EARTII AND PLANETARY SCIENCE LETTERS 5 (1968) 20%210. NORT!bHOLLAND PUBLISIIING COMP.. AMSTIqRDA~dl

23OTIh DAT~tNG O F V O L C A N I C R O C K S : A C O M M E N T

Ci .ALLEGRE Groupe de,Recherches G~ochimiques Louis garrab~ et lnstitut de PtLvsique d ~ Globe,

Facultb des Sciences de Paris. 9. Quaa St. Bernard, Peris V~me

Received 31 ,October 1968

In a recent paper, Taddeucci, Broecker and Thurber [11 have made several measurements of uranium dis- equilibrium series in Separated rainera j, volcanic rocks.

They concluded that the hornblende-glass pairs are the most useful milzeral pairs to give an age ofvolean. ism and they proved that except in one case this meth-

od agrees moderately with 40Ar/40K dating. 1 should like to suggest another wa) in which these resuhs can be utilized.

The disequilibrium equation of" 230Th (notation in activity) is;

bin

23e u I - 23~rh

Fig. I .

210 C.J.ALLEGRE

(230Th)_ (238U) + (230 Th} exp.Xlot (232Th) (232Th) (I-exp-X/ot) (23.Th)o

If we suppose that at the beginning of the system tl~ere is an isotopic homogeuization between different minerals we can plot the experimental results on a

('/-!'~'0Th) l Ve'~) Ii L(232Th)J ' L( 232T o J }

diagram as is done for Rb and Sr data. Upon crystallization the different minerals (corres-

pondhlg to different U/Th ratios) fall aloag a horizon- tal straight line.

After a time t the different points define a linear array whose slope is ( ]-exp-k/ot) and which has an in~ercep~t of (230Th/232Th)~;~lot. The only diff,.'rencc from the 87Rb-87Sr pIot is that, in this case, the inter. cept is a function of time.

I f we plot the resut!ts of Taddeucci et al. on such a dia- gram we observe that: Quartz -. olivine and hornblende

fall along a single straight line. The slope indicates an age of 25,000 years and the intercept is (230Th)/ (232Th)o = 1.008.

The glass point falls below this straight line. I suggest a two s~age mechanism for this volcanism:

I ) Crystallization of mineraL,; at depth. 2) Eruption with partial meh:ing of minerals. The joint glass-hornblende line gives approximately the erutption age. The mineral-isochron gives the age of crystallization of minerals at depth.

Several other measurements are necessary to prove this assumption.

REFERENCE

[ 1 ] A. Taddeuccl, W.S.Broecker and D.L.Thurber, 230Th dat- ing of volcanic rocks, Earth Ptanet. Sci. Letters (1968) 338.