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Pergamon Geochimica et Cosmochimica Acta, Vol. 59, No. 10, pp. 2115-2130, 1995

Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved

0016.7037/95 $9.50 + .JO

0016-7037(95)00131-X

Precise determination of the isotopic composition of potassium: Application to terrestrial rocks and lunar soils

MUNIR HUMAYUN*.’ and ROBERT N. CLAYTON’.’

‘Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA ‘Department of Chemistry and the Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA

(Received August 4, 1994; accepted in revised$mn February 10, 1995)

Abstract-We detail a method for the precise and accurate determination of isotopic variations in the “K/ 39K ratio with a precision of ?OS%o (2u,). Purified potassium is chemically extracted from rocks, soils, minerals, or solutions by ion exchange chromatography. Complete chemical yields (>99.8%) are achieved in order to avoid laboratory-induced isotopic fractionations. The purified potassium is converted to a glass by melting with barium borate flux, and the resultant bead is mounted for ion probe analysis. The SIMS method utilized by the ion probe produces extremely stable Kf ion beams, with no measurable temporal variability in the isotope ratio. The instrumental fractionation is steady at about -4%0, and is corrected for by measurement of a standard. The measurement of gravimetrically prepared isotopic standards indicates that the method is accurate at the stated level of precision and free of egregious errors. Analysis of terrestrial samples including peridotite, basalts, granites, carbonatite, biotite schists, and seawater, indicate the complete absence of isotopic variations in 6°K among terrestrial materials at the OS%0 level. Application to lunar soils and a regolith breccia confirms previously observed large isotopic fractionation effects (Gamer et al., 1975a; Church et al., 1976). Some lunar soils, e.g., 14163, are shown to have large sample heterogeneity (=7%0), while others, e.g., soils and a regolith breccia at several Apollo 15 sites (Station 7/9), are homogeneous at the level of analytical precision. The presence of potassium isotopic effects in bulk soils (up to + 12.7%0 in this study) with magnitudes comparable to the Rayleigh fractionation factor ( 25%0) indicates that volatility during micrometeorite impact melting played a large role in lunar regolith formation. As much as 15% of the regolith potassium has been lost from the Moon, through the tenuous lunar atmosphere.

INTRODUCTION

Large, kinetically induced mass fractionation of the isotopic compositions of Mg, Si, and 0, and smaller effects in Ca and Ti, are known from some Ca-Al inclusions (CAIs) (Clayton et al., 1988). It has been experimentally demonstrated for a variety of compositions that this is the result of partial vaporization (Wang et al., 1994). If partial vaporization has played a significant role in determining the volatile element fractionations in meteorites and planetary compositions (Ringwood, 1966; Wanke et al., 1981), then kinetic isotope effects are to be expected in volatile elements, particularly B, Cl, S, K, and Zn (Humayun and Clayton, 1995). Since B, Cl, and S exhibit isotopic variability due to a variety of other processes as well (cosmic-ray spallation, low temperature alteration, etc.), an unambiguous test would be to analyze the isotopic composition of potassium and compare this with chemical depletions determined from K/La or K/U ratios (Taylor, 1979; Wanke and Dreibus, 1988; Palme and Boynton, 1993). Precise determination of potassium isotopes is an analytically challeng- ing measurement and the present paper details the method, tests of its accuracy, and some of its applications that lay the foundation for potassium isotope cosmochemistry (Humayun and Clayton, 1995 )

Equilibrium stable isotope variations are common in light elements that are covalently bonded (H, B, C, N, 0, Si, S,

*Present uddress: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA.

2115

Cl) but have never been reliably documented in elements of comparable mass that are ionically bonded, e.g., Mg, K, and Ca. The largest equilibrium effects are expected at low tem- peratures, and so mineral-solution partitioning of potassium gives a limit on the extent of equilibrium isotope fractionation. Kinetic isotope effects by definition are proportional to the inverse square root of the masses of the isotopic species involved, e.g., for atomic or ionic species of potassium, this would give (Y = 0.9753 or =25%0. Processes that could effect kinetic isotopic fractionation in potassium include evaporation and diffusion. There is no kinetic isotope effect to be expected for equilibrium (thermochemical) condensation.

Naturally occurring kinetic isotope effects in potassium have been reported in the lunar soils and in metasomatic rocks. The lunar soil is subject to intense micrometeorite bom- bardment, resulting in the formation of melt-bound aggregates of particles termed agglutinates (McKay et al., 1991 ). In lunar soils, heavy-isotopic enrichments of potassium of up to +8%0 have been found (Barnes et al., 1973; Gamer et al., 1975a), and effects up to = +20%0 were reported in agglutinate-rich fractions by Church et al. (1976). Light isotope enrichments in potassium of up to -30%0 were reported from within a few centimetres of potassium-metasomatizedgranitic contact rocks from South Africa by Schreiner and Verbeek ( 1965 ), Verbeek and Schreiner ( 1967), and Schreiner and Welke ( 1971). Small isotopic effects of < -2.5%0 were reported by Hinton et al. ( 1987, 1988 ) in feldspars from the Colomera BE iron meteorite, lunar breccias ( 14305, 14321) and a terrestrial occurrence, and attributed to kinetic effects

2116 M. Humayun and R. N. Clayton

for the extraterrestrial feldspars. A discussion of the many erroneous isotopic effects reported in the literature will not be attempted. The interested reader may consult Kendall ( 1960)

Potassium has three naturally occurring isotopes: ‘“K (93.2581%), 4”K (0.01167%), and 4’K (6.7302%). Apart from radioactive 40K (half-life = 1.28 Ga), all other radioactive potassium isotopes have short half-lives (43K = 22.3 h, 42K = 12.36 h), making a double spike based on radio- isotopes technically unfeasible. Due to the low abundance of 4”K and the presence of a significant interference from 40Ca, we can only measure the isotope ratio 4’K/‘9K. This makes it impossible to uniquely decide whether a measured isotopic difference in the 41K/‘9K ratio is due to mass-dependent fractionation or nuclear effects (extinct 4’Ca, cosmic-ray spallation, etc.), but nuclear effects are rare at best, and have already been investigated by a number of researchers using TIMS (Burnett et al., 1966; Gamer et al., 1975a; Begemann and Stegmann, 1976; Birck et al., 1977; Stegmann and Begemann, 1979). The TIMS precision for three-isotope determinations of potassium is severely limited to t5-20%o.

We have developed a mass spectrometric procedure to measure differences in the isotopic ratio 4’K/“K with a precision comparable to that of any element done by double spike TIMS (0.2%o/a.m.u.), and an accuracy as good as the precision. This paper reports the methods for precise and accurate determination of the stable isotopic composition of potassium, and presents results for terrestrial rocks of a variety of compositions as a test of the method. Lunar soils previously reported as fractionated (Gamer et al., 1975a; Church et al., 1976) were also analyzed and the results are discussed in terms of interlaboratory comparison, with some implications for the origin of the isotopic fractionations. A companion paper details the findings for a variety of chondrites, achondrites, and lunar materials and the cosmochemical consequences of these results (Humayun and Clayton, 1995).

EXPERIMENTAL PROCEDURES

Potassium was extracted from natural materials such as rocks, minerals, or seawater solids by acid dissolution followed by ion exchange chromatographic separation. The KNO, salt obtained was melted with barium borate to form a glass bead, mounted on a glass slide along with standards, polished, Au-coated, and analyzed by ion microprobe. A protocol was carefully followed during mass spectrometric measurement, with standards and samples alternately analyzed. The extraction and measurement procedures are described in detail below. An illustrated summary is given in Fig. 1. The factors affecting precision and accuracy are discussed in detail, and results are presented on gravimetrically prepared spiked standards as a test of precision and accuracy. The procedures in an early form were outlined by Humayun et al. ( 1991) and Humayun and Clayton ( 1993 ) , and further details are given by Humayun ( 1994).

Chemical Extraction

It is important to obtain relatively pure potassium to avoid matrix effects due to the variability of chemical composition between different samples, and to remove elements that interfere with potassium during mass spectrometry. There are no stable nuclides other than those of potassium at masses 39 and 41, and thus, there are no elemental interferences at these masses. Molecular interferences are important, since these are not adequately resolved by the AEI IM20 ion microprobe used in this study. The principal species are 23Na’hO’, =Mg’hO+~

4ZCa40Ca ++ , ‘“CaH+. and potentially 2xSi “B + The MgO+ interference requires K/Mg > 100. The chemical extraction

Procedure

Samples: Rocks, minerals, seawater

Sample - HF / twoa - ;z$t;;; + cation exchange chromatography

AG5OW-X8

90 ml

Na+ K+ Mg++ - ci s K

0 -I

- Volume eluted

Collected

t Ba borate glass + 2 % K

t Ion Probe grain mount

t Data collection, etc.

FIG. 1. An illustrated flow chart of the procedure used to extract, mount, and isotopically analyze potassium.

procedure outlined below gave Kh4g > 1000 for most samples. Large amounts of Al and Cr were found to adversely affect the matrix composition, and doubtlessly, this is true for any other element present in percentage level concentrations. The matrix effects of Al and Cr are discussed further, where relevant, in a companion paper (Hu- mayun and Clayton, 1995).

Sample dissolution

Rocks were dissolved in HF/HCI/HNO, acids, evaporated to dryness, and taken up in dilute HNO, to obtain a clear solution. Seawater was evaporated to dryness and reacted with concentrated HN03. Syl- vite and camallite salts were similarly converted to nitrate form. El- emental abundances were determined by AAS (see below).

Blanks

Blank levels of potassium were kept at about 0.2% of the sample potassium by the use of high purity reagents. Doubly distilled acids obtained from NIST (Moody et al., 1989) were used for extraterrestrial and low-potassium samples. Blank barium borate glass obtained by the fusion of Suprapur@ barium nitrate and Suprapur@ boric acid contained =20 pg/g potassium, which contributed 0.1% blank for a glass containing 2% sample potassium. The blank levels attained during chemistry were minimized as much as possible, but the technique is not very sensitive to blanks as long as blanks are less than several percent. It is reasonable to assume that the isotopic composition of the blank is identical to the terrestrial composition, and thus, addition of blanks to an unknown acts to dilute any isotopic difference where present. Because of the small differences involved, this experiment is fairly tolerant of blanks, i.e., 10% blank contribution to a 5%0 sample induces a lowering of the isotopic composition by only O.S%o, comparable to the analytical precision.

Atomic absorption .spectrcwnet~~

All analyses of aqueous solutions were carried out using a Perkin- Elmer@ 306 Atomic Absorption Spectrophotometer. The AAS tech-

High-precision isotopic analysis of K 2117

nique is very sensitive for Na (0.012 pg/mL), K (0.043 lg/mL), and Mg (0.0078 PglmL), the elements of principal interest. Potas- sium and sodium were measured by flame emission photometry using the emission lines at 766.5 nm (K) and 589.0 nm (Na) under standard operating conditions with air-acetylene flames. Magnesium (285.0 nm) and calcium (422.0 nm) were determined by absorption using a hollow-cathode lamp. Aluminum was determined using a nitrous oxide-acetylene (reducing red) flame, and was determined only in samples having high concentrations of Al. Calibration curves were prepared from measured intensities of elemental lines from diluted high- purity Spex TM solutions of known concentration. Precision was about l-2%, determined by repeated analyses of each sample and standard solution, and accuracy determined by measuring USGS standards and gravimetrically prepared solutions was better than 5%.

KNO,) were mounted as grain mounts using a 4 X 3 grid on a 1 inch glass slide, polished, coated with 0100 nm Au, and loaded in the sample chamber of the Chicago AEI-IM20 ion microprobe.

Sample size requirements

Ion exchange chromatography

The ion exchange procedure was modified from Strelow et al. ( 1970). A large ion exchange Pyrex’z’ ( 1992) or quartz ( 1993 ) column, ID = 2.2 cm, fitted with a glass fritted disc at the base, was filled with 90 mL of 100-200 mesh Bio-Rade AG 5OW-X8 sul- phonated polystyrene divinyl benzene cation exchange resin (wet bed). This column provided adequate separation of potassium from Mg with sample sizes of about I g of basalt, peridotite, or chondrite. A smaller quartz column ( 1993). ID = 1.1 cm, was similarly prepared with 11 mL of resin (wet bed), for use with smaller sample sizes (>50 mg of rock).

This technique was optimized for precision, not minimal sample size. The sample size required for precise potassium isotopic analysis was determined by the amount of potassium required for proper procedural handling (particularly ion exchange separation). The actual amount of potassium consumed by the ion probe beam (for = 10 ,um spot, 1 pm deep) was several picograms per point and less than a nanogram in total, but the beam accesses a very small amount of the total grain mount. Typically, a400 pg ( = 10 pmoles) of potassium were mounted in =25 mg bead. Smaller amounts of potassium ( = 10 pg. e.g., PCC-I ) were used for samples where less potassium was available. The trade-off is lower precision if the extraction and mounting were less than optimal.

Mass Spectrometry

Potassium was eluted with 0.5 N HNOl, at an elution rate of 3-5 mL per min, and collected in the 700-1000 mL region. Fractions were collected at 50 mL intervals between peaks to ensure a good separation of potassium from Na. During collection all fractions were measured by atomic absorption spectromehy (AAS) to determine the exact beginning and end points of the potassium peak (for complete recovery) and to ensure that the potassium fraction was free of any potential interferences from Na and Mg. Other elements were strongly retained by the column and were released after potassium had been completely extracted by eluting with either 4 N HNO, or 6 N HCI. For certain samples, some Cr and Ti were eluted in the potassium fractions, and for lunar samples and other low-potassium rocks, this contributed a significant matrix difference. The procedure was modified by the addition of oxalic acid as a complexing agent for the removal of Ti, Al, and Fe, following Gast et al. ( 1970). This procedure removed all observable Ti, but was not quantitative for Al. The problem, once recognized, was remedied by running the recovered potassium fraction through a small ( 11 mL) ion exchange column, effecting complete removal of Al and Cr from potassium. With this procedure, potassium can be quantitatively extracted with a column yield ~99.8% and with interfering elements at blank levels.

The glasses were analyzed using the Chicago AEI-IM20 ion microprobe (Banner and Stimpson, 1975; Steele et al., 1977; Hutcheon, 1982; Scatena-Wachel, 1986) at low mass resolving power (M/m = 300) The potassium concentration in the glasses (2% potassium) was sufficient to give a maximum allowable count rate of 500,000 counts/s (cps) for j9K with the entrance slit partially closed. The general operation of the ion probe is outlined briefly below, followed by the analytical protocols for the determination of potassium isotopes.

Ion microprobe

The AEI-IM20 at Chicago has a primary ion source consisting of a duoplasmatron that produces a focusable beam of I60 ions, mass filtered by the primary magnet. The primary beam of 160- ions strikes the sample with 20 KeV, 6-8 nA, to produce secondary ions that are extracted by a series of electrostatic lenses into a double-focusing Mattauch-Herzog geometry mass spectrometer and detected by an electron multiplier operating in pulse-counting mode. The sample is placed face down, and inclined such that the beam strikes the surface at 73”10’, and the secondary ions are extracted at 30”. The repeller wire is at a potential of 500-550 V above specimen potential ( 10 kV) and provides a deflection of the secondary ions into the extractor lens ( = 1600 V below sample potential). The extractor and dustbin lenses focus the secondary ion beam onto the entrance slit of the electrostatic sector of the mass spectrometer.

Ion Probe Mounting Measurement protocol

The extracted potassium was incorporated into barium borate glass prepared by melting together E. Merck Suprapure grade reagents, Ba( N03)2 and HIBO,. The in-house potassium standard was prepared from E. Merck Suprapur@ KNO,. Glass composition was ap- proximately ( BaO)c,.4 ( B,03)0.6, a eutectic at 869°C on the B,O,-rich side of barium tetraborate, with 2% potassium added. Barium borate was chosen over other forms of substrate, because of its absence of isobaric interferences, low-potassium blank, and resistance to hydration. High-resolution mass scans revealed the presence of “B ‘“B “O+ at m/e = 39 at -IO-’ of the “K+ peak, which is negligible.

Glass beads were prepared as follows: enough barium borate glass ( = IO-40 mg) to give 2% potassium for the amount of potassium available was weighed into a Au crucible (-30 PL volume). The sample KNOX solution was evaporated to several tens of PL volume in a Teflon@ FEP beaker, then carefully pipetted onto the glass pow- der and dried. The sample mixture was then placed into a muffle furnace at 900-950°C and heated for 10 min, exactly. This was sufficient to melt and homogenize the glass, and long enough to allow any NO, gases evolved from the decomposition of KNOl to escape. The charge was quenched in air. The glass thus formed was quite resistant to hydration for months in air, but was nonetheless stored in a desiccator. Glass beads of both samples and standard (Suprapur”

The samples were mounted on a 4 x 3 grid with grains of standard-bearing glass mounted on either side of two unknowns in a row of four. A spot was selected for analysis by optical examination of the surface of the grain mount. Data were acquired under com- puter control after burn-in and tuning, a procedure that was given three minutes to achieve standardization. Forty cycles of the magnet, with a counting time of 5 s/peak, in the order 41-39-4 l-39 , were collected and stored, and this constituted a single data point. A cycle consisted of measurements of the two peaks and the time taken for adjusting the magnetic field, and was 12.7 s long. Back- ground count rates were near zero (~0.01 cps) in the vicinity of the 41 peak, and =O. 1 cps (from the 39 peak tail) in the vicinity of the 39 peak, and did not require any correction. A deadtime correction of 20.8 ns (1991), 22.0 ns (1992), or 26.5 ns (1993) was applied, but is not critical since both sample and standard were measured at comparable count rates. At the end of a run of forty cycles, control was returned to manual, a new spot was selected, and the process repeated. For a single data point collected at a given spot, the approximate count rates were 4’Ki = 35,000 cps, ‘9K+ = 500,000 cps. Forty cycles gave total counts for “K+ = 7,000,OOO and ‘9K+ = 100,000,000 with an error due to counting statistics of ?0.7-0.8%0 (20) for the 4’K/‘9K ratio.


Data points were recorded in the sequence: RESULTS

SI x 1’ .yz y X s,.

where s, , sZr were the standards at each end of the row of four, and X, y, were the samples between these. The sequence was repeated and as many data points as could be collected during a working day (primary beam permitting) were collected, with a range of IO- 16 points per sample per day. The mean of IO- 16 point analyses had a standard error of ~0.6%” (2~~). Data collection for a particular set of samples was repeated once or twice, usually not on consecutive working days. No systematic differences were observed between daily sets of data collected months apart on the same grain mounts of the same samples, or between replicate analyses of one set of grain mounts and another. For example, BCR- 1, SRM-985, Juvinas, Viga- rano, and Semarkona were samples from which different grain mounts were prepared on separate slides with new partners and standards. No dependence of the isotopic ratios was found for such new arrangements.

Precision

The precision (all errors are quoted as 2~7, unless otherwise indicated) can be judged from the data presented in Tables 1, 3. The internal precision (standard error of the mean of forty cycles) is 1 .O- 1.6%0 (compared with 0.7-0.8%0 from counting statistics alone) in the dataset shown in Fig. 2. The external precision (the standard deviation of a dataset of n points) determined from sets of lo-16 points is =2%0, in general, and 1.8-2.6%0 in Fig. 2. The mean of the dataset has a precision of 0,/h, the standard error of the mean, which is OS- 0.8%0 in Table 1.

The additional source of error between external and internal precision arises from variations in factors that affect secondary beam stability (primary beam stability, stability of extraction optics voltages, etc.) and it is necessary to determine its magnitude and reproducibility for the present purpose. It was found that a dataset of lo- 16 points was sufficiently large to determine the external precision for the particular working surface on a given day. The mean precision of a sample-to- standard ratio determined from 1777 point analyses measured on natural samples is ?2.8%0 (=2~,\ln). This error scales as \in to give the precision of a day’s work (n = lo-16 = 0

Calculation of 6°K

Following Werner ( 1975). we write the secondary beam ion intensities as

‘ni, = i,4rck‘n(nS+); (1)

“j, = j,‘9CK~9(nS+) (2)

for each beam measured, where i, = secondary ion beam intensity, i, = primary ion beam intensity, C k = concentration of isotope of potassium in atom units, and (as’) = product of mass spectrometer transmission and positive ion yield for each isotope of potassium. Dividing the two equations gives the measured intensity ratio as

~=(~)[q$+]. which is rearranged to give

where R,b,o,.,, = 4’C,l”C, is the absolute isotopic ratio of potassium, and F,,,,,,.,,r = J’(nS+)/29(nS+) is the instrumental fractionation factor, which can be written as,

Now the measured ratio of the isotopic composition of the sample to that of the standard is

where the subscripts represent the following: x = sample (or unknown), std = Suprapur@ KNO, standard, m = measured, and abs = absolute. The ratio of the transmission and detection efficiencies should be equal for both standard and unknown since the tunings were the same. Likewise, the ratios of source fractionations should be equal since the sample and standard were of similar matrix composition. In other words, the ratio of the source and instrumental fractionations between sample and standard cancel out. We can then state all results as

where R, = mean of 4’K/“K ratios for all point analyses on a given day for sample X, RItd = as above for standard, 0, = standard error of R,. and o,,~ = standard error of RIld

Table 1. Gravimetric and ion probe results of enriched standards.

Standard N tilK f 20 (%o) s41K zk 20 (%o)

Gravimetric Ion orobe

- lO%o

z

- 5 %o

I:

- 2 %o

- 1 %o

+ 1 %o

+ 2 %o

+5%0

+ lO%o

x

+ 20 700

+ 30 %o

35

13

48

14

14

28

38

46

55

36

22

25

38

14

77

26

30

- 9.39 f 0.19 %o

- 4.56 + 0.09 %o

- 1.90 f 0.08 %o

- 0.96 f 0.04 %o

+ l.OOfO.O1 %o

+ 1.94 f 0.01 %o

+ 4.95 f 0.02 %o

+ 10.29 + 0.06 %o

+ 20.62 f 0.09 %o

+ 30.37 f 0.22 %o

- 9.3 * 0.6 %o

- 8.5 f 0.9 %s

-9.lf0.5%0

- 5.1 f l.O%o

- 4.6 ?I 0.6 %o

- 4.1* 0.5 %o

-2.1 f0.4%0

- 1.1 kO.4%0

+ 1.2f0.4%o

+ 2.3 + 0.5 %o

+ 5.2 f 0.8 %a

+ 10.2 f 0.7 %o

+ 10.2 f 0.5 %o

+ 9.9 f 0.9 %o

+ 10.1 f 0.4 %o

+ 20.1 f 0.8 %o

+ 30.7 f 0.9 %o

Each standard prepared by mixing gravimetrically determined amounts of solutions prepared from enriched 39K spike (99.913%) or 4*K spike (98.88%) with Suprapur@ KN03 standard. N= number of points analyzed. All standards were analyzed in 199 1, while some were repeated in 1992 and 1993, and are given as separate entries. Means are shown for such standards.


15 ,

TT ??y TT

T TT~ 1 c TT ;; T T

10 - &IT Tll?i 11 i 7 11 I 0

N Spiked standard: + 10.29 %.

Standard: Suprapur@ KNO,

-EC Lunar soil: 14163

FIG. 2. The results of day (S/04/93), featuring Suprapur@ KNO, standard, spiked standard (+ 10.29%~1), and lunar soil 14163. The points from different samples have been grouped together for clarity, but overlap in time. The abscissa merely serves to order these. Each group shows 14 points (15 for standard) measured, followed by the mean +- 20 standard deviation, mean -t 20, standard error of the mean, and b4’K t 20,. The ratios are normalized to the mean value of the standard, and a4’K is calculated as given in the text. The difference between internal and external precision can be seen by comparing the error bars of the individual points with those of the standard deviation of the mean.

= ?0.9-0.7%0), and indicates that a value of u = ?0.4- 0.5%0 requires n = 30-50 spot analyses. The diminishing returns and the possibility of individual biases from the separation technique or matrix effects do not justify more than 3-4 days of data collection per sample.

The mean instrumental fractionation (normalized to absolute isotopic composition of potassium) was -4.2 2 2.4%0 (two standard deviations). Our day-to-day precision was comparable to or better than TIMS precision for potassium isotopes. This high degree of reproducibility for the ion microprobe over standard TIMS techniques is attributable to the differences in source isotopic fractionation accompanying the different mechanisms by which the K+ ion beams are produced in secondary ion mass spectrometry (SIMS) vs. the thermal ionization following surface evaporation technique used by TIMS. The surface evaporation process results in Rayleigh fractionation of the residual potassium on the filament, and not only is the isotopic ratio variable in time but the extent of variation is sensitive to many parameters, including temperature and impurities (Eberhardt et al., 1964). Reproducibility is determined by both high purity of the sample loaded onto the filament and on the heating schedule of the filament. Maintaining these as close as possible is a very difficult task indeed. Isotopic fractionation takes place during SIMS analysis as well, and produces a “mixed layer” which is sputtered away during the analysis and replenished by fresh material (Haff and Switkowski, 1977; Gnaser and Hutcheon, 1988). Gnaser and Hutcheon (1988) show that steady state is attained fairly quickly (essentially during bum-in for the large primary beam currents used in this study), and a con- stant fractionation factor applies throughout the remainder of the analysis period. This has been tested in the present system by collecting data at a single point for several hours, shown in Fig. 3, where no systematic change in the 4’K/39K with time can be discerned. On well-polished and well-coated surfaces the SIMS emission is extremely stable and reproducible from point to point.

Precision and accuracy determined on enriched standards

All mass spectrometric methods for the precise and accurate determination of isotopic ratios control the instrumental mass fractionation by one of two kinds of procedures: those which eliminate instrumental fractionation by internal controls (normalization in systems with three isotopes, or double spike determinations in systems with four or more isotopes), and those which control instrumental fractionation by the recognition and elimination of systematic errors. Our procedures are of the latter sort, and the accuracy of any measurement is determined by the rigor with which the various sources of error have been controlled or eliminated. This can only be shown by preparing isotopically enriched materials with a high degree of accuracy and demonstrating the ability of the ion microprobe to measure the isotopic differences. Highly enriched potassium isotopes (“‘K = 98.88% and 39K = 99.913%) were obtained from Oak Ridge National Laboratory (ORNL) A series of enriched standards in the range - 10%0 to - I%o and + 1%0 to +30%0, was prepared by mixing gravimetrically determined quantities of each spiked solution with a solution prepared from our in-house standard (Suprapure KN03). A MettlerTM UM3 ultramicrobalance capable of weighing =l g Pyrex beakers to better than + 1 pg was used for weighing out the spikes. The gravimetric procedure attained an accuracy better than 1% of the isotopic enrichment induced. Ion microprobe measurements of these enriched standards were carried out and the results are given in Table 1 and shown as Fig. 4a,b. Mass spectrometric precision is determined by the number of points analyzed and is given in Table 1. As can be seen in Fig. 4a,b, the ion probe is able to reproduce the isotopic differences within the stated mass spectrometric errors. This is a necessary and sufficient demonstration of accuracy of the mass spectrometric technique, and demon- strates as well that the total external precision of measurement is the mass spectrometric reproducibility.


5 10

Point A

15

FIG. 3. Isotopic composition of potassium as a function of time measured at a single spot over several hours. The measurements were divided up into groups of 40 scans (508 s) termed points here, each of about IO min duration, to compare with normal measurements. Fifteen points have heen determined, and no dependence on time is discemable. The mean isotope ratio is used to normalize all @‘K, and mean is shown with k2u (standard deviation) and with ?2a, (standard error).

Laboratory induced isotopic fractionation

The accuracy of measurements of natural samples is then limited by the ability to reduce laboratory sources of isotopic fractionation to zero. The two potential sources of error are the isotopic fractionation induced by ion exchange separation and the fractionation that accompanies evaporation of potassium from molten barium borate glasses. Fractionation during melting of the barium borate glass was tested by allowing two test charges to remain molten for 40 and 95 hours, which fractionated to the extent of b4’K = 1.0 and 2.1%~ respectively. Interpolation of the results for 10 minutes of melting gave ??‘K 5 0.004%0, entirely negligible compared with tOS%o mass spectrometric errors. Such an effect must have been systematically produced in both samples and standards, and thus cancels out.

Kinetic isotopic fractionation in ion exchange columns was demonstrated by Taylor and Urey ( 1938) for Li, N (as NH:), and K. It has since been described by other workers in Li (Lee and Begun, 1958), Na radionuclides (Betts et al., 1956), Ca (Russell and Papanastassiou, 1978), and Ga (Machlan and Gramlich, 1988), and it seems likely to occur in all elements subjected to ion exchange separation in large columns. It was essential to prevent isotopic fractionation during chemical separation of potassium on the ion exchange column. This was achieved by the complete recovery of all potassium loaded on the ion exchange column. It has been recognized that extreme fractionation is encountered only in the leading and trailing edges of the eluted elemental peak, and that the effect was small enough to be negligible on the whole if recovery was better than 85% (Russell and Papan- astassiou, 1978). We have attained a recovery of ~99.8% for most samples, and over 97% for all samples.

Precision Sensitivity

The present measurements of the isotopic composition of potassium are the most precise measurements ever performed

on this element. It is useful to compare this with the precision attained for other elements by other techniques (i.e., compare signal-to-noise ratio). This can be approached by comparing the precision attained with the single-stage fractionation factor (cy - 1 ), derived from the square root of the masses, which we define as the precision sensitivity as follows:

precision sensitivity = ((Y - 1 )/a,

where, (Y = J(&m,), e.g., for K, (Y = 0.9753, and (Y - 1 = 12.3%o/a.m.u., and 0 is the la precision in the same units. Since the term ((Y - 1) controls both the extent of kinetic isotopic fractionation in nature and the source fractionation in a mass spectrometer (and hence the precision, a), the precision sensitivity as defined can be used to intercompare elements. Table 2 gives the precision sensitivities of various elements attained by different techniques, and compares the present study with previous determinations of potassium. The precision sensitivity attained in the present study is comparable to that obtained by double-spike isotope dilution mass spectrometric analysis of calcium isotopes (Russell et al., 1978), and is within a factor of 3 to 5 of the precision sensitivities of most light stable isotopes (excluding ‘3C/‘2C) analyzed by the double-inlet gas source mass spectrometric technique.

Potassium Isotopic Composition of Terrestrial Materials

Terrestrial samples

Where possible, well-known and well-characterized samples have been used, e.g. USGS analytical standards PCC-I, BCR-1, and G-2 (Flanagan, 1973). BCR-1 is a continental flood basalt from the Columbia Province. BCR-1 (irrad.) is a separate aliquot of this standard, which was irradiated (June 17, 1977) for RNAA along with Shergotty and Zagami. MORB is represented by MAR 58-27. OIB is represented by samples from Hawaii and Iceland. IKI-22, a Hawaiian picrite


30

8 20 P

5

r, N 10

ii

i 0

-10 0 10 20 30

S4’K (%) k 2 o Gravimetric

1

I . [ 1 I /I

T* I- . . . 0 1

. I 0. II

11 i . -1 - -10

,I 0 10 20 30

S4k (X) k 2 (I Gravimetric

PIG. 4. Potassium isotopic composition of enriched standards. (a) Ion probe measurements vs. gravimetric values, with 45” line for comparison. (b) Ordinate shows the difference between IMP measurement and gravimetric value. Precision can be judged from the size of the error bars, while mass spectrometric accuracy is judged by the fact that none of the measured values deviates by more than the assigned errors.

with 20% MgO and high potassium content, was collected immediately after eruption by members of the Hawaiian Vol- canological Observatory (Murata and Richter, 1966). Ice- landic sample R-l 1 is a picrite from the Reykjanes Ridge that has 6’*0 = 5.6%0 (Muehlenbachs et al., 1974), typical of fresh mantle basalts but unusual from Iceland where most samples have been lowered in 6 “0 by hydrothermal activity. It also has a very low potassium content of 80-100 ppm. Basalt L-22, from a fissure eruption, Lakagigar 1783, has the lowest 6’*0 = 3.1%0 of any Icelandic basalt (Muehlenbachs et al., 1974), and a high potassium content.

Island arc basalts are represented by Okmok (Aleutian Is- lands) and PB-29 (Pagan Volcano, Marianas Islands), from two of the best studied intra-oceanic island arcs. Both arc basalts have higher potassium contents than MORB, and the sources of both Aleutian and Marianas island arcs are known to be enriched in K, Rb, Cs, and other incompatible elements by metasomatic processes occurring in association with the subduction zones (Morris and Hart, 1983; Stolper and New- man, 1994). Samples from the subcontinental mantle are provided by Sco-I, a phlogopite lamprophyre from Scofield,

Utah, and BD 114, an alkali carbonatite lava (7.58 wt % KrO) recovered from Oldoinyo Lengai, Tanzania (Dawson, 1962).

Seawater was collected from the Pacific Ocean beyond Fri- day Harbor Marine Laboratory (Washington State) at a depth of several hundred feet, in a IL Nalgenee HDPE bottle (S. M. Kidwell, pers. commun., 1991). The salinity was x28%0 (from Na, Mg, Ca, and K contents) which is typical of north- em Pacific seawater, and the potassium content is accordingly lower than that at 35%~. Sylvite (KCI) and camallite (KC1 MgCl* . 6H20) from the Permian Stassfurt deposit represent salts crystallized from ancient oceanwater. These provide a measure of the low temperature equilibrium isotopic fractionation of potassium, which is determined to be A“‘K (sylvite-seawater) = -0.2 ? 0.6%0 (20,). Since sylvite contains some NaCl, a fraction of sylvite was mn through the ion exchange column, while a second fraction (Sylvite’) was mounted without ion exchange separation or any other chemical treatment. The camallite was run through the ion exchange column to separate K from Mg. The results (of somewhat lower quality for sylvite’ ) do not indicate any systematic deviation from seawater values (or bulk terrestrial values). The continental crust is represented partly by seawater, and partly by rock samples, including USGS G-2 (Westerly granite) and metamorphic biotite schist, C-l I, from Chapleau, Ontario. Altogether, this includes fifteen terrestrial samples, with two duplicates.

Isotopic results and chemical compositions of the analyzed terrestrial samples are given in Table 3 and shown in Fig. 5. The mean isotopic composition of terrestrial potassium obtained from this dataset is b4’K = +0.28 + 0.21%0 (20,).

Qualify

The measurements of terrestrial samples provide an excel- lent test of the external precision of S4’K determinations on natural samples. The variance of the dataset can be given as follows

2 2 g data = 0 InaSS spectmmettic + afiaNral + ff tnknown >

where ai, = variance of the data, (T,$,,, spect,,,metic = variance of the measurement errors assigned ( 1 /C[ 1 /(T: 1, where cri are the individual sample errors), c&tml = variance of any small fractionations of potassium in nature treated as if these were normally distributed, atnhown = variance of any errors arising for particular samples due to unaccounted laboratory-induced fractionation. The mass spectrometric error is determined during the measurement process. The variance of the data can be estimated from Table 3 as the standard error of the average of all seventeen samples, and the weighted mean error is calculated from all 414 points. This allows us to determine the sum of the other variances as

2 gnatural + c&known = &, - &, specho,,,etnc = 0.0245,

which is equivalent to an additional error of ~0.16%0. This indicates that the there are few other sources of error besides reproducibility of the mass spectrometric ratios, and that there are no discernable fractionations in terrestrial materials. Thus, the mean of b4’K measurements of all terrestrial samples yields a result almost identical to that which would be obtained from the repeated measurement of an equivalent num-


Table 2. A comparison of precision sensitivities for various isotopic techniques.

Element a-l Precision Precision %da.m.u. %da.m.u. Sensitivity

Technique*

H B C N 0

MS Si S Cl K II

414.0 48.6 41.0 35.0 30.4 20.4 17.5 15.1 14.1 12.3

II

Ca Fe

11.9 9.1

Zn 7.6

Rb 5.9

AS 4.7 Re 2.7 OS 2.7 Ir 2.6 Pb 2.4 U 2.1

* 1.0 414 0 fO.l 486 o + 0.025 1640 CJ f0.13 270 o fO.l 3040 f 1.0 20 ls fO.l 175 0 f 0.03 5000 f 0.05 282 o +0.12 1000 f 0.4 300 f 1.2 10 0 +0.1 1190 * 1.0 90

f 1.0

f 1.0 f 1.0 f 1.2 * 1.0 f 1.2 f 1.0 + 1.5

80

60 TIMS (MINSTER and ALLEGRE, 1976) 50 TIMS (KELLY et al., 1978) 20 TIMS (CREASER et al., 1992) 30 TIMS (CREASER et al., 1992) 20 TIMS (CREASER et al., 1992) 20 TIMS (TERA and WASSERB~JRG, 1975) 20 TIMS DS (CHEN and WASSERLI~XG, 1981)

GS (SHEPPARD aad EPSTEIN, 1970) TIMS (SPIVACK, 1986) GS (HALBOW et al., 1986) GS (BECKER end CLAYTON, 1975) GS (CLAYTON and MAYEDA, 1983) TIMS (LEE et al., 1976) GS (MOLINI-VELSKO et al., 1986) GS (GAO & THIEMENS, 1993) GS (KAUFMANN et al., 1984) IMP (This study) TIMS (GARNER et al., 1975a) TIMS (PAPANASTASSIOU, pets. comm.) TIMS DS (RUSSELL et al., 1978) TIMS (V~LKENING and PAPANASTASSIOU, 1989) TIMS (V~LKENING and PAPANASTASSIOU, 1990)

Precision sensitivity = (a-l)@ a = i(m*/mt) in %da.m.u. *GS = gas source, TIMS = thermal ionization mass spectrometry, DS = double spike, IMP = ion microprobe

ber of points on a single sample, with an error within a factor of two of the mass spectrometric error ( -tO.l4%0) alone. Since these samples cover every major terrestrial composition, this result implies that errors arising from laboratory-induced fractionations ( rrunhown ) have been successfully minimized to the extent that these no longer influence the outcome of the experiment.

Potassium Isotopic Composition of South African Metasomatic Rocks

Schreiner and Verbeek ( 1965 ) reported large ( + 16%0 to -30%0, with = ?5%0 reproducibility) isotopic effects in potassium from metasomatic rocks from a contact between the Cape granite and the Malmesbuty shale in South Africa. Five of these samples were subjected to our usual procedure of chemical separation (using the small ion exchange column) and isotopic analysis, and the results are reported in Table 4. Analysis of the Cape granite, unmetasomatized shale (S3) and two granitized xenoliths, GZl and GZ2 (cited as GXI and GX2 in the original paper), reveal no discernable isotopic fractionation at the level of 5 1 %O or better, contrary to the findings of Schreiner and Verbeek ( 1965). The shale sample, S3, was run in duplicate with no difference in the results. Our results are consistent with later unpublished measurements (A. A. Verbeek, written commun., 1993). It is not possible to explain the discrepancy, even with present hindsight.

Potassium Isotopic Composition of the Samples of Hinton et al. (1987, 1988)

Hinton et al. ( 1987, 1988) reported isotopic fractionation of potassium in several natural materials, including feldspars from the Colomera IIE iron meteorite, lunar breccias, and a terrestrial authigenic sanidine. These measurements were made with the Chicago ion probe, by in situ analysis of potassium-rich minerals. Since the lunar feldspars were found only in thin section, no attempt was made to reproduce these results. Our measurement of the authigenic feldspar from the St. Peter sandstone shows this to be indistinguishable from other terrestrial materials (Table 5 ) Two samples from the Colomera IIE iron meteorite were analyzed: K-feldspar from the same batch (C-5 ) used by Hinton et al. ( 1987)) originally separated for Rb/Sr dating (Sanz et al., 1970), and a bulk silicate sample provided by I. D. Hutcheon. Potassium was chemically separated from the dissolved samples using the 11 mL column, and had minimal interferences, allowing fairly broad cuts to be applied. Thus, isotopic fractionation in the laboratory should not be an issue. Isotopic analysis of separated potassium yielded b4’K = -1.8 2 0.7%0 and -2.2 2 0.9%0 for the silicate and feldspar fractions, respectively (Table 5 ) , essentially in agreement with the result of Hinton et al. ( 1987). Their original interpretations (Hinton et al., 1987, 1988) no longer hold since our measurements of chondrites (Humayun and Clayton, 1995) show that the Colomera


Table 3. Potassium isotopic composition of terrestrial samples.

Sample N K @pm) K (pg) 841K*2 0,

USGS G-2

USGS BCR-1

USGS BCR-.l (mad)

USGS PCC- 1

MAR 58-27 (MORB)

XI-22 (Hawaiian picrite)

R-l I (Icelandic picrite) *

R- 11 (1993 repeat)

L-22 (Lakigigar)

Okmok (Aleutian Is.)

PB-29 (Marianas Is.)

Sco- 1 (phlogopite perid.)

BD 114 (Oldoinyo

Lengai)

C-l 1 (biotite schist)

Seawater (Pacific)

Sylvite (Stassfurt) ‘92

Sylvite’ (Stassfurt) ‘93

Camallite (Stassfurt)

20

82

21

22

37

26

19

5

13

12

27

12

18

13

34

44

8

20

36420

14280

13*3

898

3320

80

106

3570

7230

5490

45100

31600

318

6960 -0.1 f0.7%0

5900 0.0 f 0.3 %o

1360 -0.1 f0.6%0

9.5 + 0.2 f 0.5 %o

728 + 0.4 f 0.6 %o

1280 + 0.2 f 0.5 %o

425 t 2.8f0.7%0

118 +0.4* 1.3%0

1680 + 0.7 * 0.9 %o

2730 O.Of 1.1 %0

1070 + 0.3 f 0.6 %o

2140 + 0.8 It 1.0 %o

20880 +O.l f0.7%0

2770 + 0.4 + 0.8 %o

1562 + 0.4 f 0.5 %o

19920 + 0.2 f 0.4 %o

1027 +1.3*1.2%0

7430 - 0.6 f 0.7 %o

Weighted mean 414 +0.18f0.14%0

Average of samples 17 + 0.28 f 0.21 lo

* Fist measurement of R-l 1 with =90% K recovery, from =7 g sample, with a repeated measurement given below. Here, and in other data tables, N= number of ion probe points measured, K (ppm)= K concentration in sample determined by AAS, K (up)= amount of K recovered.

results are not typical of chondritic material, as they had as- sumed.

Potassium Isotopic Composition of Lunar Soils and Breccias

Isotopic analysis of potassium by TIMS in lunar rocks and soils has indicated the existence of large isotopic effects in 64’K of 5 to 20%0 (Barnes et al., 1973; Garner et al., 1975a; Church et al., 1976). To provide a basis for comparison between the Chicago IMP technique and the earlier TIMS measurements we analyzed five samples previously determined by TIMS at NBS (Garner et al., 1975a) and at UC Santa Barbara (Church et al., 1976). These included three soils and two rocks, one of which was found to be isotopically fractionated by Garner et al. (1975a). These results, and measurements of three Apollo 16 impact melt coatings, are given in Table 6. We also analyzed NBS SRM 985 (reagent standard), which we find to be isotopically distinct from our in- house standard by cY4’K = +0.7 2 0.4%0. For comparison purposes, we present all Chicago analyses relative to our value for SRM 985 in Table 7. It can be seen that our results are in good agreement with TIMS results for both fractionated and unfractionated samples. Agreement is best with rock samples, including the regolith breccia 15505, which is isotopi-

Terrestrial samples

Terrestrial Average

USGS G-2

USGS BCR-1

USGS BCR-1 (irrad.)

USGS PCC-1

MAR 58-27 (MORB)

IKI-22 Hawaii(OlB)

R-l 1 Iceland (OIB)

L-22 Iceland (OIB)

Okmok, Aleutian Is. (IAB)

PB-29, Marianas Is. (IAB)

Sco-1 lamproite

BD 114 carbonatite

C-l 1 micaschist

Pacific seawater

Sylvite (Stassfurt) ‘92

Sylvite ‘93

Carnallite (Stassfurt)

+t m---I

t

????

????

-4 -2 0 2 4

s4’K (%e) & 2 o

FIG. 5. Potassium isotopic composition of terrestrial samples. 64’K values are relative to Suprap@ KN03 standard.

tally homogeneous within error of the present measurements, and therefore, is a good standard of comparison.

The differences among lunar soils, particularly 14163, are conspicuous. These soils, which consist of a mixture of rock and mineral debris, glass, and agglutinates, are likely to be isotopically heterogeneous, and this can be best seen in 14163

Table 4. Potassium isotopic composition of S. African rocks.

Sample N K(ppm) K (pg) 641K*2~,

Cape granite, G2 2 32150 2090 + 1.5 f 1.7 %o

Cape granite, G3 22 33600 2220 + 0.2 * 0.7 %o

Malmesbury Shale, S3(a) 14 38900 1990 + 0.3 f 0.9 %o

Malmesbury Shale, S3(b) 5 1350 - 1.05 1.3 %o

Granitized xenolith, GZl 14 35400 3220 + 0.5 f 0.6 %o

Granitized xenolith, GZ2 11 33600 3950 + 0.5 * 0.8 %o


Table 5. Potassium isotopic composition of Hinton’s samples.

Sample N K (ppm) K@g) 641K*2 o,

Madagascar sanidine

St. Peter sanidine

Colomera IDH

Colomera C-5

12 132000 3870 +O.lf0.9%o

28 118000 11090 - 0.5 f 0.7 %o

28 36080 920 1.8 k 0.7 %o

13 145OQO 349 2.2 f 0.9 %o

Colomera C-5 (K-fsp)* 9 - 2.2 f 0.7 %o

* Measured on same grain mount used by HINTON et al. (1987). by comparing isotopic ratios collected on C-5 K-feldspar grains with those collected on a Madagascar sanidine fragment.

where a range of b4’K of 7%0 is observed among the three laboratories. If this was a real reflection of precision of measurement (i.e., indicate the presence of egregious errors), then the nine NBS analyses of lunar rocks would almost certainly show such a spread. Only one reported NBS lunar “rock” analysis, 1550.5, is isotopically distinct and that has been confirmed by this laboratory. Regardless of the differences, bulk soils show a distinctly heavy potassium isotopic composition, up to b4’K = + 13%0. The mean of nine lunar rocks analyzed by Gamer et al. ( 1975a) is identical within errors to the mean of eleven lunar rocks analyzed by Humayun and Clayton ( 1995 ), with 15495 being the only sample in common. It is concluded that this spread is larger than laboratory effects and in all likelihood represents isotopic variability in the sample as the result of imperfect mixing of an isotopically fractionated component and a normal component (KREEP frag- men&), which will be discussed further below.

These workers reported light isotope enrichment (maximum = -30%0) within a distance of ~60 cm from the Cape Gran- ite-xenolith contact due to diffusive fractionation of potassium. They showed that samples of the granite ( + 16 ? 3%0), and of shale ( - 1 t 2%0), far removed from the contact had internally consistent, though distinct isotopic compositions, expressed relative to a laboratory reagent standard. Senftle and Bracken (1955) showed that for two isotopes, the ratio of the diffusion coefficients can be described by D, I D2 = ( m2/

m,)“*, and they calculated profiles for the diffusive separation of isotopes in geological processes involving both intra- crystalline diffusion and diffusion in a tluid phase. A believ- able demonstration of the process has not been shown to our knowledge, even though diffusive separation of isotopes in the gaseous phase is a widely used process for the separation of 235U from *j’U. It was, therefore, important to confirm the findings of Schreiner and Verbeek ( 1965). Our analyses of these samples indicate no isotopic effects at a higher level of precision than obtained by the original workers. Thus, no evidence for diffusive fractionation of potassium isotopes in nature has yet been conclusively demonstrated.

Another report of isotopically fractionated potassium in a terrestrial sample was also investigated, this latter measurement being of precision comparable to that attained in the present study. An authigenic sanidine from the St. Peter sandstone was found to be - 1.3%0 relative to three other terrestrial feldspars (Hinton et al., 1987 ) Our measurement of this sample does not show any fractionation within errors, and the case for low temperature solid-aqueous solution partitioning of potassium isotopes can be rejected on the basis of the measured sylvite-seawater fractionation, A4’K~siylv,te_seawater) = -0.2 2 0.6%0 (28,). We conclude that there is no conclusive evidence for the existence of isotopically fractionated potassium on Earth at levels greater than 0.5%“.

DISCUSSION

Isotopic Fractionation on Earth An Isotopic Effect in the Colomera IIE Iron Meteorite

What, if any, evidence can be found for the existence of isotopically distinct potassium on Earth? Potassium has had a long history of reported isotope fractionations which, wher- ever reinvestigated, have been found to be analytical artifacts arising from uncontrolled source fractionation in the thermal ionization process. Brewer (1936) made an early report of isotopically fractionated potassium in kelp (=5-lo%), which could not be confirmed by a careful study by Cook ( 1943 ) with a precision of -t 1%. Since then, there have been many reported fractionations that have never been retracted, nor has any effort been made to duplicate these measurements, although Kendall ( 1960) analyzed a variety of natural materials in an effort to investigate natural isotopic fractionation in potassium. At his level of precision, ?2-3%0, no effects could be discerned (Kendall, 1960). But the spectre of isotopic fractionation on the planet has persisted for years, despite a lack of convincing evidence of a single, well-documented, reproducible instance of potassium isotopic fractionation.

An isotopic effect has been observed in feldspar and silicate fractions from silicate inclusions in the IIE iron meteorite, Colomera. Hinton et al. (1987) measured 64’K = -2.4

Table 6. Potassium isotopic composition of lunar soils, an Apollo 15 regolith breccia, and Apollo 16 impact melt coatings @MC).

Perhaps one of the most physically plausible arguments for potassium isotopic fractionation in terrestrial rocks was made by Schreiner and Verbeek ( 1965), who investigated potassium metasomatism around the Cape granite of South Africa.

Sample N K (ppm) K (pn) s41K*2 om

14163 soil 14 4360 2920 + 7.8 f 0.8 %o

15041 soil 11 2230 1677 + 12.7 f 0.7 %o

64801 soil 38 950 862 + 5.0 f 0.5 %o

15505,103 breccia 17 1560 1556 + 7.8 f 0.8 %o

15505,100 IMC 11 1480 873 + 8.3 f 0.8 %o

60015,747 IMC 22 546 460 + 0.5 f 0.7 %o

61016,418 Ih4C 8 605 688 +0.4* 1.1 %o

64435,297 Ih4C 19 784 838 - 0.3 f 0.6 %o

Corrections applied for matrix interference from Al: 60015,747= -0.8?0.3%o,61016,418= -0.7?0.3%0,64435,297= -0.4+0.16%0.


Table 7. Chicago-NBS (NIST) intercomparison.

Sample

BCR-l*

15495 mare basalt

14 163 KREEP soil* 0

15041 KREEP soil*

15505,103 Breccia

15505,100 melt splash

64801 highlands soil

Mean Earth?

Mean Lunart

NBS SRM 985

-0.7 f 0.5 %o -0.2 * 1.3 %o -0.5 f 2.1 %o

-0.4 f 0.6 %o +l.l+lS%a -1.4*2.4%0

+7.1*0.9%0 + 10.0 f 1.5 %o - 2.9 f 2.6 %o

+ 7.1 * 0.9 %0 + 3.3 * 1.5 %o + 3.8 f 2.6 %o

+ 12.0 f 0.8 %o + 17.4 f 1.5 %o - 5.4 f 2.6 %o

+ 7.1 f 0.7 %o +4.9+ 1.5%0 + 2.2 + 2.5 %o

+ 1.6 f 0.9 %o +7.5+1.5%0 +0.1+2.6%0

+4.3 zlz 0.7 %o +6.2f1.5%o -1.9f2.5%o

-0.4 * 0.4 %o +0.3 f 0.2 %o -0.7 + 0.7 %o

- 0.2 f 0.5 %o + 0.2 f 0.5 %a - 0.4 + 1.0 %0

= 0.00 = 0.00

All Chicago data normalized to mean value of NBS SRM 985, A41K= Zi’JlK - h41Ksm.

* Values are calculated from the data of CHURCH et al. (1976), who used methods closely based on those of the NBS, with NBS SRM 985 for normalization.

t Chicago data based on means of 17 terrestrial samples and of SRM 985; NBS (NIST) data are unpublished, quoted by GARNER et al. (1975b) as a bias in their data set of about 70 samples of terrestrial minerals. We estimated their precision as = 2%0 divided by 4N (N=70). We interpret the calculated difference as an interlaboratory bias, but observe that any systematic error in our mass spectrometry of this magnitude at our better precisions should have been revealed by the measurements of the enriched standards. On their part, GARNER et al. (1975b) state that “This difference is attributed to an altered fractionation pattern rather than to real differences in isotopic abundance between the minerals and reference standard.”

$ Chicago result calculated from mean of 11 lunar samples and of SRM 985, and NBS (NIST) result from mean of 9 lunar rocks in GARNER et al. (1975a).

N.B. The former National Bureau of Standards (NBS) is now the National Institute of Standards and Technology (NIST).

+ 0.4%0 on a feldspar separate relative to a terrestrial sanidine by directly obtaining K’ ion beams from feldspar mineral mounts. The existence of this effect has been confirmed by two analyses in the present study. This result is surprising in view of the relation of IIE irons to H-chondrites which do not show any potassium isotope fractionation (Humayun and Clayton, 1995). The silicate portions of IIE irons and H-chondrites have similarities of oxygen isotopes, chemical composition, and exposure ages (Clayton and Mayeda, 1978; Olsen et al., 1994; Wasson and Wang, 1986). It is difficult to interpret the Colomera K isotope composition as either a nebular effect or a parent body effect, even discarding the H-chondrite connection. A means of making isotopically light potassium relative to the normal composition is not well known.

The discovery of a lunar anorthosite breccia with S4’K = -3.9 2 0.9%0 (Humayun and Clayton, 1995), and the widespread evidence for mobilization of alkalis (Palme, 1977; Warren and Wasson, 1979) and Pb (Tera et al., 1974) in lunar breccias is circumstantial evidence for redeposition of vapor- ized potassium. It is conceivable that melting of impact ejecta may produce localized potassium-rich lithologies in which a significant fraction of the potassium has been scavenged from the redeposited isotopically light vapor. Such a orocess would

explain the presence of the effect in a volatile element, without requiring major elements to be isotopically fractionated, but it appears to require a more effective mechanism than so far required for anorthosite 60015 (because of the low potassium content of the latter). It is a mechanism that can be tested for lunar breccias and lunar granitic differentiates (e.g., 12013, 14305, 14321, etc.), and confirmation of such a mechanism operating on the Colomera parent body (H-chondrite regolith?) should shed light on the genesis of this enigmatic clan of iron meteorites. It is clear, though, that the extreme differentiation represented by the silicate phase in IIE irons involves a process beyond simple partial melting/fractional crystallization.

Lunar Breccia, 15505

During the course of our investigations of impact melts it became clear that we had inadvertently analyzed a regolith breccia, 15505. The isotopic data are presented in Table 6 and Fig. 6, and compared with some data from other investigators. It is important to note that our two analyses of 15505 (interior = ,103 and exterior = ,100) are isotopically identical, and within errors these are also identical to the determinations by


Lunar soils

CHICAGO 14163

15041

64801 15505,103 15505,100

NBS 10084 12033 12070

14163

15301 15501 15511

15515

64801 67601

68501 72501

75081 78500

UCSB 14163

14259

15041 68501

1

0 5 10 15 20

s4’K (%a) f 2 o

FIG. 6. Potassium isotopic composition of lunar soils from three different laboratories. The NBS data are from Garner et al. (1975a) and the UCSB data are from Church et al. ( 1976). Note the homo- geneity of the Apollo 15 soils in the NBS data. All values are relative to the NBS SRM-985 standard.

Gamer et al. ( 1975a) given in Table 7. Gamer et al. ( 1975a) first drew attention to the presence of a glassy coating of possible impact melt origin and suggested that this could be the source of the isotopically fractionated potassium in this sample. They suggested that “. . the potassium isotopic composition of the surface vs. the interior bulk sample would be of considerable interest.”

Our analyses of 15505 were carried out on an interior gray chip (15505, 103) free of black impact melt, and on small chips of black impact melt ( 15505, 100). Based on minera- logical and chemical investigations of Apollo 15 breccias, En- gelhardt et al. ( 1973) and Simon et al. ( 1986a) described this sample as a regolith breccia, consisting of local soil compo- nents. In contrast to the heterogeneity observed in soil 14163, the interior and exterior of 15505 are homogeneous with re- spect to b4’K, even though the exterior is an impact melt coating. Moore et al. ( 1973) showed that a similar impact melt coating, the black slag covering 60015, has three times as

much carbon as the underlying anorthosite, and Morgan et al. ( 197 1 ) showed that a similar “glaze” on an Apollo 12 boul- der contains a signiticant enhancement of meteoritic contam- ination. These dark glassy impact melt coatings are not derived from the underlying rock, but represent extraneous materials (probably local) with an admixture of meteoritic component (Morgan et al., 197 I ) The Apollo 16 boulders are covered by isotopically normal material (Table 6), with high potassium contents derived principally from the ubiqui- tous KREEP. No isotopic fractionation of potassium is, therefore, involved in the production of these impact melts.

The isotopic uniformity of potassium in and on 15505 seems perplexing unless the Apollo I5 site, at which the glass- forming impact occurred, is isotopically uniform in potassium. To examine this, all available data, for five soils and the 15505 soil breccia, were compared (data from Gamer et al., 1975a; Church et al., 1976; this study). Samples 15301 (soil from Station 7) and 15501 (soil), 15511 (soil), and 1551.5 (friable soil clod) from Station Y are identical within I% to breccia 15505 (Station 9). Only soil 15041 (Station 8) is isotopically distinct (b4’K = +12.7’%). It was collected from ejecta from either Aristillus or Autolycus ray material recognized as a stratigraphically distinct unit (Swarm et al., 197 I ) Thus, the material from which the 15505, 100 glass is derived must come from the local (Station 7/9) soil with S4’K = + 8 2 1x0, an example of the use of potassium isotopes as tracers of lunar surface materials.

Isotopic Fractionation in Lunar Soils

Lunar soils have been shown to be isotopically fractionated in oxygen and silicon ( Epstein and Taylor, 197 1; Taylor and Epstein. 1973; Clayton et al., 1974). sulfur (Rees and Thode, 1972; Thode and Rees, 1976, lY79), and potassium (Barnes et al., 1973; Garner et al.. 1975a; Church et al., 1976). Iso- topic analysis of Ca reveals small fractionations (Russell et al.. 1977), while Mg exhibits little or no fractionation (Esat and Taylor, 1992). Several processes have been proposed to explain these observations, including

I ) ion sputtering of grain surfaces (Haff et al., 1977; Swit- kowski et al.. 1977 ),

2) micrometeorite impact volatilization (Clayton et al., 1974; Housley. 1979). and

3 ) redeposition of sputtered or volatilized material after gravitational fractionation (Haff et al., 1977; Housley, 1979). Switkowski et al. ( 1977 ) calculated the isotopic effects induced on the surfaces of lunar soil grains by sputtering and redepositional processes and found that all elements should be affected in a manner such that elements of similar mass are affected to similar extents, e.g.. b, = &,. Predicted enrichments for K and Ca are in the range of IO-20%la.m.u. Volatilization is expected to affect these elements in the order S > K > Si = Mg = 0 > Ca, while redeposition of either sputtered or volatilized material after gravitational isotope separation in the lunar atmosphere acts to enhance the initial surface enrichments. Since none of these processes appears to adequately account for the pattern of isotopic fractionation observed, Esat and Taylor ( 1992) concluded that the large effects seen in potassium and in oxygen and silicon may sep- arately be the results of procedural artifacts. Potassium iso-


topic data on lunar soils from three laboratories are shown in Fig. 6. Our analyses of bulk lunar soils indicate that potassium indeed shows large heavy isotopic enrichment, consistent with the findings of Barnes et al. (1973), Garner et al. (1975a), and Church et al. ( 1976). In view of this result, a careful examination of the available evidence is warranted.

potassium seen in bulk soils. Calcium is also enriched in the finest fractions along with potassium, and yet calcium isotope enrichment is severely limited.

All three proposed mechanisms would produce heavy isotopic enrichments in a small portion of the lunar soil (either on grain surfaces or in volatilization residues), which is then diluted with isotopically normal material (grain interiors or unvaporized grains). The relation between elemental loss and heavy isotope enrichment is then given by a mixing line between a point on a Rayleigh fractionation curve and the origin. This is illustrated for potassium in Fig. 7, where K/U ratios and potassium isotopic compositions determined on several fractions of 14163 and 14259 by Church et al. (1976) show an apparently linear mixing trend, Since lunar rocks show variable K/U (Schonfeld, 1974) with mare basalts ranging up to 3000 and average KREEP at 1380 (Warren and Wasson, 1979), any mixture (e.g., as in soils) must have intermediate K/U. Due to the tenfold higher K and U abundances of KREEP relative to mare basalts, the KREEP component will dominate the soil K/U when present in proportions greater than 50%. We have thus compared only the two KREEPiest soils, 14163 and 14259, with average KREEP (Warren and Wasson, 1979) to obtain an estimate of the original K/U in these soils. Decreases in K/U of up to 15% can then be seen in these soils relative to average KREEP. These depletions are linearly correlated with increasing b4’K and extrapolate to an endmember at about b4’K = +60%0 with a K/U of about 100.

This is a first order indication of the importance of volatility in controlling the isotope fractionations seen in the soils. Ex- perimental impact melting of soils confirms the presence of chemical biases due to preferential melting and incorporation of the finest fractions, but large depletions of Na and K are observed in the impact melts (Simon et al., 1985, 1986b). Volatility, as the principal process, also accounts for the observations of Na and K in the lunar exosphere (Potter and Morgan, 1988; Sprague, 1990), since both Ca and Mg atoms would be easily detectable if present in column densities comparable to or larger than those of the alkalis. It appears that the Mg and Ca effects are better interpreted as upper limits on the magnitude of solar-wind sputtering effects.

This is comparable to the magnitude of the oxygen and silicon effects measured in the initial fractions of fluorinated lunar soils (Epstein and Taylor, 1971; Clayton et al., 1974). But the potassium isotopic effects are clearly visible in bulk soils (up to 13%0 in this study), as are those of sulfur (up to 10.8%0), while the oxygen (0.3%0) and silicon (0.1%0) effects are barely visible, and no Mg or Ca effects can be discerned in bulk soils (Esat and Taylor, 1992; Russell et al., 1977). Why are the potassium isotope enrichments so large? There are two factors:

Although volatility can be invoked as a means of decou- pling the large potassium isotope effects from the smaller effects in oxygen and silicon, a more exact description of the nature of the vaporization process must await further experimental development of impact-induced vaporization. Further, with the present dataset, it is difficult to distinguish between vaporization residues and gravitational separation in the lunar atmosphere followed by reimplantation of the heavy isotope- enriched vapor into the lunar soil, as the cause of the large potassium isotopic effects. Given the complex nature of the lunar soil, a potassium isotopic anatomy of the lunar soil would contribute significantly to our knowledge of the phys- ical processes involved in selenopedology and the nature of the long-term exposure of materials in near-Earth space. Since the regolith is the ultimate source of alkali metals in the lunar atmosphere, further understanding of the processes that control lunar atmospheric abundances should lead to a better understanding of the environments of other airless bodies in the solar system (Sprague, 1990).

CONCLUSIONS

1) the greater volatility of potassium relative to other elements (De Maria et al., 1971), and

2) chemical bias, in that the process has acted upon fractions of the soil significantly richer in potassium (and sulfur) than the bulk soil.

A new analytical technique has been developed for the measurement of stable isotope abundances of potassium. It is based on SIMS analysis of chemically separated and purified potassium, and is applicable to all types of geological, lunar, and meteoritic materials. Precision of measurement of isotopic compositions is +0.5%0 (2u,), which is a factor of 3- 4 better than the best published results obtained by conven- tional TIMS methods.

Studies of chemical fractionation in the comminution pro- Isotopic analysis of a wide variety of terrestrial igneous, cess indicate that potassium is strongly enriched in the finest metamorphic, and sedimentary rocks, as well as seawater, has fractions of the lunar soils, while Mg, SC, and other elements revealed no analytically resolvable variations in 4’K/39K ra- of the mafic constituents are depleted (Papike et al., 1981; tios. The lack of isotopic variability in terrestrial potassium, HGrz et al., 1984; Laul et al., 1987). The K/Mg enrichment in contrast to sulfur, which has similar atomic mass, reflects relative to the bulk soil is a factor of about 1.5 in the < 10 the monotonous chemistry of potassium in the terrestrial en- pm fraction, but rises to =6 in the <2 pm fraction and pre- vironment: always existing as a monovalent cation, and al- sumably increases further in finer fractions, as the result of ways similarly coordinated usually by oxide ions or water comminution of feldspar and mesostasis to finer grain sizes molecules. In this manner, potassium is similar to calcium, than pyroxene. It is these finest fractions that melt preferen- which also shows little evidence of isotopic fractionation on tially during micrometeorite impact to produce agglutinate Earth (Russell et al., 1978). Although this uniformity of iso- glass (Papike, 1981; Walker and Papike, 1981; Laul et al., topic composition eliminates the possibility of using potas- 1987). Such chemical biases alone, however, are insufficient sium isotopes as natural tracers of geochemical processes, it to account for the more pronounced isotopic fractionation of provides the basis for establishing a global value of 4’K/39K

2128

60

40

D N

2

Y

%

20

0

M. Humayun and R. N. Clayton

0 ?? 14163

0 14259

0 + 15041

v 64801

Mare basalts

I iA .I k

600 1000 1500 2000 2500 3000

KnJ

FIG. 7. Potassium isotopic composition of bulk soils and separated soil fractions. Dark symbols are Chicago rock and bulk soil analyses; gray symbols are separated soil fractions from Church et al. (1976) (UCSB). K/U values for Chicago analyses are from the literature. A Rayleigh fractionation curve is drawn through the average KREEP K/U of 1380 (Warren and Wasson, 1979). The dashed line is a mixing line drawn through average KREEP and the separated soil fractions of the KREEPiest soils and indicates an enriched component of about 60%0. The starting compositions of other soils are not well-constrained but lie along the KREEP-mare mixing line.

for the Earth, which can be used in studying the cosmochemical relationship between Earth and other solar system bodies.

Potassium isotope abundances in lunar soils and soil breccias are variably enriched in the heavy isotope relative to lunar rocks, with enrichments up to b4’K = +13%0, as previously observed by Garner et al. ( 1975a) and Church et al. ( 1976). In contrast to their geochemical behaviour in the terrestrial environment, potassium and sulfur isotopic variations are of similar magnitude in the lunar soils, which are much larger than those of magnesium and calcium. This difference in behaviour reflects the importance of vaporization processes on the lunar surface, associated with micrometeorite bombard- ment and gravitational fractionation in the transient lunar atmosphere. The absence of a complementary isotopically light reservoir for potassium and sulfur requires a net loss of these elements from the lunar surface to space. A detailed study of the distribution of isotopic abundances among the constituents of lunar soils (mineral grains, glass particles, surfaces, etc.) may provide new information on the evolution of the regolith surfaces of the Moon and Mercury.

Further discussion of the cosmochemical significance of potassium isotope abundances is presented in an accompanying paper (Humayun and Clayton, 1995 ) .

Acknowledgments-Lab space, equipment, advice, and assistance were generously provided by M. C. Monaghan, K. Wielgoz, R. S. Lewis, A. M. Davis, R. Draus, 0. Draughn, and T. K. Mayeda. We are indebted to the following for generously providing vitally needed samples: .I. Gooding and the JSC curatorial staff (lunar samples), J. B. Dawson (BDll4). E. J. Olsen FMNH (Stassfurt sylvite and camallite), S. M. Kidwell (Pacific Seawater), H. H. Woodard (St. Peter sanidine), A. T. Anderson and D. Zhang (MAR 58-27, IKI-22, L-22, R-l 1, Okmok, PB-29, and Sco-1 ), R. S. Lewis (BCR-1 irrad.), and I. D. Hutcheon (Colomera silicate). A. A. Verbeek kindly sup-

plied the South African samples. Discussions with E. L. Beary, D. S. Burnett, T. M. Esat, R. S. Lewis, J. R. Moody, D. A. Papanastassiou. G. J. Wasserburg, and many other colleagues are gratefully acknowl- edged. The paper benefited from reviews by S. B. Simon, G. Herzog, A. J. Fahey, and an anonymous reviewer. C. Koeberl is thanked for editorial handling. This work was supported by NASA grants NAG 9-5 1 and NAGW-3345 to RNC. The present investigation was carried out as part of the Ph.D. Thesis of MH.

Editorial handling: C. Koeberl

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