Geo log i cal Quar terly, 2001, 45 (3): 315–317
K/Ar dat ing and sta ble iso tope anal y sis of the Baszkówka and Mt. Tazerzait L5 chondrites
Stanisław HAŁAS and Artur WÓJTOWICZ
Hałas S. and Wójtowicz A. (2001) — K/Ar dat ing and sta ble iso tope anal y sis of the Baszkówka and Mt. Tazerzait L5 chondrites. Geol.
Quart., 45 (3): 315–317. Warszawa.
We have de ter mined the con tent of 40Ar* (1.49 nmol/g) and 4He (1.75 nmol/g) in the Baszkówka me te or ite by static vac uum mass spec - trom e try. The ra dio genic ar gon con tent was cal cu lated from the mea sured ar gon spec trum us ing the equa tion: 40Ar* = 40Ar – 295.536Ar as sum ing a po tas sium con tent of 680 ppm wgt. We have ob tained K/Ar ages of 3.78 Ga for the chon drules and 3.47 Ga for a bulk sam ple of Baszkówka. A sim i lar 4He con tent (1.47 nmol/g) but larger 40Ar* (2.32 nmol/g) con tent was found for the Mt. Tazerzait bulk sam ple.
The K/Ar age of this me te or ite on the ba sis of the K con tent (732 ppm) is 4.34 Ga. Sul phur iso tope anal y sis of troilite spec i mens from both me te or ites re veals es sen tially neg a tive δ34S val ues: –1.25±0.06‰ for Baszkówka and –1.18±0.06‰ for Mt. Tazerzait ex pressed on the V-CDT scale. This in di cates that both chondrites may have a com mon or i gin. The ox y gen iso tope data (δ18O = 4.88±0.03‰ and δ17O
= 3.66±0.10‰) sug gests that Baszkówka be longs to the L chondrites.
Stanisław Hałas, In sti tute of Phys ics, Maria Cu rie-Skłodowska Uni ver sity, Pl. M. Cu rie-Skłodowskiej 1, PL-20-031 Lublin, Po land;
Artur Wójtowicz, Pol ish Geo log i cal In sti tute, Rakowiecka 4, PL-00-975 Warszawa, Po land (re ceived: July 27, 2000; ac cepted: Oc to - ber: 31, 2000).
Key words: Baszkówka, Mt. Tazerzait, chondrites, K/Ar ages, ox y gen iso topes, sul phur iso topes.
INTRODUCTION
As part of the 6PO4D 031 13 pro ject we have ana lysed
40Ar, 38Ar, 36Ar and 4He iso topes in three spec i mens of the Baszkówka me te or ite and one spec i men of the Mt. Tazerzait meteorite. The main goals of this study were K/Ar dat ing of these L5 chondrites and de ter mi na tion of their sta ble iso tope ra tios of sul phur and ox y gen.
The pre vi ous study of no ble gases by Wlotzka et al. (1997) in di cate that both me te or ites are char ac ter ised by long ex po sure ages, about 70 Ma, and es ti mated K/Ar ages, cal cu lated assuming that the K con tent is 800 ppm, are 4.3 and > 4.6 Ga for Baszkówka and Mt. Tazerzait re spec tively. Re cal cu lated using known K con tents (given be low) the ages are 4.57 and
>4.75 Ga re spec tively. The two meteorites share a num ber of un usual fea tures con cern ing their pe trog ra phy and no ble gas
com po si tion. In this study we up date the age de ter mi na tions and dis cuss the new values of the sta ble iso tope ra tios of sul - phur and ox y gen.
ANALYSIS OF ARGON ISOTOPES AND OTHER NOBLE GASES
The static vac uum spec trom e ter MS-10 with an orig i nal mag net of 0.12 T (for 4He mea sure ment) and with a stron ger Nd-Fe-B mag net of 0.437 T (for the re main ing no ble gases) was em ployed. The an a lyt i cal pro ce dures were de scribed by Hałas (1995).
Aliquots of me te or ite frag ments of about 60–80 mg each were wrapped in Al-foil and loaded into the ex trac - tion-purification line, where gases were re leased at a tem per a - ture of about 1500°C. The no ble gases were pu ri fied by
adsorbtion of other gases. The peaks of the no ble gases were mea sured by static-vacuum mass spec trom e try.
In this study we were able to es ti mate the 4He con tent and to ana lyse ar gon iso topes by means of an unspiked method and by us ing a pure 38Ar spike. All sam ples were mea sured us ing the
38Ar spike method ex cept for bulk sam ples of Baszkówka and Mt. Tazerzait, which were re peated by the unspiked method. In these cases 38Ar re leased from me te or ites was re corded. The ra - dio genic ar gon con tent V40, Ar* was cal cu lated from mea sured
40Ar by cor rect ing for at mo spheric con tent (295.536Ar con tent).
Be cause we did not have any the he lium spike, we de ter - mined the he lium con tent of the spec i mens us ing the sen si tiv ity of the mass spec trom e ter given in the MS-10 man ual. The ar - gon and he lium ion cur rents were mea sured and the He con tent was cal cu lated from the sen si tiv ity ra tio and from the vol ume ra tio of in di vid ual parts of the vac uum line de ter mined by means of a por tion of 38Ar spike.
NOBLE GAS ANALYSES
The re sults ob tained are dis played in Ta ble 1. The mea - sured 4He con tent is com pa ra ble to that of 40Ar. We have also ob served the Ne iso topes (only 21Ne and 22Ne were de tected be - cause 20Ne was over lapped by the 40Ar++ peak) at the de tec tion level of our mass spec trom e ter, which is about 0.03 pmol. Our de ter mi na tion of the 4He con tent is con sis tent with the pre vi ous mea sure ment made by Scherer (1996, pers. comm.) for a bulk spec i men only. The re main ing re sults were some what higher.
We have ob tained an age for the me te or ites close to the age of the so lar sys tem (4.60 Ga). The large er rors on our dates arise mainly from the po tas sium de ter mi na tion. We plan to re - de ter mine the % K by the iso tope di lu tion method (Hałas and Maciocha, 2000).
316 Stanisław Hałas and Artur Wójtowicz
T a b l e 1 An a lytic of data
Sam ple Ka
[%]
40Ar [nmol/g]
38Ar [pmol/g]
36Ar [pmol/g]
40Ar* [nmol/g]
40Ar* [%]
Age [Ga]
4He [nmol/g]
Baszkówka chon drules
0.0680±0.0075
1.730 na 0.83 1.486 85.9 3.75±0.62
1.75
1.736b 4.54 0.63 1.550 89.3 3.81±0.52
Baszkówka bulk
1.723 na 1.61 1.246 72.3 3.47±0.47 1.12
Baszkówka
shell 1.568 na 3.94 0.403 25.7 1.92±0.38 na
Mt. Tazerzait bulk
0.0732±0.0110 2.747 na 1.33 2.354 85.7 4.37±0.72
1.47
2.586b 6.27 1.04 2.278 88.1 4.31±0.72
40Ar* — ra dio genic ar gon; a — de ter mined by R. Dybczyński; b — anal y sis re peated us ing an unspiked method; na — not ana lysed
Fig. 1. Iso tope com po si tion of troilite sul phur from Baszkówka and Mt.
Tazerzait against a back ground of data from Kaplan and Hulston (1966)
SULPHUR ISOTOPES
Two par ti cles of troilite (FeS) were sep a rated from both me te or ites by hand pick ing un der a bin oc u lar mi cro scope for mass spec tro met ric de ter mi na tion of δ34S. Aliquots of ca. 10 mg FeS were oxi dised by Cu2O at 850°C in a vac uum line ac - cord ing to the pro ce dure de scribed by Rob in son and Kusakabe (1975). The SO2 gas pro duced was ana lysed on a dual in let, tri - ple col lec tor mass spec trom e ter which was cal i brated to the V-CDT scale by means of SO2 gas produced from an IAEA-S1 ref er ence Ag2S spec i men of known δ34S = –0.30‰.
The re sults ob tained are:
— Baszkówka — δ34S = –1.25±0.06‰;
— Mt. Tazerzait — δ34S = –1.18±0.06‰.
The above re sults may be con sid ered as iden ti cal within er - ror. How ever, they sig nif i cantly de part from most val ues ob - tained for troilite from me te or ites. In Fig ure 1 the above re sults are com pared with data ob tained at McMaster Uni ver sity, where Kaplan and Hulston (1966) have in ves ti gated sev eral dozen sam ples taken from var i ous stony me te or ites, siderites and one spec i men of ter res trial FeS.
As is seen in Fig ure 1 both me te or ites in ves ti gated here have the most neg a tive δ34S val ues for troilite sul phur yet ob - tained. More over, their iden ti cal sul phur iso to pic com po si tion in di cates a com mon or i gin for these bod ies.
OXYGEN ISOTOPE COMPOSITION
A whole-rock spec i men of the Baszkówka me te or ite was ana lysed at the Open Uni ver sity in Eng land by Dr. Ian A.
Franchi. Re spec tive delta val ues were ex pressed on the V-SMOW scale.
The re sults ob tained are:
— δ18O = 4.88±0.03‰;
— δ17O = 3.66±0.10‰.
These re sults are plot ted on Fig ure 2 along with data ob - tained for var i ous me te or ites at the Uni ver sity of Chi cago (Clay ton et al., 1976). It is clearly seen that Baszkówka falls into class L and LL. Hence this body rep re sents neg li gi bly al - tered orig i nal mat ter of the so lar sys tem. For the sig nif i cance of the ox y gen iso tope vari a tion the reader is re ferred to Clay ton (1981) and the re view by Shima (1986).
Ac knowl edge ments. For the ex cel lent per for mance of our MS-10 we are in debted to Dr. T. Durakiewicz who in stalled his own de sign elec tron emis sion con trol ler. We are grate ful to Dr.
A. E. Fallick and Dr. I. A. Franchi for ox y gen iso tope anal y sis of a Baszkówka sam ple. The manu script was im proved by sug - ges tions from Pro fes sor Z. M. Migaszewski and Dr.
T. Durakiewicz.
REFERENCES
CLAYTON R. N. (1981) — Iso to pic vari a tions in prim i tive me te or ites.
Phil. Trans. R. Soc. Lon don, A, 303: 339–349.
CLAYTON R. N., ONUMA N. and MAYEDA T. K. (1976) — A clas si fi ca - tion of me te or ites based on ox y gen iso topes. Earth. Planet. Sc. Lett., 30: 10–18.
HAŁAS S. (1995) — Iso tope geo chron ol ogy based on ra dio ac tive dis in te - gra tion of 40 K (in Pol ish only). Prz. Geol., 43 (12): 993–998.
HAŁAS S. and MACIOCHA T. (2000) — Ap pli ca tion of iso tope di lu tion for de ter mi na tion of po tas sium con cen tra tion (in Pol ish only). In:
Dating of Min erals and Rocks: 17–23. Con fer ence ab stracts, 10–12 Feb ru ary 2000, Kraków.
KAPLAN I. R. and HULSTON J. R. (1966) — The iso to pic abun dance and con tent of sul fur in me te or ites. Geochim. Cosmochim. Acta, 30:
479–496.
ROBINSON B. W. and KUSAKABE M. (1975) — Quan ti ta tive prep a ra - tion of sul fur di ox ide for 34S/32S anal y ses, from sul fides by com bus - tion with cuprous ox ide. Anal. Chem., 47: 1179–1181.
SHIMA M. (1986) — A sum mary of ex tremes of iso to pic vari a tions in ex - tra-terrestrial ma te ri als. Geochim. Cosmochim. Acta, 50: 577–584.
WLOTZKA F., SCHERER P., SCHULTZ L., OTTO J. and STĘPNIEWSKI M. (1997) — Pe trog ra phy and no ble gases of the un - usual L5 chondrites Baszkówka and Mt. Tazerzait. 60th Me te or ite Meet ing, July 21–25, Maui, Ha waii, USA.
K/Ar dating and stable isotope analysis of the Baszkówka and Mt. Tazerzait L5 chondrites 317
Fig. 2. Ox y gen iso tope com po si tion of Baszkówka chon drules against a back ground of data ob tained by Clay ton et al. (1976)
