33. K/Ar AND 40Ar/39Ar DATING OF BASALTIC ROCKS FROMDEEP SEA DRILLING PROJECT LEG 59

John F. Sutter and Lawrence W. Snee, Department of Geology and Mineralogy,Ohio State University, Columbus, Ohio

INTRODUCTION

Six basaltic whole-rock samples from DSDP Leg 59in the Philippine Sea (Fig. 1) have been dated by a com-bination of K/Ar and 4OAr/39Ar techniques. Two sam-ples were collected from Hole 447A of the West Philip-pine Basin, three from Holes 448 and 448A on thePalau-Kyushu Ridge (Fig. 2), and one from Hole 450 inthe Parece Vela Basin. We followed a general schemefirst to identify samples that are potentially closed K/Arsystems and then to refine further the established age ofthese closed-system samples. Each sample of basalt thatshowed little or no alteration of K-bearing phases in thinsection was prepared and then irradiated with fast neu-trons to produce 39Ar from 39K (see Dalrymple and Lan-phere, 1971, 1974; and Lanphere and Dalrymple, 1971for details of this analytical technique). After irradia-tion, gases (including argon) were extracted from eachsample in six successively higher temperature steps, thelast of which was above the melting point of the sample(called the FUSE step). The argon contained in eachtemperature step was purified and isotopically analyzedin order to produce six sets of argon isotopic data versustemperature for each of the samples analyzed. Thesedata were then used to generate an age spectrum andisochron plot for each sample (see Figs. 3-5). Each rec-tangular box in the age spectrum plot represents theargon data for one temperature step. The width of eachbox is the percentage of the K-derived 39Ar released atthat temperature compared to the total K-derived 39Arreleased in all six temperature steps. The height of eachbox represents the analytical precision of the argon iso-topic data released at that temperature. The analyticalprecision represents a standard deviation calculated atthe 67% confidence level. If all temperature steps, eachcontaining more than 5°7o of the total K-derived 39Arreleased, have the same apparent age (using the criticalvalue test of Dalrymple and Lanphere, 1969), then theage spectrum is said to be "concordant" (Fleck et al.,1977). The spectrum is "discordant" if these criteria arenot met. The spectrum contains a "plateau" if adjacenttemperature steps, each containing more than 5% of thetotal K-derived 39Ar, are the same age (using the criticalvalue test just cited) and together constitute more than50% of the total K-derived 39Ar. The terms "disturbedspectrum" and "undisturbed spectrum" are only usedwhen a geologic interpretation is made. For instance, anage spectrum would be termed "undisturbed" if the pla-teau portion defined an age that, together with othergeologic information, could be interpreted as the crys-

120° 135= 150"

Figure 1. Site locations for DSDP Leg 59.

tallization age of the sample (hence a closed K/Ar iso-topic system). An age spectrum is termed "disturbed"if, for instance, it (1) has no portion that defines aplateau, or (2) it has a plateau portion, but what itdefines can be interpreted as having no geologic mean-ing or representing something other than the crystalli-zation age of the sample.

Isochron plots of radiogenic plus atmospheric ^Ardivided by atmospheric 36Ar versus K-derived 39Ardivided by atmospheric 36Ar (i.e., 40ArR+A/

36ArA versus39ArK/

36ArA) were prepared for most samples (e.g., Fig.3). The argon isotopic data from each temperature stepwere plotted, and a least-squares linear regression wasfitted to the data points. The slope of this line is the bestfit 40ArR/

39ArK ratio for the sample, and the intercept isthe initial ^Ar/^Ar ratio of the sample prior to thedecay of ^K to ^Ar. If no loss or gain of ^Ar or 36Aroccurred during or after crystallization of the sample,this initial ratio should be indistinguishable from the ac-cepted atmospheric value of 40Ar/36Ar—295.5.

729

J. F. SUTTER, L. W. SNEE

intercept values may be statistically differentfrom the atmospheric value and/or they may showmarked scatter about the regression line; such argonisochrons are indicative of a disturbed or open K/Arisotopic system.

Under ideal circumstances, samples that have un-disturbed K/Ar isotopic systems should have concor-dant ^Ar/^Ar age spectra as well as isochron plotscharacterized by strong colinearity of the data pointsthat define 4OAr/36Ar intercepts as indistinguishablefrom the atmosphere. If these two criteria are met, the"age" of the sample can be refined by multiple conven-tional K/Ar analyses.

The analytical techniques and constants used aresummarized in the Appendix.

ANALYTICAL RESULTS

In the following tables and figures we present the^Ar/^Ar and K/Ar age data by hole.

Holes 448 and 448A

Holes 448 and 448A were drilled on the Palau-Kyushu Ridge (Fig. 1); the strata penetrated are dia-grammatically represented in Figure 2. Three basaltsamples were selected for dating. Table 1 contains the^Ar/^Ar data, Table 2 contains the K/Ar data, andFigures 3 through 5 show the ^Ar/^Ar age spectra andisochrons. The uppermost sample (448-61-3, 78-85 cm)is a plagioclase-clinopyroxene-orthopyroxene-pigeonite-phyric basalt flow from Unit 13. It is our opinion thatthe age data from this sample from Hole 448 indicatesevere loss of ^Ar and that the isochron line is actually amixing line between potassium sites that have sufferedvarious amounts of ^Ar loss. Hence we feel that nogeologic meaning can be attached to the age of this sam-ple. Samples lower in the section (448A-62-1, 70-83 cmand 448A-64-3, 2-12 cm) are a plagioclase-clinopyrox-ene-orthopyroxene-phyric basaltic dike and an aphyricdike, respectively. The age data from these two samplessatisfy all the criteria for a reliable K/Ar system. Wefeel that with the data available, the "best age estimate"for each sample is an average of the 4OAr/39Ar age spec-trum and the conventional K/Ar data. Thus the "bestage estimate" for the sample from Core 62 is 31.45 ±1.52 m.y. and that for the sample from Core 64 is 34.88±1.60 m.y. It is our interpretation that these ages areprobably close to the crystallization age for these rockunits.

Hole 447A

This hole was drilled in the West Philippine Basin.Figure 6 is a generalized diagram of the rock columnrecovered from Hole 447A showing the positions of tworocks analyzed by the argon techniques. The 40Ar/39Ardata are listed in Table 3 and shown graphically inFigure 7. Unfortunately, none of the samples selectedfor dating meet the criteria for a closed K/Ar system,and it is our interpretation that none of the data havegeologic meaning.

Palau-Kyushu RidgeSite 448

nU

200

550

800

Unit

MarineSediments

-

Marine Sedimentsand

Vitric Tuff

* Λ

Basalt Flowsand

Pillow Lavas

Interbeddedwith

VolcaniclasticBreccias

withAbundant

Dikes and SillsNear Bottom

Age

Miocene

late Oligocene

__ _ — _ __ —

Disturbed- ^ — Ar Spectrum

middle Oligocene

— —31.5 m.y.-**— 34.9 m.y.

Figure 2. Highly generalized diagrammatic stratigraphic section ofHoles 448 and 448A showing position of dated samples (see arrowsin age columm). (Notice that between 200 and 400 m sub-bottomand 400 and 800 m sub-bottom the columm is broken to savespace. The uppermost sample from Core 61 of Hole 448 is at 550.3m sub-bottom and the lower two are from Cores 62 and 64 of Hole448A at 887.8 m sub-bottom and 903.1 m sub-bottom, respec-tively.)

Hole 450This hole was drilled in the Parece Vela Basin (Fig.

1). One sample was selected for dating; the 40Ar/39Ardata are listed in Table 4. The stratigraphic position ofthe sample is shown in Figure 8. The age spectrum isplotted in Figure 9, and the conventional K/Ar datalisted in Table 5. The data from this sample do not meetthe criteria for a closed K/Ar system; thus no meaningcan be attached to the age data. This is dramatically

730

K/Ar AND ^ A r / ^ A r DATING OF BASALTIC ROCKS

Table 1. 4 ( )Ar/3 9Ar data for samples from Holes 448 and 448A on the Palau-Kyushu Ridge.

Temperature(measured)

3 7 Ar/ 3 9 Ar(measured and

corrected)

3 9

3 6 Ar/ 3 9 Ar(measured)

ArK(% oftotal)

40(ArR 3 6 A r C a

(%) (

J. F. SUTTER, L. W. SNEE

a:<

CD

cr<

o

700

500

300

-650°C

525°CJ5soo^c^V**^

^ ^ " ^ FUSE

•̂ "*̂ Isochron Age =

( 4 0 Ar / 3 6 AR)0

1050°C

16.58 m.y.

= 341

900 -

700 -

CD

+ 500

<o

300

-

-

400

1050°

°C

c ^s

Isochron

(40Ar/36

650

800°C

Age =

Ar)n

= 32.26 m.y.

= 299

°C

50 100 150 200 250 100 200

3 0 I

1400 C

20

10 Total Gas Age = 21.42 m.y.

3950

Ar,,, Cumulative %

11050 Ci;

100

Figure 3. ^Ar/^Ar isochron diagram and age spectrum of Sample448-61-3, 78-85 cm.

60

50

I 40>J 308.

< 20

10

400°C

, .. :

FUSE-

1650 CJ800°C

|1O5O°C?

Spectral Age = 32.29 ± 2.17 m.y.

3950

Ar.., Cumulative %

100

Figure 4. 40AR/39AR isochron diagram and age spectrum of Sample448A-62-1, 70-83 cm.

732

K/Ar AND 40Ar/39Ar DATING OF BASALTIC ROCKS

500

400

300

200

-

-

400°C

-

FUSE

800°C

1050°C

- " ^ 5 2 5 ° C

Isochron Age

( 4 0 A r /3 6 A r ) o

= 33.01

= 301

650°C

^ ^

m.y.

West Philippine BasinHole 447A

Unit Age

35 70

Spectral Age = 33.40 ± 2.61 m.y

200-

MarineSediments

Volcaniclastic Breccia

Basalt Flowsand

Pillow Lavas

Early Miocene?

Middle Oligocene

Disturbed- ^ — Ar Spectrum

Disturbed- •*—Ar Spectrum

100

296.5

Figure 6. Diagrammatic stratigraphic section of Hole 447A showingposition of dated sample (by arrow).

Cumulative % Ar,, Released

Figure 5. 40Ar/39Ar isochron diagram and age spectrum of Sample448A-64-3, 2-12 cm.

Table 3. 4 0Ar/3 9Ar data for samples from Hole 447A drilled in the West Philippine Basin.

Temperature(°C)

^ A r / ^ A r(measured)

3 7Ar/3 9Ar(measured and

corrected)

Sample 447A-17-3, 144-148 cm (WR basalt)400525650800

1050FuseTotal Gas

15.8010.534.1214.1994.3506.533

1.2455.426

22.1928.9135.1484.26

447A-25-1, 28-33 cm (WR basalt)400525650800

1050FuseTotal Gas

583.3321.0243.4197.8

97.5964.80

28.0272.30

115.9133.4198.8576.0

36 A r / 39 A r(measured)

0.034340.024960.017360.016280.018270.03939

1.9571.0550.83530.68380.36270.3387

F

5.7543.5950.7821.7381.8091.752

7.46515.936.2827.0187.313

17.40

3 9ArK(°/o oftotal)

J16.9527.798.91

12.6624.429.27

J12.017.20

11.6718.2737.8213.03

4 0 % R 3 6 % C a

= 0.00603036.3834.0318.7140.6540.6725.39

0.995.91

34.7848.3152.3158.18

= 0.0065701.264.732.393.256.55

17.06

0.391.863.775.31

14.9246.30

Age(m.y.)

61.54 ± 6.6338.69 ± 4.108.48 ± 11.13

18.81 ± 6.2519.57 ± 3.5518.95 ± 7.6831.10

86.38 ± 101.4179.6 ± 118.372.96 ± 105.781.32 ± 86.4184.66 ± 34.79195.3 ± 62.76101.2

Note: See Table 1 for explanation of abbreviations.

733

J. F. SUTTER, L. W. SNEE

Table 4. 4 0Ar/3 9Ar data for samples from Hole 450 drilled in the Parece Vela Basin.

Temperature( ° Q

^Ar / 3 9 Ar(measured)

3 7Ar/3 9Ar(measured and

corrected)

450-36-3, 117-126 cm (WR basalt)400525650800

1050Fuse

Total Gas

9.40812.87512.92814.12011.1635.778

1.4528.001

22.76630.67830.90471.732

3^Ar/39Ar(measured)

0.0269040.0428160.0473250.0500020.0412620.038407

F

1.5710.8650.7791.8401.4770.197

3 9ArK(% oftotal)

38.4219.4313.357.909.17

11.74

4°ArR(%)

3 6 Ar C a

J = 0.00657016.686.685.94

12.7812.983.26

1.475.08

13.0816.6920.3750.80

Age(m.y.)

18.52 ± 2.9910.22 ± 6.209.21 ± 6.83

21.68 ± 10.6617.42 ± 8.772.34 ± 8.18

13.98

Note: See Table 1 for an explanation of the abbreviations.

Table 5. K/Ar data for sample from Hole 450 in the Parece VelaBasin.

Sample(interval in cm)[size fraction]

450-36-3, 117-126[#13, 80 to 60 mesh]450-36-3, 117-126[#12, 30 to 48 mesh]

Mineral

WR

WR

K

0.4040

0.3316

Moles ^ A T R

x 10~12

6.3863

4.2225

A r R •̂ ioo

11.4

14.0

Age(m.y.)

9.09 ± 0.20

7.33 ± 0.84

500

4

1