charac- granular groundmass of plagioclase and clinopyroxene. Olivine terized by Macdonald as "slightly to moderately decomposed." Ac- microphenocrysts are completely altered in both rocks; iddingsite cording to Gramme and Vine (1972), the first interval contains parts rims are present, while the interiors of the crystals are altered to of three flow units; the second contains parts of two flow units. Basalts green montmorillonite. Vesicle walls have thin interior linings that in both intervals are tholeiite (Macdonald, 1969). consist of delicate alternating bands of green and brown mont- In the first interval, the freshest rocks were found at the top and morillonite. The green montmorillonite may correspond to what bottom of the subsection. Petrographic descriptions of the core at 3 84 Macdonald (1969) described as serpentine:. We were unable to

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The general range of elemental compositions is in good agreement with the analyses of tholeiites from the Leeward Hawaiian Islands reported by Washington and Keyes (1926). These workers also analyzed one alkalic rock from Nihoa, two from Necker, and one from La Perouse Pinnacle. Their one analyzed sample from Gard- ner Pinnacles is alkalic, as is ours. Macdonald's (1969) analyses of samples of the Midway core, when recalculated to a dry reduced basis, compare favorably with ours.

POTASSIUM-ARGON AGES Argon measurements were made using equipment and isotope- dilution techniques described previously (Dalrymple and Lanphere, 1969). Mass analyses were done with Reynolds-type 4.5-in. radius and Nier-type 6-in. radius rare-gas mass spectrometers. Potassium was measured by flame photometry using the lithium metaborate fusion technique (Suhr and Ingamells, 1966; Ingamells, 1970). The results on 20 samples from 16 volcanic units from Nihoa, Necker, French Frigate, and Midway volcanoes are given in Table

% Si02 3. The poor reproducibility of potassium and argon measurements Figure 4. Alkali-silica diagram for Hawaiian rocks. Dashed line separates rocks of on small samples of Hawaiian tholeiite is a general problem (Funk- the alkalic suite (above) from tholeiitic rocks (below). After Figure 1 of Macdonald houser and others, 1968; Doell and Dalrymple, 1973), and the pre- (1968). Points represent dry reduced values of analyses in Table 2. Symbols are the sent study was no exception. Potassium measurements on 100-mg same as those of Figure 3. aliquants of powdered basalt samples weighing 1 to 2 g sometimes identify serpentine in x-ray diffraction patterns of either the altered disagreed by as much as 25 percent. Where reproducibility was un- microphenocrysts or the vesicle-filling material. The groundmass satisfactory, larger samples weighing 20 to 40 g were crushed to materials of both samples are fresh, except for devitrification of — 100 mesh and carefully mixed. The precision of potassium meas- very minor amounts of intersertal glass and slight alteration of a urements on 100-mg aliquants of this material was much better, few plagioclase laths. which indicates that the difficulty of reproducibility is, in this case, Thin sections cut from samples in the second interval, at 486, caused by small-scale inhomogeneities in the rocks. 487.5, and 488 m (1,594, 1,600, and 1,602 ft, respectively), indi- Similar problems with precision occurred in some of the argon cate tholeiitic basalts that are petrographically distinct from, and measurements. The problem was especially severe for highly vesicu- less altered than, those of the first interva' (Table 1). One of these lar samples, where duplicate measurements disagreed by a factor of rocks is aphyric; the other two contain very minor amounts of mi- as much as three in one extreme case (8G133). Trace amounts of crophenocrysts of olivine and augite. Olivine microphenocrysts, biotite sometimes occur in the vesicles of Hawaiian tholeiites, and where present, are altered to iddingsite; augite microphenocrysts we suspect that this may be one contributor to the difficulties we are fresh. Vesicles in these rocks frequently have thin linings of had with these rocks. banded green and brown clay minerals. Groundmass minerals in Because of the severity of the sampling problem, we have used these rocks appear completely unaltered. Chemical analyses of reproducibility and internal consistency as primary criteria in these samples are given in Table 2. evaluating the results. The five Nihoa samples on which only single Electron microprobe examination of the five dated Midway argon measurements were made are all highly vesicular and were samples shows that the potassium in these samples is concentrated analyzed before the severity of the sampling problem for these in the fresh interstitial groundmass phases, mostly in intergranular rocks was fully appreciated. Once the problem of dating vesicular potassium feldspar. There is no detectable potassium in either the samples was recognized, we concentrated work on the more mas- iddingsite or the montmorillonite, and thus the presence of these sive basalts when possible and did not make additional measure- alteration products should not adversely affect the K-Ar ages ments on the five highly vesicular Nihoa samples. The ages for (Mankinen and Dalrymple, 1972). Two of the rocks (R-1261 and these five samples are considered unreliable (Table 3). R-1277) contain very small amounts of devitrified glass and slightly A mean age for each island (Table 3) was calculated by weighting altered plagioclase (Table 1) but probably not in sufficient quan- the mean ages of each flow according to the inverse of its estimated tities to invalidate the calculated K-Ar ages. variance. Where more than one sample from a flow was dated, the mean age of the flow was calculated by a similar weight-averaging CHEMICAL COMPOSITION procedure. The use of a mean age for an island probably is All but one (from Gardner Pinnacles) of the rocks analyzed are justifiable because Hawaiian volcanoes form rapidly when in the tholeiites within the general range of those found in the south- tholeiitic shield-building stage (McDougall, 1964; Moore, 1970; eastern Hawaiian Islands. The tholeiites fall close to Kilauea and Jackson and Wright, 1970; Swanson, 1972; Jackson and others, Mauna Loa lavas on MgO variation diagrams (Fig. 3; Wright, 1972). Weight-averaging by the inverse of the variance allows the 1971) and appear to be related by olivine control. The samples fall quality of individual age measurements to be taken into account, on the tholeiitic side of the total alkali-silica diagrams of Mac- whereas a simple arithmetic mean gives equal weight to each meas- donald (1968) for Hawaiian lavas (Fig. 4). urement regardless of its analytical uncertainty. As a group, the rocks tend to be a little poorer in silica and a little For Necker Island and French Frigate Shoals, all data were used richer in iron, titanium, and phosphorus than are the Kilauea and to obtain the weighted mean. For Nihoa Island, only data from the Mauna Loa lavas. Except for phosphorus, however, the analyses four samples on which multiple Ar measurements were made are are not outside the ranges of tholeiites from the southeastern part included in the best weighted mean. The best weighted mean age of the Hawaiian chain (Macdonald and Katsura, 1964). We sup- for Midway volcano was calculated after excluding the data from pose that the high phosphorus values are related to the fact that sample R-1277. Duplicate argon measurements on this sample are these small islets are notorious as popular resting places for birds. in good agreement, and there is no a priori reason for suspecting

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the analytical data. The calculated age of 10.8 ± 0.6 m.y., how- the principal Hawaiian Islands, and that the ages of these volcanoes ever, is considerably less than the ages of samples stratigraphically increase northwestward away from Kilauea. Gardner Pinnacles above and below, and it was excluded solely on the basis of this probably represents an alkalic capping on a tholeiitic shield, a rela- stratigraphic inconsistency. Although argon loss was immediately tion typical of many shields in the principal Hawaiian Islands. suspected, nothing in the petrography of R-1277 suggests a higher These observations sup port the concept of a common origin for the probability of argon loss than from the other Midway samp'es. volcanoes in the Hawaiian chain and are consistent with the gen- Potassium gain seems equally unlikely because the electron probe eral melting-spot hypothesis. examination shows that the alteration products in this sample con- The new age data for the Leeward Hawaiian Islands are plotted tain a negligible proportion of the potassium in the rock. The inter- as a function of distance from the active volcano of Kilauea in Fig- val from -388 to 393 m (1,274 to 1,290 ft) in the reef hole is mod- ure 1C. Published ages for nine additional volcanoes in the erately vesicular and appears to be a flow that is bounded top and Hawaiian chain and one in the southern part of the Emperor chain bottom by oxidized and clinkery zones (Macdonald, 1969; are also shown. It is immediately apparent that no single straight Gromme and Vine, 1972), which seems to eliminate the possibility line can be drawn through all of the points. Jackson and others that R-1277 is from a younger sill. A final possibility is that the age (1972) have shown that the oldest ages of tholeiitic lava on the of R-1277 is correct and the age of R-1261 is anomalously high, principal Hawaiian volcanoes, from Kilauea to Kauai, fall on a but this is unlikely because of (1) the rapidity with which sections smooth curve indicating that the rate of propagation of volcanism of Hawaiian tholeiite normally accumulate, (2) the observation along the two most recent loci has been accelerating from 5.6 m.y. that large age differences have never been found in a single section to the present. of Hawaiian tholeiite (Doell and Dalrymple, 1973), and (3) the ab- Shaw (1973) has observed that the acceleration described by sence of any evidence for a long interval of erosion between R-1277 Jackson and others (1972) was accompanied by a systematic in- and R-1594. Thus, the anomalous age of R-1277 is unexplained. crease in both the eruption rate and the volume of lava produced. Our best weighted mean age of 7.0 ± 0.3 m.y. for Nihoa Island He proposed that the relation between age, lava volume, and erup- agrees with the age of 7.5 ± 0.4 m.y. for a single basalt sample re- tion rate is caused by shear melting that incorporates thermal feed- ported by Funkhouser and others (1968). The weighted mean age back as an integral part of the mechanism According to his of 10.0 ± 0.4 m.y. for Necker Island is somewhat younger than tbe hypothesis, heat is produced by shear in a region near the base of age of 11.3 ± 0.6 m.y. measured by Funkhouser and others (1968), the lithosphere as the Pacific plate moves over the asthenosphere. which, however, is based on a single measurement on one basalt As the temperature in this region increases, the viscosity decreases, sample. In view of the difficulties in dating Hawaiian tholeiites, we and the shear rate in :urn increases until melting is initiated. The consider the age of 10.0 ± 0.4 m.y., which is based on multiple melting produces lava that carries off excess heat, the temperature measurements on two samples, more reliable. If the general drops, the viscosity increases, and the cycle is complete. Shaw pre- melting-spot hypothesis is correct, we would expect French Frigate dicted that although volcanism generally progresses east-south- to be slightly older than Necker, and the weighted mean age of 11.7 eastward along the chain, it does so in a series of episodes during ± 0.4 m.y. for French Frigate Shoals thus is consistent with the po- each of which volcanism accelerates then halts; abruptly. The ages sition of the islands in the chain. of individual volcanoes, therefore, are closely dependent on their Ages of 15.7 ± 0.9 and 16.6 ± 0.9 m.y. for depths of 485 and exact position along a volcanic locus or set of adjacent loci. Ac- 487 m (1,594 and 1,600 ft), respectively, in the reef hole on Mid- cording to Shaw (1973), the plot of ages of volcanoes along the way Atoll (Ladd and others, 1967) were measured by us during the Hawaiian-Emperor chain versus distance frorr. Kilauea will not be initial phase of this study. In their discussion of the geology of linear but instead will lie along a series of en echelon curves, each Midway, Ladd and his colleagues asserted that the samples of representing a distinct volcanic episode. basalt from the reef hole used for dating had lost argon, and they The available data Tig. 1C) strongly suggest a nonlinear rate of concluded that the measured ages were too young. This conclusion propagation of volcanism along the Hawaiian-Emperor chain be- was based on an apparent conflict between the K-Ar ages and the tween Kilauea Volcano and Koko Seamount as predicted by Shaw ages assigned to Foraminifera in sedimentary rocks that overlie the (1973). Detailed age relations are known only for the volcanoes on basalt. Larger Foraminifera (Cole, 1969) and smaller Foraminifera the adjacent pair of loci from Kauai to Kilauea, and Jackson and (Todd and Low, 1970) that are index fossils for the East Indies Ter- others (1972, Fig. 3) have shown that they def.nitely are nonlinear. tiary e stage occur below depths of 179 m in the reef hole. No The new data suggest that this nonlinear relation may hold for species of Foraminifera that are diagnostic of stages older than Ter- other loci along the chain. This is especially apparent for the tiary e occur in the reef hole between depths of 179 and 384 m (590 Niihau-Nihoa locus but is also indicated by the Midway age, which and 1,261 ft), the top of the basalt section. From these fossils, Ladd lies well below a line that passes through the other Hawaiian and others (1967) concluded that the basalt is pre-Miocene in age, points. In addition, the age of 46.4 ±1.1 m.y. for Koko Seamount, but a more realistic interpretation of the paleontological informa- 300 km north of the Hawaiian-Emperor bend, lies above any tion is that the basalt is older than part of the Tertiary e stage. In reasonable straight line through the Hawaiian data, even if the 1967, no radiometric ages had been measured on the East Indies Midway point is not considered. Tertiary stages. Recent studies have now shown that the younger Although not exp.icitly stated, the models of Wilson (1963), limit of the upper Tertiary e stage is ~15 m.y. (Page and McDougall (1971), and Morgan (1972a, 1972b) imply that the McDougall, 1970). The boundary between the upper Tertiary e rate of propagation of volcanism directly reflects the relative mo- and the overlying lower Tertiary f stages is approximately equival- tion between the melting spot and the Pacific plate. This is in con- ent to the Saucesian-Relizian boundary of western North America, trast to the models o': Shaw (1973) and Shaw and Jackson (1973), which has an age of 15.3 m.y. according to Turner (1970). Thus, at in which the volcanic propagation rate is a second-order phenome- present there seems to be no conflict between the fossil evidence non and can be used to infer plate velocity only if enough data are and the K-Ar ages for Midway, a conclusion also reached oy available to average out the nonlinear propags.tion along individual McDougall (1971) and Jackson and others (1972). loci. So far, however, reliable age data are available for volcanoes on only five loci, and two of these dated volcanoes, Midway and DISCUSSION AND CONCLUSIONS Koko, are on single loci and lie more than 1,000 km from the The results confirm that Nihoa, Necker, French Frigate, and nearest dated island or seamount (Fig. 1C). Midway volcanoes are tholeiitic shields similar to those that form Although several authors have interpreted Hawaiian age data

with linear models (Morgan, 1972a; McDougall, 1971; Jackson La Perouse Pinnacle (French Frigate Shoals): Samples collected in 1971 and others, 1972), the difficulties in doing so are apparent from the by David L. Olsen. wide range of results that can be obtained from such calculations LPP-E-15: Lava flow on east end, 5 m above sea level. LPP-W-20: Lava flow 1.5 m thick at west end, 6 m above sea level. by making only minor modifications in the data used. For example, LPP-W-30: Lava flow 2 m thick at west end, 9 m above sea level. a least-squares straight-line fit through a'.l data, from Kilauea to LPP-W-35: Lava flow 3 m thick at west end, 11m above sea level. Koko, gives an average volcanic propagation rate of 9.2 cm per yr Gardner Pinnacles: Sample GP-1B collected by E. Kridler in 1971 from and an age of 39.8 ±5.1 m.y. (95 percent confidence level) for the west side of main rock near small cove, 5 m above sea level. Hawaiian-Emperor bend. If the Koko age is excluded, the rate is Midway Atoll: Core samples R-1261, R-1277, R-1594, R-1600, and increased to 12.6 cm per yr and the bend age decreased to 29 ± 4 R-1602 were recovered during drilling of the reef hole in 1965 (Ladd and m.y.; exclusion of the Midway age decreases the average propaga- others, 1967, 1970). Sample numbers indicate depth (in feet) in hole. tion rate to 8.4 cm per yr and increases the bend age to 43.6 ± 2.3 m.y. It is clear that the distribution of available age data, the obvi- REFERENCES CITED ous nonlinearity of age as a function of distance from Kilauea, and Chase, T. E., Menard, H. W., and Mammerickx, J., 1970, Bathymetry of the theoretical work of Shaw (1973) and Shaw and Jackson (1973) the North Pacific, Charts 2, 7, 8: La Jolla, Calif., Inst. Marine Re- indicate that reliable estimates of the velocity of the Pacific plate sources, Scripps Inst. Oceanography. Clague, D. A., and Dalrymple, G. B., 1973, Age of Koko Seamount, Em- relative to the melting spot or of the age of the Hawaiian-Emperor peror Seamount chains: Earth and Planetary Sci. Letters, v. 17, p. bend cannot yet be calculated from the present age data. 411-415. It is reasonably certain that volcanoes in the vicinity of the Cole, W. S., 1969, Larger Foraminifera from deep drill holes on Midway Hawaiian-Emperor bend are probably older than Midway and Atoll: U.S. Geol. Survey Prof. Paper 680-C, 15 p. younger than Koko. Because of the proximity of the bend to Koko Cox, A., and Dalrymple, G. B., 1967, Statistical analysis of geomagnetic Seamount and the distance from the bend to Midway, the bend is reversal data and the precision of potassium-argon dating: Jour. probably only slightly younger than Koko. An age of 42 to 44 m.y. Geophys. Research, v. 72, p. 2603-2614. for the bend, as estimated by Clague and Dalrymple (1973), and an Dalrymple, G. B., 1971, Potassium-argon ages on the Pololu Volcanic average rate of 8 to 10 cm per yr for relative motion between the Series, Kohala Volcano, Hawaii: Geol. Soc. America Bull., v. 82, p. 1997-2000. Pacific plate and the Hawaiian melting spot may be reasonable Dalrymple, G. B., and Lanphere, M. A., 1969, Potassium-argon dating: San figures. Francisco, W. H. Freeman and Co., 258 p. Dana, J. D., 1849, Geology, Vol. 10 of United States exploring expedition ACKNOWLEDGMENTS during the years 1838-1839, 1840, 1841, 1842: Philadelphia, C. Sampling on Nihoa and Necker Islands by G. B. Dalrymple and Sherman, 756 p. R. R. Doell was made possible by the permission and assistance of 1890, Characteristics of volcanoes: New York, Dodd, Mead, and Co., Eugene Kridler, Wildlife Administrator of the Hawaiian Islands 399 p. National Wildlife Refuge, U.S. Bureau of Sport Fisheries and Wild- Doell, R. R., 1972, Paleomagnetism of volcanic rocks from Niihau, Nihoa, and Necker Islands, Hawaii: Jour. Geophys. Research, v. 77, p. life. The officers and crew of the U.S. Coast Guard Cutter 3725-3730. Buttonwood, Comdr. Henry Haugen commanding, provided the Doell, R. R., and Dalrymple, G. B., 1973, Potassium-argon ages and transportation to, and some exciting landings on, these islands. paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Eugene Kridler and David L. Olsen sampled Gardner and La Hawaii: Geol. Soc. America Bull., v. 84, p. 1217-1242. Perouse Pinnacles for us at considerable personal risk. G. A. Mac- Funkhouser, J. G., Barnes, I. L., and Naughton, J. J., 1968, The determina- donald and H. S. Ladd made thin sections and samples of the Mid- tion of a series of ages of Hawaiian volcanoes by the potassium-argon way basalts available to us; G. O. Riddle and L. C. Peck did the method: Pacific Sci., v. 22, p. 369-372. chemical analyses, L. B. Schlocker the potassium measurements, E. Grommé, C. S., and Vine, F. J., 1972, Paleomagnetism of Midway Atoll A. Mankinen the electron probe analyses of the Midway basalts, lavas and northward movement of the Pacific plate: Earth and Planet- and K. E. Bargar, J. C. Von Essen, S. J. Kover, and A. H. Atkinson ary Sci. Letters, v. 17, p. 159-168. Ingamells, C. O., 1970, Lithium metaborate flux in silicate analysis: Anal. assisted with sample preparation, argon measurements, and data Chim. Acta, v. 52, p. 323-334. reduction. D. A. Swanson and E. A. Silver reviewed the paper and Jackson, E. D., and Wright, T. L., 1970, Xenoliths in the Honolulu Vol- made many helpful suggestions. canic Series, Hawaii: Jour. Petrology, v. 11, p. 405-430. Jackson, E. D., Silver, E. A., and Dalrymple, G. B., 1972, The Hawaiian- APPENDIX 1. LOCALITIES OF ANALYZED SAMPLES Emperor chain and its relation to Cenozoic circumpacific tectonics: Nihoa Island: Samples collected in 1968 by R. R. Doell and G. B. Dal- Geol. Soc. America Bull., v. 83, p. 601-618. rymple in the small gully that runs north-northwest from Adams Bay to Kinoshita, W. T., Koyanagi, R. Y., Wright, T. L., and Fiske, R. S., 1969, Millers Peak (see Fig. 2A). Kilauea Volcano — The 1967-1968 summit eruption: Science, v. 166, 8G101: Lava flow 4 m below Millers Peak a': 295 m (885 ft) elev. p. 459^168. 8C104: Lava flow at 30 m (100 ft) elev. Unii 1 of Doell (1972). Ladd, H. S., Tracey, J. I., Jr., and Gross, M. G., 1967, Drilling on Midway 8G126: Dike 5 m thick at 58 m (190 ft) elev. Atoll, Hawaii: Science, v. 156, p. 1088-1094. 8G133, 135: Lava flow 3 m thick at 72 m (235 ft) elev. Unit 4 of Doell 1970, Deep drilling on Midway Atoll: U.S. Geol. Survey Prof. Paper (1972). 280-A, 22 p. 8G140: Lava flow 2 m thick at 82 m (270 ft) elev. Unit 5 of Doell (1972). Macdonald, G. A., 1949, Hawaiian pétrographie province: Geol. Soc. 8G210, 211,212: Lava flow 2 m thick at 185 m (605 ft) elev. Unit 10 of America Bull., v. 60, p. 1541-1596. Doell (1972). 1968, Composition and origin of Hawaiian lavas, in Coats, R. R., Necker Island: Samples collected by R. R. Doell and G. B. Dalrymple in Hay, R. L., and Anderson, C. A., eds., Studies in volcanology: Geol. 1968 (see Fig. 2B). Soc. America Mem. 116, p. 477-522. 8G332: Lava flow 1 m thick, 4 m below and on north side of unnamed 1969, Petrology of the basalt cores from Midway Atoll: U.S. Geol. hill on easternmost end of island, 69 m (225 ft) elev. Survey Prof. Paper 680-B, 10 p. 8G333: Lava flow 1 m thick, 4 m below 8G332. Macdonald, G. A., and Katsura, T., 1964, Chemical composition of 8G334: Lava flow 2 m thick, 7 m below 8G332. Hawaiian lavas: Jour. Petrology, v. 5, p. 82-133. 8G347: Lava flow 1 m thick, 24 m (80 ft) elev. on northwest side of is- Mankinen, E. A., and Dalrymple, G. B., 1972, Electron microprobe evalua- land, 100 m 025° from Annexation Peak. Unit 6 of Doell (1972). tion of terrestrial basalts for whole-rock K-Ar dating: Earth and 8G355: Lava flow 0.5 m thick, directly above 8G347. Unit 7 of Doell Planetary Sci. Letters, v. 17, p. 89-94. (1972). McDougall, I., 1964, Potassium-argon ages from lavas of the Hawaiian Is-

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lands: Geol. Soc. Amer ca Bull., v. 75, p. 107-128. Shaw, H. R., and Jackson, E. D., 1973, Linear island chains in the Pacific: McDougall, I., 1971, Volcanic island chains and sea floor spreading: Na- Result of thermal plumes or gravitational anchors?: Jour. Geophys. ture, v. 231, p. 141-144. Research, v. 78, p. ¡;634-8652. McDougall, I., and Swanson, D. A., 1972, Potassium-argon ages of lavas Stearns, H. T., 1946, Geology of the Hawaiian Islands: Hawaii Div. from the Hawi and Po olu Volcanic Series, Kohala Volcano, Hawaii: Hydrography Bull., v. 8, 106 p. Geol. Soc. America Bui'.., v. 83, p. 3731-3738. 1966, Geology of the State of Hawaii: Palo Alto, Pacific Books, 226 p. Moore, J. G., 1970, Relationship between subsidence and volcanic load, Suhr, N. H., and lngamells, C. O., 1966, Solution technique for analysis of Hawaii: Bull. Volcano!., v. 34, p. 562-576. silicates: Anal. Chemistry, v. 38, p. 730-734. Morgan, W. J., 1971, Convection plumes in the lower mantle: Nature, Swanson, D. A., 1972, Magma supply rate at Kilauea Volcano, v. 230, p. 42-43. 1952-1971: Science, v. 175, p. 169-170. 1972a, Plate motions and deep mantle convection, in Shagam, R., and Todd, R., and Low, D., 1970, Smaller Foraminifera from Midway drill others, eds., Studies in earth and space sciences (Hess Vol.): Geol. Soc. holes: U.S. Geol. Survey Prof. Paper 680-E, 49 p. America Mem. 132, p. 7-22. Turner, D. L., 1970, Patassium-argon dating of Pacific coast Miocene 1972b, Deep mantle convection plumes and plate motions: Am. foraminiferal stages in Bandy, O. L., ed., Radiometric dating and Assoc. Petroleum Geologists Bull., v. 56, p. 203-213. paleontologic zonation: Geol. Soc. America Spec. Paper 124, p. Ozima, M., Kaneoka, I., and Aramaki, S., 1970, K-Ar ages of submarine 91-129. basalts dredged from scamounts in the Western Pacific area and dis- Washington, H. S., and Keyes, M. G., 1926, Petrology of the Hawaiian Is- cussion of oceanic crust: Earth and Planetary Sci. Letters, v. 8, p. lands: V. The Leeward Islands: Am. Jour. Sci., 5th ser., v. 12, p. 237-244. 336-352. Page, R. W., and McDougall, I., 1970, Potassium-argon dating of the Ter- Wilson, J. T., 1963, A possible origin of the Hawaiian Islands: Canadian tiary f|_2 stage in New Guinea and its bearing on the geological time- Jour. Physics, v. 41, p. 863-870. scale: Am. Jour. Sci., v. 269, p. 321-342. Wright, T. L., 1971, Chemistry of Kilauea and Manna Loa lava in space Palmer, H. S., 1927, Geology of Kaula, Nihoa, Necker, and Gardner Is- and time: U.S. Geol. Survey Prof. Paper 735, 40 p. lands, and French Frigate Shoals: Bernice P. Bishop Museum Bull. 5 Wright, T. L., and Fiske. R. S., 1971, Origin of the differentiated and hy- (Tanager Exped. Pub. No. 4), 35 p. brid lavas of Kilauea Volcano, Hawaii: Jour. Petrology, v. 12, Shaw, H. R., 1973, Mantle convection and volcanic periodicity in the p. 1-65. Pacific; evidence from Hawaii: Geol. Soc. America Bull., v. 84, p. 1505-1526. MANUSCRIPT RECEIVED B Y THE SOCIETY SEPTEMBER 4,1973

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