The isotopic composition and elemental abundance of gallium in meteorites and in terrestrial samples

Ckochimica et Coemochimica Acta, 1972, Vol. 86. pp. 735 to 743. Pergamon Press. Printedin Northern Ireland

The isotopic composition and elemental abundance of gallium in meteorites and in terrestrial samples J. R. DE LAETER Department of Physics, Western Australian Institute of Technology, South Bentley, Western Australia (Received

2 November

1971; accepted in revised form

24 Januaq 1972)

AbstractThe isotopic composition of gallium in six iron meteorites and a terrestrial standard were measured using a solid source mass spectrometer. Isotopic abundances of meteoritic and terrestrial gallium agree to within 0.11 per cent. The concentration of gallium in 21 iron and 6 stone meteorites and in 13 standard rocks was determinedusing the method of isotope dilution. In general, the agreement between this work and other published data is excellent. INTRODUCTION A COMYARATIVEstudy of the isotopic

composition of various elements in terrestrial, meteoritic and lunar samples is of great potential value to our understanding of the early history of the solar system and to processes of nucleo-synthesis. Isotopic differences between terrestrial and meteoritic samples may be produced by isotope fractionation or by incomplete mixing of material with different nucleosynthetic histories during the formation of the solar system. In fact the only conErmed variation in isotopic composition between terrestrial and meteoritic samples which is not caused by long-lived natural radioactivity or cosmic ray effects is found in the rare gases (see review of ANDERS, 1971). REYNOLDS (1967) reviewed a number of investigations concerned with the isotopic composition of solid and gaseous elements in meteorites. He concluded that the uniformity in isotopic composition of the heavy, non-gaseous elements implies that very effective mixing of the solar system material has taken place. The only comparative study of meteoritic and terrestrial Ga was carried out by INGHRAM et al. (1948), who isolated several milligrams of Ga from 1 kg of the Canyon Diablo iron meteorite and compared the Ga@/Gavl ratio with that of a terrestrial standard. They concluded that the isotopic composition of terrestrial and meteoritic Ga was identical to within O-2 per cent. In point of fact very few measurements have been made on the isotopic composition of Ga since the early work of ASTON (1935) and SAMPSON and BLEAKNEY (1936). Apart from INGIHRAM et al. (1948), who used thermal ionisation techniques, only HIBBS (1949) and ANTKIW and DIBELER (1953) who employed electron impact ionisation, have studied the isotopic composition of Ga. On the other hand a considerable amount of work has been published on the abundance and distribution of Ga, particularly in iron meteorites. GOLDBERG et al. (1951) discovered that the Ga concentrations in iron meteorites were quantised in three, apparently well-defined regions. LOVERING et al. (1957) extended the earlier work and found that the Ga and Ge concentrations were highly correlated, and that iron meteorites could be classified into four Ga-Ge groups. It WMI also realised that the existence of these discrete groups might have resulted from differing formation histories of the meteorites. 736

736

J. R. DE LAETER

Wasson and coworkers determined the abundances of Ga and other elements in order to elucidate the cIass~cation of iron meteorites, (WASSO~, 1967; WASSOW and KIMBERLIN,1967; WASSON, 1969; WASSO~, 1970; W~so~ and SCHAUDU, 1971). WMSON and KIMBERLIN(1967) noted that genetic groups of iron meteorites are charaoterised by (1) limited concentration ranges of all elements compared to the range found in the iron meteorites as a whole; (2) smooth variations in the concentration of one element plotted against that of another; (3) similar structures. On the basis of analytical data on some 460 iron meteorites, eleven resolved chemical groups have been defined. The present work has used the stable isotope dilution technique to measure the Ga concentration in 21 iron meteorites and in 5 stony meteorites. The elemental abundance of Ga in 13 standard rocks has also been measured. As far as can be ascertained, this is the first time that the isotope dilution technique has been used to determine the abundance of Ga either in meteorites or in standard rocks. EXPERIMENTAL PROCEDURE A 1 to 2 g sample was sawed from a fresh, inclusion-free piece of each iron meteorite analyzed. After careful cleaning to xemove any surface contamination, the sample was weighed and placed in a quartz flask fitted with a teflon top. A gentle stream of dry nitrogen was allowed to fill the flask while the sample was being digested in pure HCl. The concentration and amount of the HCl used was sufficient to ensure that the concentration of the final solution was between 6-7 M. The solution was then quickly transferred to an anion exchange column (I.5 cm x 15 cm, packed with Dowex AGI x 10, IO!&-200mesh), previously equ~ibrated with 6 M HCI. Several column volumes of 6 M HCl were sufficient to remove the ferrous Fe, Ni and Co, whilst ferric Fe together with Ga were still strongly adsorbed (&%AUS and NELSON, 1958). The ferric Fe and Ga were then eluted using 2 columns of O-1M HCl. Under the existing digestion conditions most of the Fe could be kept in the ferrous state, thus minimising the amount of contaminant eluted with the Ga. The Ga fraction was evaporated on a water bath, thus ensuring that the gallium chloride was not lost. The residue was redissolved in a minimum of 4 M HCI and placed on a small cation exchange column (05 cm x 5 cm, packed with Dowex AC 50 x 8, 200-400 mesh), equilibrated with 4 M HCl. It was found that at this concentration the ferris Fe was eluted just ahead of the Ga. Immediately after the ferric iron was eluted, 2 column volumes of 4 M HCl were collected and taken to dryness with a few drops of concentrated HNO,. The residue was then taken up in a drop of HNO, and transferred to the side filament of a conventiona triple filament source. Previously outgassed Re filaments were used throughout the analysis. No evidence of Ca oont~~ation from the filaments or ion source were detected. Blank analyses of the complete extraction procedure showed that the contamination level was of the order of 4 x 10Wsg, which is insignificant in comparison with the amount of Ga contained in the samples. The Ga concentrations were, however, corrected for this small blank. Silicate samples were first crushed in an agate mortar to 200 mesh size and then weighed into a teflon dish. Each sample (of size O-5 to 1 g) was dissolved with a HF-HClO, mixture, taken to dryness, and then redissolved in 6 M HCI. This solution was then placed on the large anion column and the extraction procedure repeated as for the iron meteorites. Isotope dillction The elemental abundance of Ga in the iron and stony meteorites and in a number of standard rooks was determined by using a gallium oxide tracer, enriched to approximately Q@6% in

The isotopic composition and elemental abundance of gallium in meteorites

737

Ga’r. The gallium oxide tracer was taken into solution in 10 M NaOH and then neutralised with HCl. The concentration of this solution was determined using the isotope dilution method itself to calibrate the tracer against an accurate gravimetric standard made up from a spectroscopically pure sample of Ga metal. An accurately weighed quantity of the Ga71 tracer was added to a known amount of the sample immediately prior to dissolution. Sufficient tracer was added to ensure that the Gasg/Ga71ratio of the mixture was approximately unity.

Isotopic analyses were carried out on a 12 in. radius, 90” magnetic sector, solid source mass spectrometer, equipped with an electron multiplier. Microgram quantities of Ga produced an ion beam in excess of lo-l2 A for several hours without a marked decrease in intensity. The ion beams were amplified by a vibrating reed electrometerwith a 10s ohm input resistor whose linearity as a function of ion beam intensity was carefully checked. A voltage-to-frequency converter, followed by an electronic counter, allowed digital presentation of the data, which was fed on-line to a small oomputer. The amplifying system was periodically calibrated for scale factors, linearity and speed of response. The magnetic field was successively switched from mass 69 to 72 and back again, until each isotope was measured approximately 100 times. The computer was programmed to select a number which was representative of the height of each peak, and this notation was stored in memory until the mass spectrometer run was completed, after which the isotopic ratios were calculated. This system of data analysis enabled the mean and standard deviation of the Ga7i{Gasg ratios to be computed immediately after the completion of a run with a minimum of operator involvement, thus contributing significantly to the efficiency of data analysis. A careful examination of the mass spectrum in the vicinity of the Ga isotopes was made to detect the presence of interfering ions. No evidence of any peaks above background was ever found. Time dependent isotopia fractionation was observed in most of the analyses. The initial ratios measured when the Ga beam was first observed was always lower than the values quoted in this paper. The Ga7r/Gaavratios then increasedto a plateau value when very little change was observed over several hours of operation at a beam current of approximately 5 x lo-12 A. The samples were never taken to extinction and no ratios were taken unless a stable beam intensity was obtained. The importance of elim~at~~ variable fractionation effects in isotope abmdance studies is well known. Unfortunately it is not possible to employ the double spike technique for Ga, and additionalprecautionsneed to be taken to ensurethat samplesare measuredunder conditions which are as identical as possible. In this particular study isotope dilution measurements had already established the Ga concentrationfor the meteorites under examination. It was thereforepossible to mount samples of approximately the same size on the filament button. All samples, including the terrestrial standards, were subjected to the same chemical extraction procedure, so that the samples actually mounted were not only of similar size, but also of similar composition. The same filament button was used for a number of successiveanalyses. After the data from a particular sample had been collected, the filament currents were increased to outgassing conditions until no Ga memory was observed at the normal operating cmrents. The new sample was then loaded, and the source reassembled as for the previous run. This could be done in such a way that the focussing conditions remained unaltered from analysis to analysis. Three filament buttons were used for the 15 samples analysed between 22nd July and 13th August 1971. A standard sample was analysed for each new filament button used. In point of fact no significaut diEerencein isotopic ratio was observed for these 3 analyses. A strict time sequencewas adopted for each sample, and measurementswere not aommenced until the Gas9 ion beam had attained stability at a pre-determined electrometer reading. The electron multiplier gain was not altered during the course of the measurements; nor were the slit widths or the magnet settings. The accelerator voltage was always operated at 6 kV. Since the data was digitised and the selection of peak maxima was achieved by the computer programme, there was no possibility of operator bias affecting the results.

J. R. DE LAETEB

738

RESULTSBND DISCUSSION Table 1 lists the isotopic composition of various terrestrial Ga samples as measured by a number of workers. The value of O-6559 for Ga71/Gassas measured in this laboratory for terrestrial samples has not been corrected for mass discrimination. Experience with other elements (Ba, Rb and Sr) indicates that the mass spectrometer used in this project fractionates each mass unit by approximately 6*2x,,, the lighter isotopes being enhanced with respect to the heavier masses. This fractionation is largely produced by the different ion-electron conversion e5ciencies in the electron multiplier. Table 1. Isotopic compositionof gallium Method of ionisstion

(1935) SAMPSONand BLEAKNEY (1936) hQHF%AI el al. (1948) HIBBS (1949) ANTKIW and DIBELEB (1953) This work This work corrected for 6.26’& per m*** unit &TON

Anode rays Thermionic Thermionic Electron impaot Electron impact Thermionic

clay

61.5 61.2 60.2 60.00 f 60.4 60.1

y)0

0.07

ae”(

%)

38.5 38.8 39.8 40.00 f 39.6 -

0.05

39.9

cls”/CkP

0.6260 0.6340 0.6623 0.6667 0.6566 0.6569

f f

0.0063 0.0050

f f

0.0066 0*0006

0.6641 f

0.0005

The National Bureau of Standards Rb standard (NBS 984) has a quoted absolute Rb*‘/Rb*s ratio of O-38565 f 0.00027. Repeated measurements of this Standard Rb in this laboratory gives a value of O-38086 &- 0*00041, which represents a mass fractionation of 6~25%~per mass unit. If the measured Ga71/GaaSvalue is corrected by 12*5x,, then a value of O-6641 is obtained, which is in good agreement with the values of INQHRAM et al. (1948) and HIBBS (1949). The value of O-6623 as quoted by INO~AM et al. (1948) is generally accepted as being the “correct” value. The isotopic composition of Ga in the terrestrial standards and in 6 iron meteorites is given in Table 2. The isotopic ratios have not been corrected for mass discrimination effects. The value of each Ga71/Ga’j9ratio listed is the average of approximately 100 ratio measurements. The error quoted with each of these values is the standard deviation of the set of 100 ratio measurements. Each meteorite was measured on two separate occasions to give an indication of the precision of the measurements. The error quoted for the means of the 2 meteorite analyses is the standard deviation of the total set of all the measured ratios for that meteorite. The spectroscopically pure reagent Ga solution was measured 4 times during the period of the investigation. A terrestrial syenite sample was also analysed. The error quoted with the terrestrial mean value is the statistical error of the five terrestrial analyses at the 99 y. confidence limit. The last column in Table 2 lists the permil deviations 6 of the meteoritic values with respect to the Terrestrial Mean value where: 6 = 1000

1

(Ga71/Gass),et - (Ga71/Ga69)sTD ( Ga71/Ga6s)sTn

The maximum deviation is - l*l%, for Canyon Diablo and Mt. Dooling, the average

The isotopic composition and elemental abundance of gallium in meteorites

739

Table 2. Isotopic composition of gallium in iron meteorites Deviation from tmreetrial mean Qa”/Ga~~

Sample

Date

Reagent Ga solution

9th July 1971 22nd July 19’71 4th August 1971 12th August 1971 10th July 1971

0.6556 0.6663 0.6560 0.6561 0.6567 0.6669

f & * f f f

0.0010 0.0011 0.0009 0.0009 0.0010 0.0006

10th August 1971 13th August 1971 MeWI 4th August 1971 12th August 1971 MlXbll 22nd July 1971 6th August 1971 MeaIl 29th July 1971 11th August 1971 Mea 6th August 1971 11th August 1971 MEIVl 28th July 1971 13th August 1971 MeflZI

0.6654 0.6660 0.6652 0.6560 0.6554 0.6652 0.6556 0.6567 0.6556 0.6658 0.6656 0.6657 0.6665 0.6662 0.6553 0.6663 0.6860 0.6561

f f f f f f f f f f f f f f f f + f

0.0010 0.0013 0.0011 0*0012 0.0011 0.0011 0.0009 0.0010 0.0009 0.0014 0.0009 0.0011 O*OOll 0.0008 0.0009 0.0013 0~0011 0.0012

syenite SY-3 Terrestrial mem Cesyon Diablo

Mt. Dooling

Mt.. Stirling

Mundrabilla

Toluca

Youndegin

Average

a%,

-1.1

-1.1

-0.6

-0.3

-0.9

+0*3 -0.6

for the 6 meteorites being -0*6%,. These differences are not statistically significant in terms of the quoted errors at the 95 per cent confidence limit. Unfortunately 4 of the 6 meteorites analysed belong to Chemical Group I. The other two siderites-Mt. Dooling and Mundrabilla-are probably distant relatives of the Group I irons. In retrospect it would have been valuable to have examined the Gari/Ga@ ratio in meteorites drawn from other of the chemical groups to see if any exceptions could be found to the claim that the isotopic composition of meteoritic and terrestrial Ga is identical to within 0.11 per cent. The Ga content of the 21 iron and 5 stony meteorites given in Table 3 have all been determined by the stable isotope dilution technique. Many of the samples have been analysed in duplicate to provide an indication of the precision of the technique for Ga determinations. The accuracy of any particular analysis, as based on weighing errors and isotopic ratio measurements, is usually of the order of &l per cent, but the absolute error is certainly a factor of two larger. An australite from the Kalgoorlie region in Western Australia was also analysed to give an abundance of 5.5 ppm for Ga. This is to be compared with an average value of 8.3 ppm for 43 australites analysed by TAYLOR and SACHS(1964). However, Taylor and Sachs’ emission spectroscopic measurements were based on Ga values for the standard rocks G-l and W-l of 18 ppm and 24 ppm respectively. The deviation

presently accepted value for W-l is 16 ppm, and the renormalisation of their data using this lower value for W-l would reduce the discrepancy between the two sets of data.

J. R.

740

LtE LAETER

Table 3. Concentrtstionof gallium in meteorites (in ppm by weight)

A. Irons Avooe B&our

Chemical group? IIIA I

struatural+ Om Downs

This work

Classification

Meteorite

Om-Og

Ge concentration literature P&ms

21.9 61.4 62.8 s2:

56.4

sa=i 21.2 X6.9 16.5

74= 76.3” 81.8 79.7”

Canyon Diablo Duketon Heig

083 Om Om-Og

I IfIA IIIA

Henbury Kumerintl

Om-Og OPl

IIIA IIC

Milly Milly Mt. Dooling

Om AllOln

IIIA I-An3

=

618 51.51

Mt. Maguet Mt. Stiriing

D-Opl

Auom I

-z 88.1 8B*1

6.4& 7.70 7*63g 7*3h

587

63’ 4ge

Mundrabills Nuleri Odessa Red6elds

Om Om

Anom Anom I Auom

zi 17.4 75.3 40.6 -41.1

74.7d

IIIB I IVB IIIB IIIA I

iz 71.0 O-23 21-6 20‘6 90.6 90.7

13s 16.26 61s 6!&2b 70.@ 18.2” O-24=’ 138

ZZf -

88s 90.1

18.2 4.4 6.0 4.4 4.3

17.0k 18.33 6.4’ 4.91 5*6k

‘iiiri

%-%S

OS H

,, 39.5 39.2

608 17.2b 17.46 l@’

z3

36.80

19-9 67.5 58.1

40.8

Tieraeo Creek Toluca Warburton Range Wonyuig~a Youanmi Youndegin

B. Stones Abee Bruderheim Cocklebiddy Lake Brown woolgorong C. Special categov Australite

Of-Om Cg D Om Om OS-%g

Petrological~ E4 L6 H1 L6 L? -

64

88”

8.3%

~b,Lovx~wa et al. (1957); b, GOLDBERGet al. (1951); o. WASSON (1967); d, WASSO~Vmd KIMBERLY (1967); e, WASSO (1969); f, WASSON (1970) g, WASSON snd SOHAUDY(1971): h, Swr.~s et al. (1967); i, TANDONand WASSON (1968) j, Fonc~d and SNALES (1967); k, GREINT.AND(1966); 1, TAYLOB and SACHS (1964); m, SCHAUDY et cd.(1972). * Structural classification according to Buchwald (see WASSON, 1970). I’ Chemical groups aacordiug to WASSON (197 1) and earlier references. $ Petrological classification acoording to VAN Sc~aaos and WOOD (1967).

The Ga content of 13 of the iron meteorites studied have been analysed by other authors, and their values have been included in Table 3. The agreement between the present work and the literature values is, in general, good although there are some notable exceptions. Canyon Diablo has been analysed on numerous occasions

The isotopic composition and elemental abundance of gallium in meteorites

741

by many workers, and one could question if the spread in values from 74 ppm to 84.7 ppm is due to actual variations in the meteorite, or to analytical discrepancies. Experience in standard rock determinations indicates that the latter explanation is probably the more likely, although the size and nature of the Canyon Diablo meteorite may mean that the variations are in fact real. Henbury is a meteorite which has apparently been mislabelled (SMALESet al (1967)), and our result confirms the fact that the figure of 60 ppm obtained by Lovlemxaeta&(1957) should be ignored. The situation with respect to Mt. Stirling requires explanation as the present result is significantly higher than the data of LOVERINGI et al. (1957) and WASSON (1969). Mt. Stirling is generally considered to be part of the Youndegin shower (MCCALLand DE LAETER, 1965). In contrast to the data of WASSON(1969), the present work lends support to this hypothesis. A slice of the genuine Mt. Stirling meteorite obtained from the Western Australian Museum was recently analysed in Wasson’s laboratory and a value of 86.6 ppm was obtained. This compares favourably with the 88.6 ppm measured in this laboratory and confirms the suspicion of Wasson that his original specimen must have been mislabelled. The Ga values for Toluca also show a wide variation between different authors. The present result confirms the value of 70.6 ppm given by WASSON(1970), and verifies the suggestion of SXALESet al. (1967) that their specimen was not genuine Toluca. As far as can be ascertained, the other 8 iron meteorites listed in Table 3 have not been previously analysed for Ga. An attempt has been made to classify these meteorites into a structural class on the basis of their Widmanstatten pattern and nickel content, and into a chemical group using Ni and Ge abundances determined by X-ray fluorescence spectrometry in this laboratory. The Ga abidance of 18-2 ppm for the enstatite ohondrite Abee compares favourably with the value of 17 ppm as determined by GR~E~A~D (1965) and 18.3 ppm by Forron& and SMALES(1967). The average value of Ga for 6 enstatite ehondrites as determined by GREENLAND(1965) was in fact 15.6 ppm. The 4 ordinary chondrites listed in Table 3 have Ga abundances in the range 4.3 ppm to 5-O ppm. This compares with an average value of 5.4 ppm for 6 H and L group chondrites as determined by GREENLAND (1965), and an average value of 5.5 ppm for I.6 ordinary chondrites as determined by FOUCH&and SMALES(1967). TANDON and WASSON(1968) obtained a range of Ga values from 4.6 to 5.7 ppm for 5 L group ohondrites. The publication of FLAXAom (1969~ on the new series of samples prepared by the U.S. Geological Survey provides the first compilation of analytical data on the major, minor and trace constituents of these standard rocks. The Ga determinations cover a wide range of abundances, and under the circumstances the average values given by FLANAGAN(1969) are of little significance. In particular, the average values of PCC-1 and DTS-I. are meaningless, since the sensitivity of most of the analytical techniques used were inadequate to measure the low level of Ga in these particular rocks. The concentration of Ga in 13 standard rocks as measured in this laboratory are given in Table 4. The recommended values for G-l and W-l given by FLEISCHER (1969) are confirmed by the present work, and our result for W-l is also in good 3

J. R. DE LAETER

742

Table 4. Concentrationof gallium in standard rocks (in ppm by weight) Sample

G-1 W-l a-2 GSP-1 AGV-1 PC&l DTS-1 BCR-1 SY-2 SY-3 NBS-708 T-l OR-l

source

This work

USGS USGS USGS USGS USGS USGS USGS USGS ssc ssc NBS T&IlZtLIli8 Penn. state

19.6 16.9 22.9 22.3 20.6 0.42 0.16 19.8 24.7 25.6 17.1 18.8 3.0

Liter8tllre v&e 18

16 20.2 lw3 18.4 12.4 12.5 21.6 33 43 21 -

FLEISCHEB (1969)

FLEISCHEB (1969) FLANAGAN (1969) FLANAGAN (1969)

FLANAGAN 11969) FLANAUAN i1969j FLANAC+AN(1969) FLANAQAN (1969) GILLIEEION(1969) GILLIESON (1969) GEOL. Swv. DIV. TANZANLA (1963)

agreement with the value of 16.5 ppm by TANDONand WASSON(1968). The values of Ga for the new U.S.G.S. rock standards in Table 4 may be compared to the average Ga contents given by FLANAGAN (1969). The agreement is surprisingly good for G-2, GSP-1, AGV-1 and BCR-1, when one considers the enormous variation in the Ga abundances in these standard rocks as listed by FLANAGAN (1969). On the other hand the values of O-42 ppm and 0.15 ppm as determined for PCC-1 and DTS-1 respectively, are much lower than any of the determinations given by FLANAGAN (1969). In fact only BRUNFELT etal.(1967), who used neutron activation analysis, give a meaningful value for these 2 rocks. In addition to the 8 U.S.G.S. standards, the concentrations in 5 other standard rocks SY-2, SY-3, NBS-70a, T-l and OR-l are also given in Table 4. The fLrsttwo rocks are syenite samples distributed by the Spectroscopic Society of Canada, and our values are much lower than the provisional analyses of GILLIESON(1969). The NBS-70a K-feldspar has recently been proposed as an inter-lrtboratory Rb-Sr geochronological standard by COMPSTON etal.(1969). A value of 17 ppm is suggested as the Ga concentration of this rock. Our value for the standard tonalite T-l is in good agreement with the existing value, whilst as far as can be ascertained, our value of 3 ppm for the Adularia OR-l is the first analysis of Ga which has been reported. Ackwwledgements-Many of the meteorite samples were generously supplied by the Western Australian Museum Board to whom appreciation is expressed. Mr. R. READ and Mr. I. D. ABERCROMBIE provided technical assistance for some phases of the project. This researchwas supported by the Australian Research Grants Committee. REFERENCES ANDERS ANTKIW

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1890-1891.

F. W. (1935) The isotopic constitution and atomic weights of hafnium, thorium, rhodium, titanium, zirconium, calcium, gallium, silver, carbon, nickel, cadmium, iron and indium. Proc. R. Sot. Series A. 149, 396-405. BRUNFELTA. O., JOHANSEN0. and STEINNESE. (196’7) Determination of copper, gallium and zinc in standard rocks by neutron activation. Anal. Chim. Acta 37, 172-178. COMPSTON W., CHAPPELL B. W., ARRIENSP. A. and VERNONM. J. (1969) On the feasibility of NBS 708 K-feldspar as a Rb-Sr age referencesample. Geochim. Cosmochim. Acta 33, 763-757. ASTON

The isotopic composition

and elemental abundance of gallium in meteorites

743

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