20. radiometric ages of basaltic basement … ages of basaltic basement 800 " " "

Larson, R. L., Lancelot, Y., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 129

20. RADIOMETRIC AGES OF BASALTIC BASEMENT RECOVERED AT SITES 800, 801,AND 802, LEG 129, WESTERN PACIFIC OCEAN1

Malcolm S. Pringle2

ABSTRACT

ODP Leg 129 achieved one of its primary goals by recovering Jurassic-age ocean crust at Site 801 in the Pigafetta Basin, westernPacific Ocean. At Sites 800 and 802, although also located on presumed Jurassic-age crust, drilling terminated in the ubiquitousCretaceous-age volcanic rocks that seem to blanket the Early Cretaceous and Jurassic basement of the western Pacific Ocean.

Site 800 in the Pigafetta Basin penetrated 56 m of massive alkali dolerite sills. 40Ar/39Ar laser analyses of mineral separatesrevealed a crystallization age of 126.1 ± 0.6 Ma. The isotopic composition of these dolerite sills shows a strong HIMU component(high radiogenic Pb, low radiogenic Sr), nearly identical to similar age lavas recovered from nearby seamounts. Both the sillsand seamounts of the Pigafetta Basin are Early Cretaceous products of the South Pacific Isotopic and Thermal Anomaly (SOPITA),which is responsible for the ocean islands and thermal (?) swell found in the South Pacific today.

Site 801, drilled into the Jurassic Quiet Zone crust of the Pigafetta Basin, penetrated 131m of basaltic flows divided into anupper sequence of alkalic basalts, an altered hydrothermal deposit, and a lower tholeiitic sequence. For the lower tholeiitic basalts,whole rock incremental- and laser-heating experiments revealed an age of 166.8 ± 4.5 Ma, almost exactly that predicted by simplelinear extrapolation of the M-sequence magnetic lineation ages. The age and composition of the tholeiites offers important proofthat the "Quiet Zone" crust of the western Pacific is indeed Jurassic mid-ocean ridge basalt (MORB), and not a product ofmid-Cretaceous, intra-plate volcanic events.

For the upper alkalic basalts at Site 801,40Ar/39Ar mineral incremental-heating and laser-fusion experiments yielded an ageof 157.4 ± 0.5 Ma. About 10 m.y. younger than that predicted by extrapolation of the M-sequence anomalies, the alkalic basaltswere erupted in an off-ridge environment, which is consistent with both the geochemistry of the basalts and the Bathonian toCallovian radiolarian age of the overlying sediments. The 157.4 Ma age also serves as an important calibration point on theGeologic Time Scale (GTS), but further work may be needed to verify the correlation of the radiolarian zonations in the Pacificwith the time scale stages based on European sections.

Site 802 in the East Mariana Basin penetrated 51 m of basaltic pillow units and flows. Whole rock 40Ar/39Ar incremental-heating analysis of two samples revealed a crystallization age of 114.6 ± 3.2 Ma. Although the isotopic composition of thesebasalts indicates an ocean island basalt component, the major and trace element chemistry shows a strong MORB affinity. Thisis very similar to the contemporaneous basalts recovered at Site 462A in the Nauru Basin, and both seem more closely related tothe formation of Ontong Java Plateau, ca. 120 Ma, rather than to products of the SOPITA.

INTRODUCTION

The principal goal of Ocean Drilling Program (ODP) Leg 129 wasto recover the oldest rocks of the Pacific Ocean—a sedimentarysection from the Mesozoic superocean and Jurassic ocean crust fromthe western Pacific. This goal, which had eluded nine previous legsover two decades of drilling by ODP and the Deep Sea DrillingProject, was finally achieved at Site 801 in the Pigafetta Basin. Sites800 and 802 terminated in younger sill and flow complexes, productsof mid-Cretaceous volcanism which almost ubiquitously blankets theoldest portions of the western Pacific.

The purpose of this study was to determine radiometric ages for thevolcanic basement sampled at each of these sites. Before this study, noreliable radiometric ages for Jurassic or Early Cretaceous ocean crustfrom any ocean had been determined. In spite of the difficulty indetermining concordant crystallization ages for altered ocean crust,reliable ages were determined for volcanic rocks from each of the sitesstudied here. Also, we report what may be the first reliable ages fortholeiitic, mid-ocean ridge basalt (MORB) of any age recovered duringthe more than two decades of ocean floor drilling.

The success of this study is due in large part to the recent advancesin the 40Ar/39Ar geochronology of very small samples. The increasedsensitivity of the new analytical systems allows one to work withdatable mineral separates in quantities that previously would have

1 Larson, R. L., Lancelot, Y, et al., 1992. Proc. ODP, Sci. Results, 129: College Station,TX (Ocean Drilling Program).

2 U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, U.S.A.

been insufficient: 10-20 mg for plagioclase, less than 1 mg for morepotassic phases such as biotite. Also, the greatly reduced analyticaltime necessary for the very small sample techniques—20 min. vs. 3hr to 1 day per sample for previous systems—facilitates generatingsets of age data required for applying internal concordance testsdesigned to distinguish reliable ages. Furthermore, much of thesuccess in obtaining reliable results from whole rock samples can beattributed to careful petrographic selection of the least altered sam-ples with the most holocrystalline groundmass.

SITE LOCATION AND SUMMARY

Hole 800A was drilled on magnetic anomaly M33 in the northwestPigafetta Basin, 80 km to the northeast of Himu Seamount (Fig. 1).The oldest sediments recovered were earliest Cretaceous in age(Lancelot, Larson, et al., 1990), but the hole bottomed in 56 m (7 mrecovery) of clearly intrusive, massive dolerite sills. The dolerites areclearly younger than the sedimentary rocks that they intrude, and thevolcanic basement recovered at Hole 800A is probably closer in ageto the thick sequence of mid-Cretaceous volcaniclastics recoveredhigher in the section than to the presumed Jurassic crust locatedsomewhere below.

Site 801 was drilled 600 km to the southeast of Site 800, in theJurassic Quiet Zone 450 km to the southeast of the oldest magneticlineation identified in all of the ocean basins (anomaly M37). Holes80IB and 801C represent a composite section of more than 130 m ofpenetration into submarine basalts (Lancelot, Larson, et al., 1990). A3 m thick hydrothermal deposit recovered at Hole 801C divides the

389

M. S. PRINGLE

10'S 10'S140'E 150"E 160-E 170"E 180'E

Figure 1. Sites of Mesozoic volcanism in the western Pacific for which both reliable 40Ar/39Ar ages and radiogenic isotope analyses areavailable, including Sites 800, 801, and 802 drilled during Leg 129. Figure drawn using the GMT utilities of Wessel and Smith (1991).

section into two distinct compositional groups: all of the cooling unitsat Hole 80IB and the cooling units above the deposit at Hole 801Care alkalic basalts; all of the cooling units below the deposit atHole 801C are tholeiitic, mid-ocean ridge basalts (Floyd et al., 1991;Floyd and Castillo, this volume). The oldest dated sediments are fromthe latest Bathonian or from the Bathonian-Callovian boundary (Mat-suoka, this volume).

Site 802 was drilled in the East Mariana Basin, 400 km west of ItaMai Tai Guyot (Fig. 1). A set of Mesozoic magnetic anomalies similarto those found in the Pigafetta Basin has been identified in the EastMariana Basin. However, the identification of the East Mariana Basinmagnetic anomalies is less certain older than M33, and the magneticage of the ocean crust near Site 802 can only be assigned tentativelyto anomalies M35-M37 (Lancelot, Larson, et al., 1990). The oldestsediments recovered are younger than late Aptian, at least 40 m.y.younger than the magnetic anomaly age (Lancelot, Larson, et al.,1990). As at Site 800, the age of the recovered basement seems moreclosely related to the mid-Cretaceous volcaniclastics higher in thesection than to the apparent Jurassic crust underneath.

K-Ar GEOCHRONOLOGY

The K-Ar clock, and especially the 40Ar/39Ar technique, has beenwidely used to date oceanic lavas. However, several fundamental as-sumptions of the technique may be violated by samples that have beenaltered in the submarine environment. Both addition of potassium andloss of radiogenic 40Ar during alteration will lower the apparent K-Arage, making conventional K-Ar apparent ages minimum estimates forthe age of crystallization. 40Ar/39Ar total-fusion analyses have been usedwith some success on whole rock samples dredged from seamounts (forexample, Clague et al., 1975; Dalrymple and Clague, 1976; Schlanger etal., 1984; Watts et al., 1988). For a 40Ar/39Ar total-fusion age of an alteredvolcanic rock to be accurate, radiogenic ^Ar generated over geologictime and K-derived 39Ar generated during irradiation must both be lostfrom alteration phases that are younger than the crystallization age. Itseems that if the extent of alteration is relatively small, this loss isrelatively complete, and a significant fraction of the original K-Ar systemis undisturbed. For such samples, total-fusion ages can give reliableestimates of the crystallization age.

390

RADIOMETRIC AGES OF BASALTIC BASEMENT

No independent criteria have been developed, however, to testwhether a given sample is too altered for the 40Ar/39Ar total-fusiontechnique to yield an accurate age. Pringle (1992) has shown that, evenfor samples that are only partially disturbed (i.e., disturbed samplesthat still have concordant incremental-heating plateau), 40Ar/39Ar total-fusion ages can be as much as 10% too old or too young. Samples thatdo not have concordant incremental-heating plateau can easily havewhole rock total-fusion ages ± 30% of the crystallization age, even ifthey have been chosen using petrographic criteria such as those pro-posed by Mankinen and Dalrymple (1972). Pringle (1992) has alsoshown that some total-fusion ages of carefully cleaned plagioclasemineral separates may not give reliable age estimates.

The real power of using 40Ar/39Ar geochronology in decipheringthe crystallization age of altered rocks is that accurate crystallizationages can be differentiated from unreliable ages using a series ofinternal tests for a set of apparent ages from a given sample. It maynot be possible to differentiate why a particular age from a sample isunreliable, although alteration is certainly a contributing factor. How-ever, one can test whether the K-Ar clock of a particular sample is toodisturbed to reveal a meaningful age.

The need for this internal consistency check is especially impor-tant for the relatively low-K, moderately to highly altered samplestypical of those recovered from the ocean floor by ODP. Rocks withinterstitial glass or very fine-grained groundmass—among the mostcommon recovered from the ocean floor—do not give reliable ages,even when alteration products cannot be detected petrographically(Mankinen and Dalrymple, 1972; Fleck et al., 1977). In fact, Siede-mann (1978) concluded that it would not be possible to reliably dateany altered mid-ocean ridge basalt using the 40Ar/39Ar technique. Thisauthor is not aware of any previous study which has reliably datedsuch material, the recovery of which was one of the principal goalsof Leg 129.

Criteria for Interpreting 40Ar/39Ar Apparent Ages

To test whether a sample can reveal a reliable 40Ar/39Ar age, onemust first generate a set of ages upon which to apply the criteriapresented below. Incremental-heating experiments can be used togenerate an age spectrum, and the plateau steps can be tested with thecriteria described below (for example, Dalrymple et al., 1980). A seriesof laser-fusion ages on mineral separates, whether degassed or simpletotal-fusion analyses, can also be tested. Davis et al. (1989) used a setof whole rock total-fusion ages from a Seamount to determine that thegroup as a whole represented a concordant age for that Seamount, eventhough each of the samples exhibited disturbed whole rock incre-mental-heating experiments indicative of significant internal argonredistribution. Below, we apply a new approach for samples thatexhibit a significant amount of internal argon redistribution and loss,generating a mid-temperature (700°-1100°C) age for a series of smallwhole rock chips and applying the criteria below to that set.

Following Pringle (1992), we modify the conservative criteria ofLanphere and Dalrymple (1978) and Dalrymple et al. (1980) so thateach of the criteria involves the use of a rigorous statistical test. Weapply these criteria to the set of apparent ages for a given sample. Weaccept that this set of apparent ages represents an accurate estimateof the crystallization age of the sample only if:

1. No age difference can be detected between any of the individualages at the 95% confidence level. For an incremental-heating experi-ment, these ages should represent a well-defined, high-temperatureage plateau representing three or more contiguous steps which containat least 50% of the 39Ar released. For total-fusion analyses and agesfrom degassed phases, the selected ages should include over 50% ofthe 39Ar released, making sure to count the degassing steps in the totalamount of 39Ar.

2. A well-defined isochron exists for the set; i.e., the F variate statisticfor the regression is sufficiently small at the 95% confidence level.

3. The weighted mean age from the ages chosen in (1) and theisochron analysis of the data from (2) are not significantly differentat the 95% confidence level.

4. The 40Ar/36Ar intercept from the isochron analysis in (2) is notsignificantly different from the atmospheric value of 295.5 at the95% level.

For the test of a well-defined isochron in (2), most authors followBrooks et al. (1972) and Mclntyre et al. (1966), who suggest a generalcutoff value of 2.5 for the ratio between scatter about the isochron tohow much of that scatter can be explained within the limits ofanalytical error alone. This departs from standard practice in not usingsome level of significance to reject the model age, and does not takeinto consideration that different cutoff values should be used fordifferent numbers of data points. The more rigorous F variate statis-tical test is preferred here. For example, the proper cutoff value at the95% confidence level for isochrons with 4, 6, 8, and 11 data points is3.00, 2.37, 2.10, and 1.88, respectively. If this ratio is exceeded, thenthere is more scatter about the isochron than can be explained byanalytical error alone; i.e., some additional geologic or experimentaldisturbance is significant. Further justification of an age derived fromsuch an isochron is necessary before that age can be accepted ashaving geological meaning.

Samples Studied

Site 800

Three cooling units of massive, medium-grained, aphyric alkalicdolerite were described by Lancelot, Larson, et al. (1990) from the 7 mof recovered material at Hole 800A (56 m total basement penetration).From thin section examination, it was determined that none of thesamples were acceptable for whole rock analysis. Variable amounts ofclay, carbonate, and other low temperature alteration phases replacebetween 20% and 60% of the original crystallization phases; someindividual mineral grains are completely replaced. Samples from thelowermost unit appeared most suitable for mineral separates; twosamples of slightly different mineralogy were chosen as the freshestand coarsest-grained fractions. Sample 129B-800-61R-1, 17-24 cm,is representative of most of the unit. It is dominated by zoned plagio-clase laths 0.5 to 5.0 mm in diameter, many with altered cores, andsubhedral titanaugite prisms 0.4 to 4 mm long. Sample 58R-2, 52-60cm, from the top of the unit, is similar but also contains small butsignificant amounts of apparently primary potassium feldspar andhornblende as well as a trace of apparently secondary, fine-grainedactinolite and biotite.

Site 801

Most of the cooling units at both Sites 80IB and 801C wereinterpreted as relatively thin (0.5 to 4 m thick), aphyric basalt flows,although one unit in the tholeiitic section is unusually thick (c. 13.5 m,Lancelot, Larson, et al., 1990). Actual pillow lava margins, the mostreliable indicator of extrusive igneous activity, were recovered onlyfrom some of the tholeiitic cooling units. For the alkalic cooling units,the bottommost unit at Hole 80IB and several of the uppermost unitsat Hole 801C were clearly intrusive; they may in fact represent a singleunit (Lancelot, Larson, et al., 1990). Although the other alkalic coolingunits were interpreted as extrusive lava flows, no actual pillow marginswere recovered. Thus, it is possible that at least some of the units couldbe thin, intrusive, basalt sills. If this were true, then the age of the alkalicbasalts would be a minimum age for the "overlying" radiolarian-bear-ing sedimentary rocks, rather than a maximum age as is assumed in thediscussion below.

391

M. S. PRINGLE

None of the samples from the alkalic section of Site 801 were freshenough for whole rock analysis, but two samples were selected formineral separation. Sample 129B-801-1R-1, 109-119 cm, from theuppermost unit of Hole 801C is an aphyric, medium-grained, hy-pocrystalline basalt. It correlates to two units at Hole 80IB (units 6and 7). Matrix plagioclase, partially replaced by white clays, rangedup to 1 or 2 mm in diameter and was coarse enough to separate. Also,we were able to separate about a milligram of fine-grained biotiteflakes, 0.1 to 0.15 mm in diameter, which grew as a late-stage yetprimary igneous phase. Sample 44R-3, 13-22 cm, has 5%-10%subhedral plagioclase phenocrysts up to 2 mm in diameter in a glassygroundmass; it is from the intrusive unit at the bottom of Hole 801B,and correlates to two units at Hole 801C (units 4 and 6).

Most of the cooling units of the tholeiitic section of Site 801 arehypocrystalline to cryptocrystalline, aphyric basalts with groundmasscontaining 10%-50% smectitic clays. Samples from these units are tooglassy or altered for whole rock analysis and too fine-grained formineral separates. Several of the less-altered, coarser-grained flowinteriors were sampled for whole rock 40Ar/39Ar analysis, but nonegave any useful age information. Only the two samples selected fromthe interior of the thick flow gave concordant results. Both samples—10R-5, 53-58 cm, and 10R-6, 21-26 cm—are fine-grained yetholocrystalline, aphyric basalts with only a few percent of greensmectite after glass in an intersertal groundmass.

Sample 801C-5R3, 35-38 cm, contained 5% to 10% plagioclasephenocrysts 1 to 4 mm in diameter in a highly altered, hypocrystallinegroundmass. We were able to separate and carefully acid clean about30 mg of relatively clean plagioclase. However, we could not get anyuseful age information using either the laser or resistance furnacesystems described below, presumably because of the low potassiumcontent of the separate (<.01%), the effects of alteration not visible inthe mineral separate, or both.

Site 802

Almost 17 m of pillow basalts and thin flows representing 51mof basement penetration at Hole 802A were divided into 17 coolingunits (Lancelot, Larson, et al., 1990). The upper 16 units are relativelythin, less than 1.5 m thick, hypocrystalline basalt with abundantquench textures, too fine-grained and glassy for 40Ar/39Ar geochro-nology in spite of the low degree of alteration apparent in most of thesamples. The bottommost unit was interpreted as a 3 m thick flow andis similar to the other flows except that it grades into a fine-grainedyet holocrystalline groundmass in the center, with only a few percentof green clays after olivine microphenocryts and a trace of carbonatein some areas. Both samples selected for 40Ar/39Ar whole rock incre-mental-heating experiments were from this unit.

Techniques

Samples for mineral separation were crushed by hand in a hard-ened-steel mortar and pestle. Conventional heavy liquid and magnetictechniques were used to prepare pure mineral separates, which werefurther cleaned with appropriate acids in a 40Khz ultrasonic bath.Feldspar was cleaned with cold 5%-10% HF and warm 2.5N HC1.Hornblende was cleaned with warm 2.5N HC1. Biotite was notcleaned in any acid, but was carefully hand-picked of impurities.Whole rock samples were prepared as 6 mm diameter cores drilledfrom the freshest sections of 1 cm thick slabs of the basalt.

The whole rock cores, 0.5-0.8 g, and mineral separates, 1-15 mgencapsulated in Cu foil packets, were sealed in fused-silica vials andirradiated for 24 to 40 hours in the core of the Geological SurveyTRIGA reactor (Dalrymple et al., 1981). The samples were closelymonitored with sanidine separated from the 27.92 Ma Taylor CreekRhyolite (sample 85G003 of Dalrymple and Duffield, 1988). Theneutron flux parameter, J, was calculated from multiple analyses of

Taylor Creek sanidine monitors using the U.S. Geological Survey(USGS) laser system, and is reproducible to better than 0.3%. Sixpackets of the hornblende standard Mmhb-1 were analyzed as un-knowns during this study and gave an average age of 514.0 ± 1.4 Ma(standard error of the mean).

Three different extraction systems and mass spectrometers wereused for this study. The first system, at the USGS in Menlo Park, CA,is the GLM (Great Little Machine) continuous laser extraction systemdescribed by Dalrymple (1989). It employs an ultrasensitive, ul-traclean mass spectrometer capable of measuring the very small(0.1-15 mg) samples available for most of this study. It also includesan infrared microscope for measuring the temperature of samplesduring laser-heating. Most of the analyses reported here were ana-lyzed on the GLM. The second system, at Stanford University, issimilar to the USGS laser system but also has an all-metal Staudacher-type resistance furnace attached to the cleanup system. This systemwas used for two incremental-heating experiments on plagioclase.The third system, at the USGS, also has an all-metal resistance furnacefor incremental-heating but is attached to a Nier-type single-collectormass spectrometer and is about two orders of magnitude less sensitivethan the first two systems. The third system was used for incremental-heating experiments on the 6 mm diameter whole rock cores.

Mean ages were calculated as weighted means, where each age isweighted by the inverse of their variance (Taylor, 1982). Incremental-heating experiments were reduced as both age spectra and isochrons.Isochron ages were calculated for both 40Ar/36Ar vs. 40Ar/39Ar and36Ar/40Ar vs. 39Ar/40Ar correlation diagrams using the York2 least-squares cubic fit with correlated errors (York, 1969); SUMS/N-2 isthe F variate statistic for this regression. Dalrymple et al. (1988) foundno significant difference between the two correlations when the errorcorrelation coefficients are determined using the method recom-mended by York (as quoted in Ozima et al., 1977).

RESULTS

The ̂ Ar/^Ar age determinations for basaltic basement recovered atSites 800, 801, and 802 during Leg 129 are summarized in Table 1; theindividual age determinations are shown in Tables 2,3,4, and 5. The bestage for each site is the weighted mean of the isochron ages for each ofthe experiments described below. We prefer to use the isochron age asthe best estimate of each sample age because it (1) combines both anestimate of the degree of internal discordance - the scatter about theisochron line - and an estimate of analytical error in the final errorestimate, and (2) makes no assumption about the composition of the initialnon-radiogenic, or trapped, argon component.

Figure 2, a time calibration plot of the Mesozoic magnetic anoma-lies and the preceding Jurassic Quiet Zone, summarizes the age of theocean crust recovered during Leg 129. It includes both the radiometricages determined here and the biostratigraphic ages determined fromthe ages of the oldest sedimentary rocks overlying the basalt (Lancelot,Larson, et al., 1990; Matsuoka, this volume). Note that all of the oceancrust ages in Figure 2 except for the Leg 129 results are biostrati-graphic; i.e., no reliable radiometric ages for Jurassic or Early Creta-ceous ocean crust from any ocean were available before this study.

Hole 800A

Two 40Ar/39Ar experiments were done on the mineral separatesfrom the alkalic dolerites of Site 800. The first was a laser incremental-heating experiment using a defocussed laser beam on the K-feldsparfrom 800A-52R-2,52-60 cm. It revealed a concordant high tempera-ture plateau over almost 80% of the gas released; the isochron analysiswas also concordant (Fig. 3A, Table 1). The second was a series ofmineral laser fusion analyses conducted on plagioclase from Sample61R-1, 17-24 cm and on plagioclase, K-feldspar, and hornblendefrom Sample 52R-2, 52-60 cm (Table 2). The isochron with all of the


Table 1. Summary of Leg 129 radiometric (40Ar/39Ar) age determinations.

Plateau or weighted TF age Isochron age

Sample

Site 800

8OOA-52R-2, 52-60 cm,and61R-l, 17-24 cm

800A-52R-2, 52-60 cm

Site 801

801B-44R-3, 13-22 cm

801C-1R-1, 109-119 cm

801C-10R-5, 53-58 cm801C-10R-6, 21-26 cm

Site 802

802A-62R-2, 45-50 cm

802A-62R-3,4-12cm

Material

Plagioclaseand K-feldspar

K-feldspar

Alkalic basalts

PlagioclasePlagioclasePlagioclaseBiotitePlagioclase

Tholeiitic basalts

Basalt coreBasalt chips

Basalt coreBasalt coreBasalt core

3 9Ar(%)

b100

79.7

b68.950.3

b100b100

98.0

59.4C62.4

78.4100.090.5

Age ± 1 s.d.

αBest age:

126.1 ± 0.6 Ma

125.8 ± 0.7 Ma

aBest age:

158.1 ± 0.5 Ma155.0 ± 1.0 Ma157.6 ± 0.8 Ma161.5 ± 0.9 Ma156.2 ± 0.8 Ma

"Best age:

158.6 ± 2.7 Ma171.5 ± 1.1 Ma

aBest age:

118.3 ± 1.6 Ma114.7 ± 2.8 Ma115.8 ± 2.6 Ma

Age ± 1 s.d.

126.1 ± 0.6 Ma

126.1 ± 0.7 Ma

126.1 ± 0.9 Ma

157.4 ± 0.5 Ma

158.1 ± 0.7 Ma154.1 ± 2.6 Ma157.9 ± 1.4 Ma159.7 ± 2.1 Ma156.2 ± 0.8 Ma

166.8 ± 4.5 Ma

153.7 ± 8.0 Ma173.0 ± 5.5 Ma

114.6 ± 3.2 Ma

116.8 ± 4.8 Ma112.5 ± 4.6 Ma116.0 ± 13.1 Ma

mAn/36Asi ± 1 s.d.

279.1 ± 42.0

148.2 ± 71.4

295.0 ± 4.3305.1 ± 26.8233.2 ± 170.7313.9 ± 18.9287.3 ± 17.6

366.7 ± 108.9285.7 ± 30.9

303.2 ± 39.4306.0 ± 24.5294.4 ± 38.3

SUMS/N-2

2.16

1.30

0.310.060.950.830.40

0.031.58

2.120.400.97

N

20

11

54668

49

464

a"Best age" is calculated by weighting the individual isochron ages by the inverse variance.bLaser total-fusion analyses, including fusions of previously degassed minerals.c"Plateau" steps from individual basalt fragments heated to about 1100° after degassing at 600° and 700°

feldspar laser fusion data (n = 20) is shown in Figure 3B. Note thatthe F variate statistic for the isochron, 2.16, is slightly greater thanwould be expected from analytical error alone. This extra scatter isincluded in the calculation of the estimate of the reported error in age.The hornblende apparent ages, ca. 125 Ma, were slightly younger thanthe rest of the mineral data and had significantly lower radiogenic40Ar compositions (Table 2), apparently because of a small amountof secondary actinolite that could not be easily separated from thehornblende. Inclusion of these data in the isochron analysis resultedin a slightly higher isochron age and lower initial 40Ar/36Ar intercept,similar to the example given by Lanphere and Dalrymple (1978).Their conclusion not to include such data in the final age analysis isfollowed here. The best age for the Hole 800A dolerite sills is theweighted mean age of these two experiments, 126.1 ±0.6 Ma.

Site 801 Alkalic Basalts

Five different experiments were done on three different phasesfrom the two units studied here; all meet or exceed the reliabilitycriteria. Incremental-heating experiments using the Stanford resis-tance furnace were conducted on plagioclase from each of two units.Sample 801C-1R-1, 109-119 cm showed no signs of disturbance(Fig. 4A). Sample 801B-44R-3, 13-22 cm, showed young apparentages at both low and high temperatures, probably due to 40Ar lossfrom alteration phases and Ar redistribution mechanisms, respec-tively, but did show a good mid-temperature plateau for almost 60%of the 39Ar released (Fig. 4B). Three sets of laser fusion analyses,including co-existing biotite and plagioclase from Sample 801C-1R-1, 109-119 cm, also revealed concordant ages (Figs. 4C, D, E). Thebest age for the alkalic basalts at Site 801 is the weighted mean ageof all five experiments, 157.4 ± 0.5 Ma.

Hole 801C Tholeiitic Basalts

The two samples chosen from the 13.5 m thick flow were the onlysamples from tholeiitic sequence from Hole 801C that yielded concor-

dant age information according to the criteria described above. Bothsamples exhibited a disturbed age spectrum typical of argon redistri-bution (Fig. 5). Fine-grained, altered whole rock samples are verysusceptible to argon redistribution that has been attributed mainly tothe recoil of 39Ar from higher-potassium sites degassed at low tem-peratures to lower-potassium sites degassed at high temperatures.This results in an age spectrum with a decreasing step-like patternsuch as those found in Figures 5 and 6. The six-step incremental-heat-ing experiment on Sample 801C-10R-5, 53-58 cm, revealed a mid-temperature plateau with 59.4% of the 39Ar released (Fig. 5A).

For Sample 801C-10R-6,21-26 cm, anew approach of determin-ing concordant ages from rocks with suspect argon redistributionprofiles was tested. Because samples with such problems will revealmid-temperature plateaus when they are concordant, the mid-tem-perature release was extracted from nine individual chips weighing2-5 mg each (Fig. 5B). Using an infrared microscope to monitor thetemperature of the chips during heating, the chips were degassed at600°C and 700°C and the liberated argon was combined and meas-ured for each temperature. Next, each chip was heated to 1100°C, andthe gas released was measured for each chip. Finally, the chips werefused in three groups of three chips each and the gas for each groupwas measured. The 1100°C steps included 62% of the 39Ar released(Fig. 5B); the isochron and age spectrum are concordant (Table 1).The best age for the tholeiites at Hole 801C is the weighted meanisochron age for this experiment and the incremental-heating experi-ment described above, 166.8 ± 4.5 Ma.

Hole 802A

The three whole rock incremental-heating experiments on samplesfrom the thickest, most holocrystalline flow from Hole 802A allshowed typical argon redistribution profiles (Fig. 6). Each experimentwas concordant according to the criteria used for this study, with ca.80%, 100%, and 90% of the 39Ar released on the age plateaus. Thebest age for the lowermost flow from Hole 802A is the weighted meanage of the isochron ages, 114.3 ± 3.2 Ma.

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M. S. PRINGLE

Table 2. Incremental heating and laser fusion data for mineral separates from Hole 800 A dolerites.

Step 4C

800A-61R-1

PlagFlagPlag

dPlagdPlagdPlag

800A-58R-2,

PlagPlagPlagPlagPlag

dPlagdPlagdPlagdPlag

KsparKsparKsparKsparKspar

UhlHhlUhlHhlHhl

800A-58R-2

e0.5W0.8WLOW1.3W1.6W2.0W2.4W3.0W3.6W4.2W5.0W6.0WFuse#lFuse #2

Ar/39Ara

, 17-24 cm

6.4036.3296.3716.2956.2856.279

52-60 cm

6.3326.3086.3226.3096.2056.2736.2706.2866.283

6.3076.2496.2526.2016.291

8.2508.0488.9427.9648.065

,52-60 cm

6.5436.2756.1216.2846.3066.2946.2896.2976.2996.2906.2856.3136.3216.330

37Ar/39Ara

3.0523.3113.3563.0753.3293.426

1.78791.07281.70371.63771.17591.44791.42231.38111.2215

0.121430.095290.163900.106690.15418

4.9695.7506.6216.3225.124

0.166310.159870.134940.113450.117400.115750.118440.120400.108820.121950.120140.115600.101390.14886

36Ar/39Ara

Mineral TF

0.0015840.0014100.0015370.0010890.0011740.001212

Mineral TF

0.0009290.0006760.0008680.0008000.0004830.0005540.0006070.0005670.000526

0.0003670.0001250.0002400.0002960.000150

0.0085460.0079020.0113400.0078480.007956

K-feldspar IH

0.0018790.0003130.0003640.0001780.0005740.0000360.0000750.0005920.0002340.0003870.0005420.0003280.0003030.000180

4 0 . bA r R

96.497.597.098.798.698.5

97.898.198.098.299.199.198.899.099.0

98.399.499.098.699.4

74.176.668.377.175.8

91.698.698.399.297.499.999.797.398.998.297.598.598.699.2

40ArKb wArCa

b

J = 0.011675

0.0.0.0.0.0.

0.20.20.20.20.20.2

J = 0.011675

0.0.0.0.0.0.0.0.0.

0.0.0.0.0.

0.0.0.0.0.

0.10.10.10.10.10.10.10.10.1

0.00.00.00.00.0

0.30.40.40.40.3

J = 0.01166

0.0.0.0.0.0.0.0.0.0.0.0.0.0.

0.00.00.00.00.00.00.00.00.00.00.00.00.00.0

36ArCab

51.863.258.775.976.276.0

51.842.752.855.065.570.463.065.562.4

8.920.418.49.7

27.7

15.619.615.721.717.3

2.313.59.8

16.95.4

83.941.6

5.412.38.35.99.38.8

21.8

K/Ca

39Ar Apparent agec

(%) (Ma)

Total gas age = 125.98 ± 0.61

0.1600.1480.1460.1590.1470.143

0.2740.4560.2870.2990.4160.3380.3440.3540.401

4.035.142.994.593.18

0.0980.0850.074'0.0770.095

4.1 125.75 ± 0.398.1 125.73 ± 0.371.3 125.90 ± 0.604.6 126.55 ± 0.384.2 126.27 ± 0.393.8 126.09 ± 0.39

2.4 126.09 ± 0.444.3 125.90 ± 0.386.1 126.11 ± 0.374.0 126.14 + 0.394.1 125.17 ± 0.389.1 126.54 ± 0.368.4 126.14 ± 0.364.5 126.62 ± 0.385.7 126.52 ± 0.37

1.6 126.12 ± 0.532.5 126.33 ± 0.432.3 125.84 ± 0.443.3 124.42 ± 0.402.9 127.12 ± 0.41

2.3 124.77 ± 0.552.1 125.82 ± 0.563.2 124.86 ± 0.572.7 125.42 ± 0.522.2 124.82 ± 0.55

Total gas age = 125.76 ± 0.63

2.953.063.634.324.174.234.144.074.504.024.084.244.833.29

6.1 121.90 ± 0.607.2 125.70 ± 0.537.0 122.34 ± 0.549.6 126.60 ± 0.463.0 124.74 ± 1.035.0 127.62 ± 0.684.0 127.30 ± 0.805.7 124.45 ± 0.613.3 126.55 ± 0.943.9 125.52 ± 0.814.6 124.51 ± 0.72

13.0 126.30 ± 0.4217.1 126.57 ± 0.3910.7 127.53 ± 0.45

aCorrected for Ar and Ar decay, half-lives are 35.1 days and 259 years, respectively.bSubscripts indicate radiogenic (R), calcium-derived (Ca), and potassium-derived (K) argon.cLambda E = 0.581E-10/yr, Lambda B = 4.692E-10/yr. Errors are standard deviaton of analytical precision.dFor these analyses, the samples were previously degassed with a 4W, 5-mm-wide laser beam.eLaser power used for each of these steps, actual temperature was not measured.

DISCUSSION

Site 800

The age of the alkalic dolerite at Hole 800A is significantlyyounger than both the age inferred from the magnetization of theocean crust and the age of the oldest overlying sediments (Fig. 2).This confirms the conclusion of Lancelot, Larson, et al. (1990) thatthe 800A dolerites are sills intruded into older sediments and are notrelated to the formation of the original ocean crust, presumablylocated somewhere below the recovered section.

The 126 Ma age is similar to the 120 Ma age for the alkalic basaltsdredged from Himu Seamount, located 80 km to the southwest (Smithet al., 1989). The isotopic compositions of the 800A dolerites have astrong HIMU component (high radiogenic Pb, low radiogenic Sr),nearly identical to lavas recovered from Himu Seamount 80 to thesouthwest, Golden Dragon Seamount 150 km to the southeast, and two

80-90 Ma seamounts in the Marshall Islands (Castillo et al., this volume;Staudigel et al., 1991; Smith et al., 1989; Davis et al., 1989). Using thehotspot frame of reference, sites of Mesozoic volcanism in the westernPacific can be backtracked to the original location where they formed(Fig. 7). All four sites of Mesozoic volcanism with a HIMU isotopicsignature (solid circles and squares) backtrack to the general vicinity ofthe Austral-Cook Islands. Three out of the four locations are indistin-guishable from the present day location of the Rurutu hotspot, which haserupted lavas with a HIMU isotopic component.

Summarizing the unusual geochemical and geophysical charac-teristics of the modern day volcanism of islands and seamounts of thesouth central Pacific, Smith et al. (1989) and Staudigel et al. (1991)defined the South Pacific Isotopic and Thermal Anomaly (SOPITA,Fig. 7). They concluded that this anomalous region of the PacificBasin was also responsible for at least some of the CretaceousSeamount volcanism of the western Pacific, and extended back to atleast 120 Ma. With the new data from Site 800, we extend this back


Table 3. Incremental heating and laser fusion data for mineral separates from Site 801 alkaiic basalts.

Step

801B-44R-3,

d6306907508008909801100

Degas 1FuseOFuse 1Degas 2Fuse 2Degas 3Fuse 3

40Ar/39Ara

13-22 cm

44.4926.3825.6523.3125.9524.5432.32

217.626.0826.3323.9033.5623.3032.33

801C-1R-1, 109-119 cm

d6106907608209009701070117013301490

T. Fuse"

"

Degas @ 4WFuse

T. Fuse

"

10.7539.8679.6029.6259.7409.738

10.07510.07612.43023.39

8.0238.0418.0528.0248.0107.9808.013

10.1599.2528.9788.9139.3529.686

37Ar/39Ara

21.9022.6922.9423.4820.8821.0415.09

22.2222.9521.6623.0721.3323.2620.47

10.42810.93315.50213.2377.0575.2717.948

11.32416.13619.753

12.80212.62314.28614.08813.57813.21511.665

0.22750.146620.157290.160530.100050.2053

36Ar/39Ara

Plagioclase IH

0.093380.022200.0198540.0123420.0197220.0166910.04400

Plagioclase TF

0.67600.0197200.020340.0122550.044700.0106690.04293

Plagioclase IH

0.0064880.0039500.0045800.0038060.0024170.0020570.0040330.0047620.0143430.04638

Plagioclase TF

0.0036500.0037060.0040080.0042640.0038610.0038750.003409

Biotite TF

0.0066350.0033860.0025410.0028870.0041900.005276

4 0 . bA r R

41.981.984.292.383.986.763.5

9.084.683.792.565.794.465.8

89.896.998.799.298.498.094.494.976.248.1

98.998.699.197.998.998.598.7

80.889.291.790.586.884.0

40ArKb 39ArCa

b

7 = 0.004091

0.0 1.50.0 1.50.0 1.50.0 1.60.0 1.40.0 1.40.0 1.0

7 = 0.004091

0.0 1.50.0 1.50.0 1.50.0 1.50.0 1.40.0 1.60.0 1.4

7 = 0.009419

0.1 0.70.1 0.70.1 1.00.1 0.90.1 0.50.1 0.40.1 0.50.1 0.80.1 1.10.0 1.3

7 = 0.01146

0.1 0.90.1 0.80.1 1.00.1 0.90.1 0.90.1 0.90.1 0.8

7 = 0.011460

0.1 0.00.1 0.00.1 0.00.1 0.00.1 0.00.1 0.0

36ArCab

6.327.531.151.228.533.99.2

0.931.328.750.712.858.712.8

43.274.591.093.678.568.953.064.030.311.5

92.689.994.187.292.890.090.3

0.91.11.61.50.61.0

K/Ca

39Ar Apparent agec

(%) (Ma)

Total gas age = 148.4 ± 1.9

0.0220.0210.0210.0210.0230.0230.032

27.8 134.4 ± 6.317.4 155.1 ± 1.317.6 155.0 ± 1.18.9 154.5 ± 1.56.3 156.0 ± 2.6

14.6 152.6 ± 1.07.3 146.8 ± 2.0


0.0220.0210.0220.0210.0230.0210.024

22.5 140.9 ± 5.911.6 158.3 ± 0.722.0 157.9 ± 0.611.5 158.5 ± 0.79.3 157.9 ± 0.9

13.6 157.7 ± 0.69.5 152.4 ± 0.9


0.0470.0440.0310.0370.0690.0930.0610.0430.0300.024

1.2 158.1 ± 3.75.4 156.7 ± 1.1

35.8 155.8 ± 1.321.7 156.6 ± 0.711.9 156.6 ± 0.58.1 155.8 ± 0.88.2 155.5 ± 0.64.7 156.7 ± 1.52.3 155.7 ± 3.20.8 184.0 ± 10.0


0.0380.0380.0340.0340.0360.0370.042

6.3 158.2 ± 1.010.5 158.0 ± 0.78.7 159.1 ± 0.86.5 156.8 ± 1.0

16.8 158.0 ± 0.631.4 156.8 ± 0.519.7 157.6 ± 0.5


2.153.343.123.054.902.39

28.8 162.1 ± 0.88.7 163.0 ± 1.9

10.9 162.5 ± 1.58.1 159.4 ± 2.0

21.3 160.3 ± 1.222.1 160.7 ± 1.2

aCorrected for 37Ar and 39Ar decay, half-lives are 35.1 days and 259 years, respectively.bSubscripts indicate radiogenic (R), calcium-derived (Ca), and potassium-derived (K) argon.cLambda E = 0.581E-10/yr, Lambda B = 4.692E-10/yr. Errors are standard deviation of analytical precision.dTemperature of each incremental heating step (°C) using a resistance furnace for heating.

to at least 126 Ma, and conclude that all of the seamounts and sills ofthe Pigafetta Basin that have been studied to date are Early Cretaceousproducts of the same anomaly which is responsible for the oceanislands and thermal (?) swell found in the South Pacific today.

Site 801

Two of the most important scientific objectives of Leg 129 wereto test the assumption of linear spreading rates and tectonics for theJurassic Pacific crust, and to calibrate the oldest M-sequence mag-netic anomalies. Figure 2 shows that the 166.8 Ma age for the tholeiiticocean crust at Site 801 is almost exactly the age predicted for the oceanfloor by simple linear extrapolation of the ages of the M-sequencemagnetic lineations using the time scale of Harland et al. (1990).Major element, trace element, and isotope geochemistry of the 801tholeiites are indistinguishable from those of modern MORB (Floyd

et al, 1991; Floyd and Castillo, this volume; Castillo et al., this volume).Both the age and geochemistry of the basalts offer important proofthat the quiet zone crust of the western Pacific is indeed Jurassicmid-ocean ridge basalt, and is not a product of mid-Cretaceous vol-canic events.

Since this site was located in the Jurassic Quiet Zone, 450 kmbeyond the oldest indentified magnetic lineations (Fig. 2; Lancelot,Larson, et al., 1990), we can not directly calibrate the age of theM-sequence lineations. Also, the Site 801 tholeiite age is not preciseenough for a useful calibration of the time scale. However, a simplelinear extrapolation of the M25-M37 magnetic lineations found inthe western Pigafetta Basin is consistent with the Harland et al. (1990)Geologic Time Scale (GTS) and offers indirect confirmation of theage of that sequence. This confirmation is extremely important, sincethe oldest well-dated magnetic lineation in the western Pacific is M9,dated at Hauterivian at DSDP Site 304 (Fig. 2).

395

M. S. PRINGLE

Table 4. Incremental heating and laser fusion data for whole-rock samples from Site 801 tholeiitic basalts.

Step

801C-10R-5

60070080090010001400

801C-10R-6,

d600d700e1100:

#1#2#3#4#5#6#7ttx#9

fFuse:#l-#3#4-#6#7-#9

40Ar/39Ara

53-58 cm

33.8224.0421.9220.8118.04616.281

21-26 cm

39.2616.422

12.64813.48811.77312.72312.84312.56312.52312.84913.316

12.32911.65911.735

37Ar/39Ara

27.2046.1532.4251.73

280.3366.3

14.85520.59

36.1833.5345.8640.6840.6944.5837.8239.8639.95

149.55144.92176.75

36Ar/39Ara

Basalt core

0.046910.026670.0169110.0195750.080430.10975

Basalt

0.095270.02257

0.0187970.020970.0189620.021060.021500.022090.0199500.021900.02351

0.054490.050300.05982

65.482.488.991.991.779.6

31.369.3

78.873.783.376.475.676.276.974.271.6

65.871.269.0

4()A

J--

Kb 39ArCab

= 0.004584

0.0 1.80.0 3.10.0 2.20.0 3.50.0.

J-

18.824.5

= 0.009995

0.0 1.00.

0.0.0.0.0.0.0.0.0.

0.0.0.

1.4

2.42.23.12.72.73.02.52.72.7

10.09.7

11.8

36ArCab

15.646.551.671.193.889.8

4.224.5

51.843.065.152.050.954.351.049.045.7

73.877.579.5

K/Ca

39Ar(%)

Total gas age

0.01770.01030.01480.00910.00140.0010

35.016.314.022.1

7.05.6

Total gas age

0.0330.023

0.01320.01430.01040.01170.01170.01070.01260.01200.0119

0.00290.00310.0024

7.114.5

4.17.37.49.49.35.38.56.24.9

4.86.15.2

Apparent agec

(Ma)

= 164.1 + 2.0

177.2 ± 1.3161.6 ± 4.8157.7 ± 5.6156.8 ± 3.8161.2 ± 11.4136.7 ± 14.5

= 175.8 ± 1.1

210.9 ± 4.8196.9 ± 1.9

175.3 ± 2.8174.7 ± 2.1173.8 ± 2.1171.8 ± 1.9171.6 ± 1.9169.6 ± 2.4169.8 ± 1.9168.6 ± 2.2168.5 ± 2.4

155.6 ± 4.5158.7 ± 4.2158.5 ± 5.1

aCorrected for Ar and Ar decay, half-lives are 35.1 days and 259 years, respectively.bSubscripts indicate radiogenic (R), calcium-derived (Ca), and potassium-derived (K) argon.cLambda E = 0.581E-10/yr, Lambda B = 4.692E-10/yr. Errors are standard deviation of analytical precision.dDegas 9 basalt fragments, each weighing between 2 and 5 mg, at 600° and then 700°C.eHeat each of the previously heated basalt fragments to 1100°C, measure individually.fFuse previously heated basalt fragments, measure in three groups of three fragments each.

Harland et al. (1990) concluded that just a few biostratigraphicallycontrolled, high quality isotopic ages from the Late Jurassic couldgreatly reduce the errors in the ages of the M-sequence magneticanomalies. Thus, the 157.4 ± 0.5 Ma age for the alkalic basalts at Site801 offers an important tie point in the calibration of the GTS. Thealkalic basalts are overlain by radiolarian-bearing sediments indica-tive of the middle Tricolocapsa conexa zone, which correlates to theBathonian-Callovian boundary or upper half of the Bathonian (Mat-suoka, this volume, chapter 10). The 157.4 Ma age is younger thanthe 161.3 Ma value suggested for this boundary by Harland et al.(1990), but is within the uncertainty of 3 to 5 m.y. suggested by themfor that part of the GTS. However, this age is significantly youngerthan the 169 Ma age suggested by Kent and Gradstein (1985), andsuggests that the refinements in the GTS made by Harland et al. (1990)are a significant improvement, at least for the Late Jurassic. Becauseof ambiguities in the correlation of the Pacific Jurassic radiolarianzones with the European stages, and because of the lack of magneticanomalies older than M37, a more precise tie of this radiometric agewith the GTS may not be possible yet. The limiting constraint for thispart of the time scale now seems to be the resolution of the biostra-tigraphy rather than the accuracy of the radiometric ages.

Pessagno and Blome (1990) report a 154 ± 1.5 Ma age for theOxfordian-Kimmeridgian boundary, not significantly different from the154.7 Ma reported by Harland et al. (1990), but about 5 m.y. older thanone extrapolated from our age for the latest Bathonian. However, B.Murchey (pers, comm., 1991) concludes that the diagnostic Subzone 2Gamma of Passagno and Blome (1990), defined as between the firstoccurrence of Mirifusus and the last occurrence of Xiphostylus, is cor-relative with the Tricolopasa plicarum and Tricolapsa conexa zones ofMatsuoka and Yao (1986). Matsuoka (this volume) correlates these zoneswith the A0 and Al zones of Baumgartner (1984,1987), assigned to theBajocian through Callovian stages rather than the Oxfordian assignment

of Pessagno and Blome (1990). Also, the 157 ± 1.5 Ma U/Pb age ofSaleeby (1987) from the Rogue Formation of Oregon (U.S.A.) is consis-tent with this result when the zonations of Baumgartner (1984, 1987),Matsuoka and Yao (1986), and Matsuoka (this volume) are used ratherthan those of Pessagno and Blome (1990).

The 157.4 Ma age for the alkalic basalt is 5 to 10 m.y. younger thanthat predicted by extrapolation of the M-sequence anomalies (Fig. 2).It is 10 m.y. younger than the 166.8 Ma age of the tholeiites, and thetwo ages are significantly different at the 95% confidence level.However, the age for the alkalic basalts is consistent with Bathonian-Callovian radiolarian age of the overlying sediments, which is alsoyounger than the extrapolated crustal age (Fig. 2). The major and traceelement geochemistry of the alkalic basalts indicates a significantcontribution from an ocean island basalt (OIB) source (Floyd et al.,1991; Floyd and Castillo, this volume). The isotopic geochemistry alsoindicates a significant contribution from an ocean island basalt source,and Castillo et al. (this volume) suggest that the alkalic basalts areproducts of a mixture of the depleted mantle source of the Site 801tholeiites and the OIB mantle source of the Site 800 alkalic dolerites.However, it is important to note that the location of the melting anomalythat created the Jurassic-age alkalic basalts at Site 801, although stillwithin the SOPITA, is significantly removed from the location of thesource of the Cretaceous-age HIMU basalts of the Pigafetta Basin andMarshall Islands (Fig. 7).

The difference in age between the 801 tholeiitic and alkalic basalts,the presence of the hydrothermal deposit separating the two se-quences, and the geochemistry of the alkalic basalts all suggest thatthe alkalic basalts were erupted in an off-ridge environment. How-ever, the Site 801 alkalic basalts are only about 60 m thick, and thebathymetric and seismic data around Site 801 suggests a relativelyflat, normal seafloor (Lancelot, Larson, et al., 1990). This suggeststhat the upper sequence alkalic basalts may be more related to isolated,

396


Table 5. Incremental heating and laser fusion data for whole-rock samples from Hole 802A (East Mariana Basin).

Step 40Ar/39Ara

802A-62R-2, 45-50 cm

d60070080090010501300

34.8322.8218.79315.19214.04716.641

802A-62R-2, 45-50 cm

d60070080090010501400

34.1922.7619.45015.52414.54018.044

802A-62R-3,4-12cm

d600700800Fuse

20.2012.20212.55910.028

37Ar/39Ara

13.24718.01336.7837.3420.88111.90

13.78016.71432.7737.2219.875130.82

10.15314.48435.8888.85

36Ar/39Ara

Basalt core

0.071160.031580.028630.0177710.0098930.04705

Basalt core

0.075190.033550.029440.0193940.0114960.05694

Basalt core

0.046140.023330.032650.03881

42.665.370.584.990.969.9

38.262.268.682.187.464.3

36.452.945.856.0

4 °Ar Kb W Ar C a "

7 = 0.005181

0.0 0.90.0 1.20.0 2.50.1 2.50.1 1.40.1 7.5

J = 0.004995

0.0 0.90.0 1.10.0 2.20.1 2.50.1 1.30.1 8.8

J = 0.01107

0.0 0.70.1 1.00.1 2.40.1 6.0

36ArCab

5.015.334.656.556.864.0

4.913.429.951.646.561.8

5.916.729.661.6

K/Ca

39Ar Apparent agec

(%) (Ma)

Total gas age = 122.01 ± 1.57

0.0370.0270.01300.01280.0230.0041

8.5 134.84 ± 6.9113.1 135.84 ± 4.4718.9 122.69 ± 3.1419.2 119.61 ± 3.0821.4 117.20 ± 2.7618.9 113.80 ± 3.25

Total gas age =115.32 ± 2.93

0.0350.0290.01460.01280.0240.0034

8.0 115.03 ± 14.7312.0 124.67 ± 9.6817.4 118.95 ± 6.7018.8 114.07 ± 6.2423.0 112.52 ± 5.0820.7 111.15 ± 5.74

Total gas age =118.88 ± 2.65

0.0480.0340.01330.0052

9.5 142.26 ± 11.7211.2 125.68 ± 10.0122.9 114.11 ± 5.0056.5 115.51 ± 3.01

"Corrected for 37Ar and 39Ar decay, half-lives are 35.1 days and 259 years, respectively.bSubscripts indicate radiogenic (R), calcium-derived (Ca), and potassium-derived (K) argon.cLambda E = 0.581E-10/yr, Lambda B = 4.692E-10/yr. Errors are standard deviation of analytical precision.dTemperature of each incremental heating step (°C) using a resistance furnace for heating.

relatively small alkalic seamounts formed near mid-ocean ridgesrather than to a large intraplate Seamount or island chain (Castillo etal., this volume; Castillo and Batiza, 1989; Zindler et al., 1984).

Site 802

The age of the Site 802 basalts is indistinguishable from the 110.8±1.0 Ma age of two basalt flows from the bottom of Site 462A in theNauru Basin (Pringle, 1992). Geochemically, the two provinces aresimilar, with major and trace element compositions showing a MORBaffinity, while the isotopic compositions indicate some contributionfrom an ocean island basalt source (Castillo et al., this volume; Floydet al., this volume; Castillo et al., 1991). The isotopic compositionsof the basalts recovered from these basins (Castillo et al., 1991;Castillo et al., this volume) are significantly different than thosereported for the Cretaceous SOPITA (Staudigel et al., 1991) but aresimilar to those found for the Ontong Java Plateau (Mahoney andSpencer, 1991; Mahoney et al., in press). I suggest that both the Nauruand East Mariana Basins are the result of rifting and extension relatedto the formation of Ontong Java plateau ca. 120 Ma, perhaps analo-gous to the dynamic relationship between the North Hawaiian ArchFlow and the Hawaiian hotspot (Clague et al., 1990).

It is important to note that the backtracked locations of the meltinganomalies which formed both Ontong Java Plateau and the EastMariana and Nauru Basin basalts are significantly south of the modernSOPITA (open symbols, Fig. 7). Also, the Louisville Ridge hotspot,which has been attributed to the Ontong Java Plateau when back-tracked to around 120 Ma, has been consistently decreasing in erup-tion rate for at least 40 Ma and appears to have no recent island orSeamount volcanism associated with it. This may indicate that boththe geographic extent and ocean crust production rate of the SOPITAwas much greater during the Cretaceous than today (for example,Larson, 1991), and that the mantle source compositions of the Creta-ceous SOPITA were more diverse than suggested by Staudigel et al.(1991). However, I prefer the alternative interpretation that the On-tong Java Plateau, East Mariana Basin, and Nauru Basin basalt

provinces did not have the same origin in the SOPITA as otherCretaceous Seamount provinces found in the western Pacific today.

ACKNOWLEDGMENTS

This study would not have been possible without Pat Castillo'shelp with initial sample selection at sea, Roger Larson's answeringand fax machines, and Hubert Staudigel's advice to always avoidovercommitting oneself while convincing me to overcommit myselfand accept an invitation to join the Leg 129 scientific staff. DebbieChristianson provided timely mineral separations from limited mate-rial. Mike McWilliams and Phil Gans provided ready access to theirnew argon facility at Stanford. Walter H.F. Smith patiently instructedthe author in the use of the GMT plotting software (Wessel and Smith,1991). G. Brent Dalrymple, Bill Sliter, Roger Larson, Tracey Vallier,and John O'Conner provided helpful reviews of the manuscript.

REFERENCES

Baumgartner, R O., 1984. A Middle Jurassic-Early Cretaceous low-latituderadiolarian zonation based on Unitary Associations and age of Tethyanradiolarites. Eclogae Geol. Helv., 77:729-837.

, 1987. Age and genesis of Tethyan Jurassic radiolarites. EclogaeGeol. Helv., 80:831-879.

Brooks, C , Hart, S. R., and Wendt, I., 1972. Realistic use of two-errorregression treatments as applied to rubidium-strontium data. Rev. Geophys.Space Phys., 10:551-557.

Castillo, P. R., and Batiza, R., 1989. Sr, Nd, and Pb isotope constraints on near-ridgeSeamount production beneath the South Atlantic. Nature, 342:262-265.

Castillo, P. R., Carlson, R. W., and Batiza, R., 1991. Origin of Nauru Basinigneous complex: Sr, Nd, and Pb isotope and REE constraints. EarthPlanet. Sci. Lett., 103:200-213.

Clague, D. A., Dalrymple, G. B., and Moberly, R., 1975. Petrography and K-Arages of dredged volcanic rocks from the western Hawaiian ridge andsouthern Emperor Seamount chain. Geol. Soc. Am. Bull., 84:991-998.

Clague, D. A., Holcomb, R. T, Sinton, J. M., Detrick, R. S., and Torresan,M. E., 1990. Pliocene and Pleistocene alkalic flood basalts on the seafloornorth of the Hawaiian islands. Earth Planet. Sci. Lett., 98:175-191.

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Dalrymple, G. B., 1989. The GLM continuous laser system for ^Ar/^Ardating: description and performance characteristics. U.S. Geol. Surv. Bull.,1890:89-96.

Dalrymple, G. B., Alexander, E. C, Lanphere, M. A., and Kraker, G. P., 1981.Irradiation of samples for 40Ar/39Ar dating using the Geological SurveyTRIGA reactor. Geol. Surv. Prof. Pap. U.S., 1176:1-55.

Dalrymple, G. B., and Clague, D. A., 1976. Age of the Hawaiian-EmperorBend. Earth Planet. Sci. Lett., 31:313-329.

Dalrymple, G. B., and Duffield, W. A., 1988. High precision 40Ar/39Ar datingof Oligocene rhyolites from the Mogollon-Datil Volcanic Field using acontinuous laser system. Geophys. Res. Lett., 14:462-464.

Dalrymple, G. B., Lanphere, M. A., and Clague, D. A., 1980. Conventional and40Ar/39Ar ages of volcanic rocks from Ojin (Site 430), Nintoku (Site 432),and Suiko (Site 433) Seamounts, and the chronology of volcanic propogationalong the Hawaiian-Emperor chain. In Jackson, E. D., Koizumi, I., et al., Init.Repts. DSDP, 55: Washington (U.S. Govt. Printing Office), 659-676.

Dalrymple, G. B., Lanphere, M. A., and Pringle, M. S., 1988. Correlationdiagrams in 40Ar/ Ar dating: is there a correct choice? Geophys. Res.Lett., 15:589-591.

Davis, A. S., Pringle, M. S., Pickthorn, L.B.G., Clague, D. A., and Schwab,W. C , 1989. Petrology and age of alkalic lava from the Ratak chain of theMarshall Islands. J. Geophys. Res., 94:9757-9774.

Duncan, R. A., and Clague, D. A., 1985. Pacific plate motions recorded bylinear volcanic chains. In Nairn, A.E.M., Stehli, F. G., and Uyeda, S. (Eds.),The Ocean Basins and Margins (Vol. 7A): New York (Plenum), 89-91.

Fleck, R. J., Sutter, J. R, and Elliot, D. H., 1977. Interpretation of discordant40Ar/39Ar age spectra of Mesozoic tholeiites from Antartica. Geochim.Cosmochim. Acta, 41:15-32.

Floyd, P. A., Castillo, P. R., and Pringle, M. S., 1991. Tholeiitic and alkalicbasalts of the oldest Pacific Ocean crust. Terra Nova, 3:257-265.

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Handschumacher, D. M., Sager, W. W., Hilde, T.W.C., and Bracey, D. R., 1988.Pre-Cretaceous tectonic evolution of the Pacific plate and the extension ofthe geomagnetic time scale with implications for the origin of the Jurassic"Quiet Zone." Tectonophysics, 155:365-380.

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Date of initial receipt: 4 November 1991Date of acceptance: 31 March 1992Ms 129B-130

398


Cross Strike Distance (km) Hawaiian Lineations

1000 2000

Aalenian

Bajocain

3 Bathonian

Oxfordian

CallovianKimmeridgian

Tithonian

Berriasian

Valanginian

Aptian

Albian

Cenomanian

I I r i i i

ODP Leg 129 Radiometric Ages

r—I

Radiolarians

] Dinoflagellates

Nannofossils

Foraminifers

Range Overlap

800 Dolerites

802 Tholeiites

417D 166

802

I l l IM1

11 | i i r i i i1 i i r i HI inn i I I i15 20 25 30 M35

Mesozoic Magnetic Anomalies

180

170

160

150

<o

130

120

110

100

90

JQZ

Figure 2. Time calibration plot of the Mesozoic anomalies M0 to M37 and the preceding Jurassic magnetic quiet zone (JQZ). Modified from Lancelot, Larson, etal. (1990) using the time scale of Harland et al. (1990). Magnetic anomalies are plotted as cross-strike distance across the Hawaiian lineations for M0 to M25.Anomalies M25 to M37 are normalized to that pattern after Handschumacher et al. (1988). Ages of various DSDP holes (numbered) are shown in rectangles.Vertical lengths of rectangles show paleontologic age ranges from DSDP intial reports, except for 100 (Zotto et al., 1987) and 105 (Gradstein and Sheridan, 1983).Horizontal lengths show magnetic ages ranges from Larson and Hilde (1975) for Sites 303, 304, 166, 100, and 105, from DSDP Initial Reports for Site 387 andHole 417D, and ODP Leg 129 Vol. A for Sites 800, 801, and 802. Radiometric ages from Sites 800, 801, and 802 are shown as ties lines, ±2 s.d. long.

399

M. S. PRINGLE

A. 800A - 52R-2, 52-60 cm K-Feldspar (IH)160 60000

40000 -<

<o

20000 "

10 20 30 40 50 60 70 80 90 100

3 9Ar released (%)

B. 800A Feldspar Total Fusion60000

40000 -

I20000 "

2500 5000 7500 10000

39Ar/36Ar

2500 5000 7500 1000039Ar/36Ar

Figure 3. 40Ar/39Ar laser analyses of feldspar separates from Hole 800A, Leg 129. A. Age spectra and isochron for a laser incremental-heating experiment. B.Laser-fusion isochron for feldspar separates from both 52R-2, 52-60 cm and 61R-1, 17-24 cm.

400

A. 801C - 1R-1,109-119 cm Plagiolase (IH)200

10 20 30 40 50 60 70 80 90 100

B. 801B - 44R-3,13-22 cm Plagiolase (IH)200

12010 20 30 40 50 60 70 80 90 100

39Ar released (%)


40000

4000

3000

1200 2400 3600 4800

0 ' ' l i i i . I

0 50 100 150 200

39A r /36A r

C. 801C-1R-1,109-119 cm4000

D. 801C-1R-1,109-119 cm40000

30000 -

20000 -

10000 -

100 200 300 400 500

3 9Ar/3 6Ar

1200 2400 3600 4800

39A r /36A f

E. 801B-44R-3,13-22 cm

6000

4500 -

3000 "

1500 "

60 120 18039Ar/36Ar

240

Figure 4. 40Ar/39Ar laser fusion and resistance furnace incremental-heating experiments for plagioclase and biotite separates from the alkalic basalts recoveredat Holes 80 IB and 801C, Leg 129. A. Resistance furnace incremental-heating age spectrum and isochron. B. Resistance furnace incremental-heating age spectrumand isochron. C. Laser-fusion isochron. D. Laser-fusion isochron. E. Laser-fusion isochron.

401

M. S. PRINGLE

A. 801C - 10R-5, 53-58 cm Basalt Core220

1200 10 20 30 40 50 60 70 80 90 100

B. 801C - 10R-6, 21-26 cm Basalt Chips

^600

21ü

8, 190

2 170coαCL

< 150

130

700

-171.5 ±1.1

1 2 3 4 5 6 7 8 9

HEAT@ 11001-3 4-6 7-9

' < — FUSE-)

10 20 30 40 50 60

3 9Ar released (%)

70 80 100

4000

<to

<o

2000

1500 -

1000 "

500 -

50 100 150 200

50 100

3 9Ar/

3 6Ar

150

Figure 5. Ar/39Ar resistance furnace and laser-heating of samples from the tholeiitic basalts of Hole 801C, Leg 129. A. Resistance furnace incremental-heatingage spectrum and isochron. B. Laser-heating of nine individual basalt chips weighing 2-4 mg each. Chips were first degassed together in two steps at 600°Cand 700°C, then heated at 1100°C and measured individually, and finally fused in three groups of three each.

402


A. 802A - 62R-2,45-50 cm Basalt Core160 4000

10 20 30 40 50 60 70 80 90 100 60 120 180 240

B. 802A - 62R-2,45-50 cm Basalt Core160

140

Φ

on~ 120«α

<" 100

80

f * 4 Λ T j.n n \

c

-

-

1 1 I . I . I , ! , 1 i 1 .

10 20 30 40 50 60 70 80 90 100 50 100 150 200

C. 802A - 62R-3,4-12 cm Basalt Core

| 140

Φσ>

<- 120Φ

CO

< 100

-

-

-

10 20 30 40 50 60 70 80 90 100

3 9Ar released (%)

800

600

200

o " " •0 50 100 150 200

3 9 A r / 3 6 A r

Figure 6. 40Ar/39Ar whole rock incremental-heating experiments on three samples from Hole 802A, Leg 129. Age spectra are shown on the left, isochrons onthe right.

403

M. S. PRINGLE

140-E 160'E 180'E 16O'W 140"W 120"W

40*N

2O'N- -20'N

2O'S

40'S

-40'N

20'S

•40'S

140'E 160-E 18O'E 160#W 140'W 120 W

Figure 7. Locations of the melting anomalies which formed the eleven sites of Mesozoic volcanism shown in Figure 1, and modelhotspot traces left on the Pacific plate as the plate moved over each of the locations since those sites were formed. Model hotspottraces calculated using the rotation poles of Duncan and Clague (1985) and ages from this study for Sites 800, 801, and 802; fromMahoney et al. (in press) for Sites 289, 803, and 807; from Pringle (1992) for Himu, Hemler, and Hole 462A; and from Davis et al.(1989) for Erikub and Ratak. Figure drawn using the GMT mapping utilities of Wessel and Smith (1991).

404

20. radiometric ages of basaltic basement … ages of basaltic basement 800 " " "

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