230Th dating of volcanic rocks

EARTH AND PLANEVARY SCIENCE LETTERS 3 (1967) 338-342. NORTH-HOLLAND PUBLISHING COMP ., AMSTERDAM

230Th DATING OF VOLCANIC ROCKS

Adriano TADDEUCCI *, Wallace S. BROECKER and David L. THURBER Columbia Universitj% Lamont Geological Observatory, Palisades, New York, USA Received 15 September 1967

Measurements of 2380, 232Th, and 23 0Th have been made in separated phases from four of the Mono Craters volcanics and from one of the Inyo Craters. The results show that sufficiently large differences in U/Th ratio exist between the glass and hornblende separates to allow dating of these eruptions. Suggested ages for the Mono Craters range between 2000 and 10,000 years, and show a general agreement with K-Ar ages and geological evidence . The Inyo Crater sample has an apparent age of 35,000 years, ten times higher than the K-Ar age.

1 . INTRODUCTION '30Th (ionium), with its half-life of 75,200 years [ I ] , appears to provide a useful tool for the chronology of rocks ranging in age from a few thousand years to about 200,000 years . The first attempt to use U-Th disequilibria for age measurements of recent volcanic products has been reported by Cerrai, Dugnani Lonati, Gazzarrini and Tongiorgi 121, who worked on zircon and ilmenite from a beach sand, assumed to be natural concentrates of resistate minerals from a nearby tuff formation . More recently Kigoshi [31 has reported an ionium dating technique using leach solutions of whole rock samples. Our experiments have been carried out on rhyolitic volcanic glasses containing phenocrysts of quartz, sanidine, plagioclase, hornblende, and olivine, from the Mono Craters region of California . These rocks provide a particularly interesting study because they have been dated by Dalrymple [41 by means of K-Ar techniques. His most reliable ages. fall in the range 6000 to 11,000 years. Since the K-Ar method is subject to a * On leave from Istituto di 3eochimica della 0riversiti di ltoma, Città Universitaria, Roma, Italia .

number of potenti:il problems in such young rocks, ages based on 238îJ and 230Th measurements would support their validity . 2. METHOD The age t of a mineral can be calculated from the expression : 230*Th = 230Th .- 238U = [230Tho _ 238U1 e-At where 230* Th is the excess 230Th; 230Th° and 230 1h are, respectively, the total activities of ionium at the time of crystallization and at the time of measurement ; 238U is the activity of this nuclide, and A is the decay constant of 30Th (i.e., 0.693/75,200 years) . Since this equation contains two unknowns, t and 230Th°, it cannot be used immediately. 230Th° can be determined in the following way : uranium and thorium isotopes are measured in two different minerals coexisting in the same rock. The assumptions are made that : (1) both minerals crystallized from the magma at the same time (or, at least, that the gap of time between the crystallization of the two minerals is negligible with respect to the age of the rock), (2) both minerals had the same 230Th/232Th ratio at the moment of the crystallization . Best results will be ob-

230Th DATING OF VOLCANIC ROCKS

tained if the 238U/232Th is greatly different in the two minerals . We are therefore able to write a system of two equations (one for each mineral) in which the only two unknowns are t and the initial ratio (230Th/ 232,ß)o . We have 230Th ` r(230Th) (23KU ', ~~t 0 232 I A L232 - \232~/A ] 230Th * _ 23 2Th B

230Th o _ 238U 23 2Th) 23 2

at

/B

Recalling that 232'

I* -\32~/-\232Th>

o and eliminating ,30Th,) solving for t z3 2T 238 U \ 232h/g - \232Th r s l \2~32~h/A-\2~3 ~/B

J (230Th/232Th)o ratio An expression for the initial by elimination of may also be obtained e-At. 230T h 0 _ IF 230Th X 238U 232Th 232Th ) 232Th B )A] _ L\23~/A X \~?332Th B X

238 230Th +(232 232, )B ,)AJ 230Th ) ( [('iJ2Th A ~2Th

_i

33 9

3. ANALYTICAL PROCEDURE The activities of 238U , 232Th, and 230Th isotopes have been measured by of spectrometry, using an isotope dilution technique. Isolation and separation of U and Th are accomplished by means of ion exchange resins and liquid-liquid extractions. As the method has been described in detail in previous papers [5-71, we give only a brief resume . A proper amount of mineral sample (in general between 1 and 3 g separated from a 20-pound rock sample) is dissolved in HF and HC104, and 2 cm3 of a solution containing 22 8Th in equilibrium with 232U (a activity = 5.46 dpm/cm3) is added as spike . Aluminium and iron are precipitated with NH40H and redissolved so that the solution is 8 M in IM. After the iron has been extracted with isopropyl ether, the solution is passed through an anion resin column (Bio-Rad AG-1). Uranium is adsorbed, later to be eluted with 0.1 M HCl. The thorium fraction which passes through is treated with NH40H . Aluminium is removed by washing the precipitate several times with warm 3 M NaOH. The residue, containing the thorium, is dissolved in 4 M HCl and passed through a cation resin column (Bio-Rad AG-50W) for further purification . The adsorbed thorium is then eluted with 0.75 M oxalic acid . The eluate is evaporated to dryness with concentrated HC104 and HN03 in order to remove the oxalic acid and the residue is redissolved in HN03 (pH = 1 .5). The thorium is then extracted into 0.4 M TTA and finally mounted by evaporating the organic phase on stainless steel planchets . When a noticeable amount of iron is found in the uranium eluate, a second extraction with isopropyl ether is needed . After this operation, the uranium is purified by extracting into ethyl acetate from a 2 M HN03 solution saturated in aluminium nitrate and back-extracting into pure water. The final separation from any contaminating elements is obtained by extracting the uranium into 0.4 M TTA at pH 3.5 . The organic phase is then evaporated on stainless steel planchets in order to obtain a thin source mount. The yields obtained range between 40% and 80% for thorium and between 60% and 100% for uranium .

0.878

14.08

* Activity ratios

Age K-Ar

8200 ± 900 10,400 ±1400

0.934

0.843 ±0.029

8400 ± )Ann 9100 ±1000

0.947

19 .10

0.986

0.881

0.06

0.7 38.1

Hornblende

4700 ±2500

~

0.859 ±0 .022

0.944

0.045

0 .962

0.949

6.1 20.1

0.7 50.5

Glass

5.1

Hornblende

Crater Mtn.

230Th AgeT10,500± 1800

2301h *

238U

230Th*

232Th

23011, *

232,h

U (Ppm) Th (ppm) 238U *

Glass

1st Dome N. of Crater Mtn .

0.856

15 .56

0.843 ±0 .025

0.054

0.7 36.7

Hornblende

6800 ±,7n00

1800 ±2000

1.31

0.852

0.651

3.9 18 .5

Lass

Ist Dome S. of Crater Mtn.

Mono Craters

Table 1

0.855

13 .145

0.848 ±0.034

0.065

0.6 29 .0

Hornblende

8700 ± 1400 7700 } 400 6900 ±1000

1000 ±3000

0.970

0.855

0.880

5.8 20 .6

Glass

Ist Dome S.W . of Punch Bowl

0.907

Î i 1

1

i ;

1.063

1 .17 j

~

4.6 ; 12 .4

Glass

6.15

0.7i2 ±0 .026

0.13

14.3

0.6

Hornblende Run 2

3900 ±1300 4.2% radiogenic Ar

39,300 ±6009 35,100 ± 2500

1.017

3.43

0.740 ±0.040

0.215

0.5 6 .9

Hornblende Run 1

Dome N. of Deer Mtn .

Inyo Crater

C

m

0

to w

Go

0 0

a a

0

ä

sr 30Th smaller RESULTS four dome Th error measured, inthe excess Craters 2two quartz to Mountain) 3gU, mineral the in the are hornblende pair activity system) first for following calculated ages age analysis from youngest minerals different from glass the hornblende concentration present 1their SPerhaps than samples present for two given dating shows with 141 is appeared, step but given present of from and the 230Th the AND ten Mono and ratio pair the on U ratio the samples either Crater until inarespect has of the i0ilial values, glass, dome had and comments times of phases the such hornblende here initial general the olivine Inyo (l and the we separates, in statistical DISCUSSION our been Craters ratio are the always results stformed 30,000 results Th the did same therefore, Mountain), could sanidine, 230Th and the pairs 230Th/2 dome N higher work Crater differs the the Mono as to (quartz, 230Th/232Th content duplicated glass in not fractions agreement of auranium initial the uranium of rock and shows same the are use of from result years counting Deer has Sthorium form is seem than the sample Craters isK-Ar by Uon close pertinent one for to always been and, 2Th 230Th/232Th within glass, aand four the aU Mountain, after of and offer which (or the to factor large our and, content from ages and The isPunch the to Th ratios (dome difference is whereas eruptions be error to become rhyolitic shown K-Ar olivine, to ratios but the close purposes equilibrium in the Th the glass-hornmeasured although find fairly excess decay isotope the the aconsiderof in thoriumlarger are analyses Bowl limit best In age and 2N DATING the are to K-Ar nearby out between the the beaand conthe higher of glass of ratio equiin is bevery The the senof and exIn the byOF VOLCANIC age Activity volcanic athe enrich Lamont a40Ar Mono the from should aided NATO likely this ±Ku reason such the form volcanic and Park, apparatus very 200 ROCKS thorium ratios of does Dalrymple uncertainty provided thorium methods mineral the procedures Lake as in products olivine Geological to years argon in high also collected Grant fellowship aid An allanite, constructing be Mono not the and eruption sediment was Th/U was effort be older used by and separates magma necessarily relative crystals much interpreting GA-344 should emphasized of Craters was provided radiocarbon contained and not the the in Observatory than in the issampling useful found, Sanidine the currently core chamber Th volcanic aided separated yield to and and Uand 2000 Aaron Adriano 2uranium hornblende enriched imply by collected operating not the information in upper Geological in that in*National gave years could aoperating the long samples which recognized Kaufman results being was separate that from both Taddeucci's an Mono limits Itmade phenocrysts before The less inherit age these the isand made no the the *Science Ross Survey, and possible on than on bottom evidence the piston Lake of mineral and olivine hornpossible 230Th for minereruptoreprothe the small HoroaTelivisit one age

230Th

341

4.

Table

The which table for hornblende) In with tent able blende sitivity Table measurements samples Inyo . The Both sistent 230Th/238U librium, 230Th "the

. .

0.2 5 .8 2.4 0.6

*

.

quite of mile of 2200 The fractions als that phase blende some solve It and of amounts could tion .

.

.

. asses, .

.

.

.W.

0.5 20.6 20.1 29.0

1.148 0.880 0.378 0.065

230TIi 232Th

0.950 0.970 2.397 13.145

1 .092 0.855 0.905 0.848

.

.

.

.

. .

.

. .

ACKNOWLEDGEMENTS .

thorium tween 230Th/232Th the . was closed bearing The

Quartz Glass Olivine Hornblende

230TIh 238U

.

close hornblendes than unsupported Except Deer 230Th Dalrymple In 1 . tween is sample -0 2

238U U Th (ppm) (ppm) 232Th

Brent Menlo vided Lung analytical spectrometers witz coring sediments . Financial Foundation to by

.S. . .

. .

342

A. TADDEUCCI, W. S. BROECKER and D. L. THURBER

REFERENCES (1 ] R. W. Attree, M. J. Czbc..-ll, R. L. Cushing and J. J. Pieron, A calorimetric determination of the half-life of Th-230 and consequent revision of its neutron capture crosssection, Can. J. Phys. 40 (1962) 194 . 21 E. Cerrai, R. Dugnani Lonati, F. Gazzarrini and E. Tongiorgi, il metodo ionio-uranio per la determinazione dell'eta dei minerali vulcanici recenni, Rend . Soc. Min. Ital. 21 (1965) 47 . 131 K. Kigoshi, Ionium dating of igneous rocks, Science 156 (1967) 932 .

[4j G. B. Dalrymple, Potassium-argon ages of recent rhyolites of the Mono and Inyo Craters, California, Earth Planet. Sci. Letters 3 (1967) 289. (5] A.. Kaufman, Th230-U234 dating of carbonates from Lakes Lahontan and Bonneville, Ph .D. dissertation, Columbia University, N.Y . (1964) . [6j T. L. Ku, An evaluation of the U234__U238 method as a tool for dating pelagic sediments, J. Geophys. Res. 70 (1965) 3, 457. [7j D. L. Thurber, Anomalous U2341U238 and an investigation of the potential of U234 for Pleistocene chronology, Ph .D . dissertation, Columbia University, N.Y. (1963) .