KRb ratios in some volcanic rocks from Mauritius, Indian Ocean

1573

Notes ence from other amino acid derivatives, the isovaline derivatives could not be resolved. Resolution was obtained, however, on columns coated with Dexsil 400 GC, but the retention times of the diastereomeric derivatives of uamino-n-butyric acid were similar to the retention times for the isovaline derivatives. Consequently, thin-layer chromatography on cellulose of the amino acid~on~ining fraction from the meteorite was carried out to separate isovaline from a-amino-n-butyric acid. The isovaline prepared by this procedure was derivatized and chromatographed without interference on the gas chromatographic column coated with Dexsil 400 GC. Figure 1a shows the resolution obtained when a racemic isovaline standard as the N-TFA-So+)-2-pentyl ester derivative was chromatographed on the Dexsil4CO GC phase. Because baseline resolution was not achieved (91% resolution), the areas measured for each peak were not equal as would be expected for racemic mixtures for which complete resolution of peaks is obtained. However, the separation and area ratios were reproducible. Duplicate analyses yielded average values of R( -) 526%. S( +) 47.4% with a deviation of + 0.5%; R(-) to S( +) ratios were 1.11 + 0.02. Figure lb shows the gas chro~togram of the meteorite fraction containing isovaline that had been derivatized to form the N-TFA-S(+)-2-pentyl esters. The area of R( -) is 52.2% and S(i) is 4X:!; the ratio of R(-) to S(+) is 1.09. Figure Ic shows the gas chromatogram resulting from co-chromatography of the derivatized meteorite fraction containing isovaline and the derivatized racemic isovaline standard (Fig. la). Area calculations show 52.1% R( -) and 47.9% S(+), and the area ratio is 1.09. These results taken as a whole indicate that isovaline in the meteorite is present as a racemic mixture. To determine whether isovaline could undergo racemization, R( -)-isovaline [9603% R(-) and 3*97?; S( +)] was treated with alkali as previously described. Normally, chiral amino acids, having a hydrogen atom on the cl-carbon atom, will show a significant degree of racemization when treated under these same conditions. The isovaline reisolated from this alkaline treatment contained 95.97% R( -) and 4.03x, S( +). These determinations are well within ex~rimen~l error and show that no measurable racemization occurred. These results suggest that the stereoisomeric forms of isovaline are stable, at least under conditions which cause racemization in the usual protein amino acids. The above observations indicate that isovaline from the meteorite (1) is present as a racemic mixture, (2) was ori-

C~OL%I~K.I ct Cosmochimiw

K/Rb

ginally synthesized as a racemic mixture, and (3) was not involved in the process of racemization. If isovaline was originally synthesized as a racemic mixture, the other chiral amino acids in the meteorite were also probably synthesized as racemic mixtures. If this conclusion is correct, then it is quite unlikely that the amino acids could have arisen by means of racemization of an original population of amino acids containing an excess of one or the other enantiomeric forms. Rather, the finding of racemic isovaline in the Murchison meteorite supports the contention that in this meteorite the amino acids were originally synthesized as racemic mixtures and, therefore, are probably products of an abiotic, extraterrestrial synthesis. Acknowledgements--We

thank CARLETON B. MOORE, Center for Meteorite Studies, Arizona State University, for providing the sample of Murchison meteorite.

REFERENCES GREENSTEIN J. P. and Wr~rrz M. (1961) Chemistry of the Amino Acids, Vol. 3, p. 2575. John Wiley. KVENVOLDEN K. A., LAWLESSJ. G. and PONNAMPERUMA C. (1971) Nonnrotein amino acids in the Murchison meteor& Pr& Nat. Acad. Sci., U.S.A. 68. 486490. LAWLE.I+~ J. 0. (1973) Amino acids in the Murchison meteorite. Geochim. Cosmochim. Actn 37. 2207-2212. NEUBERGER, A. (1948) Stereochemistry of amino acids. In Advances in Protein Chemistry, Vol. IV, (editors M. L. Anson and J. T. Edsall), pp. 2977383. Academic Press. PARSONSJ. W. and TINSLEYJ. (1960) Extraction of soil organic matter with anhydrous formic acid. Soil Sci. Sot. Amer. Proc. 24, 198-201. POLLOCKG. E. (1972) Resolution by gas-liquid chromatography of diastereomers of five nonprotein amino acids known to occur in the ~urchison meteorite. Anal. Chem. 44. 2368-2372. POLLOCKG. E. (1974) Correction. Resolution by gas liquid chromatography of diastereomers of five nonprotein amino acids known to occur in the Murchison meteorite. Anal. Chem. 46. 614. POLL~CKG. E. and MIYAMOTOA. K. (1971) A desalting technique for amino acid analysis of use in soil and geochemistry. Agr. Food Chem. 19. 104-107. STEVENSON F. J. and CIIENGC. N. (1970) Amino acids in sediments: revovery by acid hydrolysis and quantitative estimation by a calorimetric procedure. Geochim. Cosmochim. Acta 34. 77-88.

Acta. 1975. Vol. 39. pp. 1573 to 1576. Pergamon Press. Printed in Grwt

Britain

ratios in some vulcauic rocks from Mauritius, Indian Ocean A. N. BAXTER* Department of Geology, University of Edinburgh, Edinburgh EH9 3JW, Scotland (Received

24 September

1974; accepted

in revised form 22 April 1975)

Abstract-K/Rb

ratios in Mauritian Qlder Series basalts range from 200 to 350. A fall to around 175 in trachytes probably relates to kaersutite fractionation. Highly variable ratios in Jntermediate

*Now

at: Department

of Geology, City of London Polytechnic, Jewry Street. London EC3N IND, England.

Notes

I.574

(126-686) and Younger (158-691) Series basalts refect wide fuctuatlons m Rb initial abundance levels. K/Ba, K/Sr and Rb/Sr ratios show similar behaviour, attributed to small scale (< lo”‘,) melting in the upper mantle.

OVER THYlast decade considerable attention has been directed on the relationship between potassium and rubidium in basic and ultrabasic rocks in an attempt to evaluate its usefulness in understanding petrogenetic processes. GAST (1965, 196X) has shown that K/Rb ratios in basic lavas are likely to be controlled by the nature and amount of partial melting in the source material and the degree and type of subsequent crystal fractionation. while several workers have suggested that mantle inhomogeneity may account for observed variations within and between basaltic provinces (GUNN, 1965; HART, 1964;HART et al., 1965: STUEBER and MURTHY, 1966). This paper presents K/Rb ratios for 223 volcanic rocks from Mauritius, one of the Mascarene island group of the western Indian Ocean, analyzed in the course of a general investigation of the petrology and geochemistry of the island (BAXTER, 1972). Apart from some peripheral reef deposits, the sub-aerial portion of Mauritius is entirely composed of volcanic rocks of Pliocene--Pleistocene age (MCDOUGALL and CHAMALAUN. 1969). Three lava series separated by major unconformities are recognised (BAXTER, 1972), the Older Series? represented by the deeply eroded remnants of a major shield structure and comprising a highly differentiated transitional basalt-trachyte suite (BAXTER. in press a). a subordinate Intermediate Series formed of alkali basalts and basanites. and the Younger Series. a voluminous sequence of mildly undersaturated alkali olivine basalts erupted from localized vents along a major fissure running N.N.E.-S.S.W. through the centre of the island. Figure 1 summarizes the range of K/Rb values in the three series. The bulk of the basic lavas of the Older Series have ratios between 200 and 350, rather lower than the transitional tholeiites of neighbouring Reunion (cu. 400). (MCDOUGALL and COMPSTON,1965) and considerably less than the mean value of ca. 500 reported for the shieldforming Hawaiian tholeiites (LFSSINGet al., 1963). A decline in K/Rb ratios with increasing differentiation is apparent in the Older Series volcanics, to a mean of ca.

175 in the trachytes. Similar trends are documented from many volcanic provinces (ERLANK. 1968) and these are normally ascribed to persistent discrimination against Rb in favour of K in crystallizing lattices. particularly in feldspars. although STRONG (1972) has suggested that volatile transport may be a significant factor. In the Older Series the decline is first apparent in hawaiitic compositions and correlates closely with the appearance of kaersutite as a phenocryst phase. HAKT and ALDRICH (1967) have shown that amphiboles in general preferentially concentrate K with respect to Rb, and KESSON and PKICF (1972) have demonstrated ratios of 100&4ooO in some kaersutites. Clearly even limited fractionation of kaersutite from liquids with normal basaltic ratios would produce a trend to decreasing K/Rb values and it is probable that this may have contributed to the decline evident in the Older Series. In contrast to the Older Series, the relatively undifferentiated basic lavas of the Intermediate and Younger Series show a much wider range of K/Rb values, from 126-686 in the former and 158-691 in the latter. extending into abnormally low ratios for oceanic basaltic rocks (c.f. GAST. 1968). As these lavas are petrographically extremely fresh. have relatively low water contents (0.5- l.O”/, HzO) and generally low oxidation ratios (FezO,/FeO + FeZO, == 0.1 -0.2). there seems little doubt that these variations are primary, rather than induced by secondary alteration processes. The only similar example of such low ratios in oceanic basaltic rocks known to the author. is in a series of basanites from Moheli. one of the Comores Islands group of the Indian Ocean (STRONG. 1972). In this case, however, the low K/Rb ratios are ascribed to a relative potassium deficiency, caused by amphibole fractionation. Both Mauritian series show marked high pressure enrichment trends at a point around IO”,, MgO for K.Rb and the other incompatible elements (BAXTER, 1972). FOI example, in the Intermediate Series KZO ranges from 038 to 1.33’?& Rb 446ppm. Ba 144571 ppm, Sr 25% 595ppm and Zr from 79 to 198ppm, while in the Younger Series K ,O varies from 0.16 to 0.59”~,. Rb i-21 ppm. Ba 85 Table

600

I. KZO, Rb, K/Rb. KiBa and K/Sr levels in selected Intermediate and Younger Series lavas X,Rb

5oot

.

100

0

03

IO

15

2.0

25

3cl

3-5

,

4.0

45

5 0

Wt %K t lg. 1. K/Rb ratios of Mauritian volcanics, plotted against K. Solid circles, Older Series; open triangles, Intermediate Series; open squares, Younger Series.

Analyst

A. N. Baxter.

Notes

1575

3 IOppm, Sr 193457 ppm and Zr from 63 to 125ppm. al. (1963). The precision of the method is estimated at betThere seems little doubt, however, that the variation in ter than 3% at 0.25% K,O. Rb was also determined by K/Rb ratios reflects relative fluctuations in Rb initial abunX-ray fluorescence, on loose undiluted rock powders dance levels rather than in K (Table I). in particular, the ground to less than -200 mesh B.S., held in special conYounger Series lavas, a distinctly potassium-per suite iainers with a replaceable mylar film base. Operating concompared with world-wide alkali basalt averages compiled ditions used were: 80kV 20mA.W tube. LiF crystal. scintilby MAN~~N(1967), are associated with some remarkably lation counter and a count time of l20 see,-and results low Rb concentrations. Petrogenetic schemes have been were calculated using the peak/background technique of put forward previously to account for the origin of the ANDERMAN and KEMP (1958). The accuracy of the method Intermediate and Younger Series (BAXTER,1972) based on is calculated at + 1ppm in the range 3-20ppm, f2ppm variable amounts of partial melting in the upper mantle at 6Oppm, and precision at 7% ai 30ppm- and 8%. at and high pressure eclogite fractionation. Whilst capable 11 oom. U.S.G.S.. Canadian Dem. of Enerav and Edinof explaining gross incompatible and major element variaburgh University standards with comparable K and Rb tion, these cannot readily account for the observed range levels were used throughout the period of analysis. of K/Rb values if ideal incompatible element behavior is Acknffw~~dge~nts-.---This study was carried out as part of assumed for both potassium and rubidium. Hypotheses which attempt to explain variation in ratios through the a general investigation of the volcanic rocks of the western involvement of amphibole, either by partial melting of an Indian Ocean, initiated by B. G. J. UPTON of Edinburgh upper mantle containing trace amounts of amphibole (e.g. University and W. J. WADSWORTH of Manchester UniverHART et al., 1965). or through subsequent amphibole frac- sity and sponsored by N.E.R.C. The authors research was tionation would appear to be ruled out in this case by completed during the tenure of a N.E.R.C. research stuthe lack of any consistently changing relationship between dentship. absolute K content and K/Rb ratios. The possibilitv that REFERENCES the variation may be a function of mantle ‘inhomogeneity ANDERMANK. and KEMP J. W. (1958) Scattered X-rays would seem highly unlikely, bearing in mind the intimate association of lavas with differing ratios in the field, and as internal standards in X-ray emission spectroscopy. Anal. Chem. 30, 1306-1309. the extent of local mantle inhomogeneity this would imply. BAXTERA. N. (1972) Magmatic evolution of Mauritius, The factors controlling the behaviour of the incompatwestern Indian Ocean. Unpublished Ph.D. Thesis, Uniible elements and their partitioning between crystals and versity of Edinburgh. liquid during partial melting in the upper mantle are only incompletely understood, and it has been pointed out by BAXTERA. N. (in press a) The petrology of the Older Series lavas from Mauritius, Indian Ocean. GAST (1968) that for less than 10% melting their enrichBAXTER A. N. (in press b) Petrogenesis of alkaline basalts ment factors are likely to be highly variable and unpredicfrom Mauritius, Indian Ocean. table. It would seem possible that the variable ratios ERLANKA. J. (1968) The terrestial abundance relationship encountered in the Intermediate and Younger Series may between potassium and rubidium. In Ori@ and ~istr~b~result from similar small degrees of melting. The wide varition ofrheElements, (editor L. H. Ahrens). Pergamon ations seem in other incompatible element ratios, such as Press. K/Ba and K/Sr (Table i), which one would also expect to change during such an event, appear to support this GAST P. W. (1965) Terrestial ratio of potassium to rubiview. Rb/Sr ratios in these lavas (BAXTER,in press b) disdium and the composition of the earth’s mantle. Science play a wide range of values, from 0.012 to 0.102 in the 147. 858-860. Intermediate Series, and 0.01 I to 0.058 in the Younger SerGAST P. W. (1968) Trace element fractionation and the ies, rising linearly with increasing Rb levels and with the origin of tholeiitic and alkaline magma types. Geochim. overall degree of incompatible element enrichment. As Rb Co&chim. Acta 32. 1057-1086. is expected to partition into the liquid phase more readily GREEN D. G. and RINGW~~D A. E. (1967) The genesis of than Sr during partial melting, these trends may refledt basaltic magmas. Contrib. mineral. Petrof. 15. 103-190. sequential. gradually increasing degrees of small-scale meltGUNN B. M. (1965) K/Rb and K/Ba ratios in Antarctic ing in the upper mantle. Alternatively, the variation could and New Zealand tholeiites. J. Geonhvs. . . Res. 70. 6241arise at some subsequent stage during the evolution of the 6247. magma. The postulated deep level enrichment processes HARRISP. G. (1957) Zone refining and the origin of potasof ‘zone refining’ (HARRIS,1957; HARRISand MIDDLEMOST, sic basalts. Geochim. Cosmochim. Acta 12. 195-208. 1970) and ‘wall rock reaction’ (GREEN and RINGWOOD. HARRISP. G. and MIDDLEMOST E. A. K. (1970) The origin 1967). both of which essentially involve continuous smallof kimberlites. Lithos 3. 77-88. scale melting of surrounding mantle material as the magma HART S. R. (1964) Ultramafic rocks of St. Pauls Island. rises towards the surface, could also give rise to the obCarnegie Inst. Wash. Yea&. 43. 330-331. served variations in these lavas. High pressure processes, HART S. R., ALDRICHL. T., TILTON F. R., DAVIS G. L., however. such as eclogite fractionation (O’HARA, 1968) KR~GH R. E. and YAMAGUCHI M. (1965) Potassiumwhich apparently discriminate equally against all the inrubidium studies of ultrabasic rocks in Japan. Carnegie compatible elements, cannot account for such variations. Inst. Wash. Yearb. 64. 293-296. HART S. R. and ALDRICHL. T. (1967) Fractionation of Aual)?tical techniques K/Rb bv amphiboles, implications regarding comThe K content of the lavas was determined by X-ray positon. icier& 155. 3255327. fluorescence, using a Philips PW1212 automatic KES~~N S. and PRICE R. C.119721 The maior and trace spectrometer, PE crystal, Cr tube, gas flow counter and element chemistry of kaersutite and its bearing on the a counting time of 30 sec. Samples were prepared for petrogenesis of alkaline rocks. Contrib. Mineral. Petrol. analysis using the fusion technique described by ROSE et 35, 2 15-210.

Notes

1576

LES%NGP.. DECKERR. W. and REYNOLDSR. C. (1963) Potassium and rubidium distribution in Hawaiian lavas. J. Geophvs. Rrs. 68. 5851-5857. MANSON’V:(1967) Geochemistry of basaltic rocks: major elements. In Basafts: the Poldervuart Treatise m Rocks of‘ Basaltic C~~p~sit~~~, (editors H. H. Hess and A. Poldervaart). Interscience. MCDOUGALI.i. and COMPSTONW. (1965) Strontium isotope composition and potassium-rubidium ratios in some rocks from Reunion and Rodriguez. Indian Ocean. Nature

207, 252-253.

MCDOUGALLI. and CHAMALAUN F. H. (1969) Isotopic dating and geomagnetic polarity studies on volcanic rocks from Mauritius, Indian Ocean. Bull. Grol. Sot. Amer. 80. 1419.-1442.

M. J. (196X) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth-&i. Rev. 4. 69-133. Ross H. J., ADLER 1. and FLANAGANF. I. (1963) X-ray fluorescence analysis of the light elements in rocks and minerals. rlppI. Specrrosc. 17, 8 f -85. STRONGD. F. (1972) Petrology of the island of Moheli, western Indian Ocean. Bull. Geol. Sot. Amer. 83. 389406. STIJEHEK A. M. and MUR~‘HYV. R. (1966) Potassium:rubidium ratio in ultramafic rocks: differentiation history of the upper mantle. Science 153. 740-741. O’HARA

Contents of eleven trace elements in ureilite achoudrites C. M. Br~z,? M. IKKAMUDDINand M. E. LIPSCHI~TY.* Department of Chemistry. Purdue University, West Lafayette. Indiana 47907, U.S.A.

Abstract---We determined Ag, Bi, Cd. Co, Cs, Ga, In. Se. Te. TI and Zn in the 6 ureilite achondrites by neutron activation analysis. All 11 elements are depleted below Cl levels and their characteristic abundance pattern differs substantially from those of chondritic groups. Thus ureilites do not represent a simple mixture of volatile-rich chondrites with achondritic material but perhaps cosmochemicallyfractionated achondritic material and a late ‘distillate’ of mobile elements. CQrri., 1974). These differences can be reconciled by ad hoc assumption of large-scale mixing of carbonaceous chonTHE:SIX known ureilite a~hondrit~s (Table I) are excepdrites with more evolved asteroidal material during the tionally carbon-rich, their C contents of 2-42, approaching high-pressure phase of the collision process (VDOVYKIN. those of Cl chondrites (VDOVYKINand MOORE, 1971). 1970) but this time is extremely brief. Furthermore, carMineralogicdlly ureilites are unusual especially in the presbonaceous chondrites are rich in other volatile elements. ence of C as graphite (and hexagonal chaoite), diamond If the carbon in the postulated carbonaceous chondritic (and hexagonal lonsdaleite) and small amounts of organic parent material remained sufficiently coherent to form material (LIPSCHLITZ and ANDERS, 1961a.b; LIPKHUTZ, visually-detectable diamond, these other elements should 1964; VDOVYKIN,1970. 1972; VD~VYKINand MWRE 1971). also be present in ureilites unless they are later lost. This association and other properties led to the suggestion Recently we found that substantial proportions of many (LIP~CHUTZand ANDERS,I96la,b: LIPSCWVTZ, 1964) that trace elements are lost during extended (7-29 days) heating diamonds in ureilites formed by shock during collisionof geologic material in a low-pressure environment at teminduced breakup of their parent body and apparently this peratures >41x)“C. While the degree of loss of a particular is now a consensus. element depends on the meteoritic or geologic sample The origin and chemical nature of the pre-shock parent (IKRAMUUDINand LIPSCHIJTZ,1975; IKRAMUDDINet al., material is debatable; it might be that this material was 1975a) and ambient atmosphere (IKRAMUDDINet al.. a direct primordial condensate (ARRHENIUSand ALFV~N. 197Sb), there are certain general patterns. After decompres1971; WIIK, 1972) or a mixture, perhaps of known meteorision of shock-loaded material residual temperatures can tic types (VDOVY~IN,1970). Coidens&ion models for ureibe quite high and, in meteoroidal material, can persist for lites have substantial difficulties (WKNKEet al.. 1972) and extended periods producing recognizable alterations (TAYmixing seems to us more attraciive conceptually. Among LORand HEYMANN,1971; JA~Nand LIPSCHUTZ,1971). Thus the known sorts of meteorites, carbonaceous chondrites post-shock conditions in fragments of the ureilite parent are the most obvious choice for a C-source but there are body might be similar to conditions in our heating expersubstantial mineralogic and major element differences iments (IKRAMUDDI~Y. and LIPSCHUTZ.1975; IKRAMLIUUIN between these and ureilites (VDOVYKIN,1970; MCCARTHY it (II., 1975a.b) and the trace element abundance pattern of ureilites could reflect the loss pattern of their parent * Also Department of Geosciences, Purdue. Author to material. There exist few trace element data for ureilites, particularly for volatile/mobile elements (VLIOVYKIN, 1970; whom correspondence should be addressed. WANKEet al., 1972; GILLCMet al.. 1972) and we therefore t Now at Chemistry Department. Loras College. Dubudecided to determine particularly important elements---Ag. gue. Iowa 52001. U.S.A. INTRODUCTION