Equilibration temperatures of the ordinary chondrites: A new evaluation

0016-7037/84,'S3.00 + ,343

NOTE

EquiIib~ationtemperatures of the ordinary chondrites: A new evaluation EDWARDJ. OLSEN Field Museum of Natural History, Chicago, IL 60605

and T. E. BUNCH Ames Research Center, National Aeronautics and Space Administration, Moffett Field, CA 9403.5 (Received November 23, 1983; accepted in revisedfom March 9, 1984) A~S~I%C+KRETZ(1982) and

LIND~LEY (1983) have each calibrated the Ca-pymxene thermometer. These newcalibrationsanappliedto24analrzedpyroxcnwfrom24H6,MandLL6chondrites.Boththermometen agree that the H group equilibrated to a significantly lower temperature, 820”-8M”C, than the L and LL groups. The two thermometers disagme on the value of the higher temperature of equilibration of the L and LL groups, giving values of 860°C (Lindsley) to 930°C (Katz). INTRODUCTION

graphically from the temperature-contoured pyroxene qu~~late~. ~te~ination of temperature using the KRETZ ( 1963) devised a theoretical mineral thermomorthopyroxene data is difficult because of the steepness eter based on the exchange of Fe++ and Mg++ beof the temperature contours on this side of the diagram; tween high calcium clinopyroxene and low calcium however, it is fairly easy to obtain temperatures from orthopyroxene. VAN SCHMUS and KOFFMAN ( 1967) the gentler slopes on the C&rich flank of the diagram. applied the Kretz thermometer to equiiibmted chonAnalyses of equilibrated chondrites by BUNCH and drite meteorites and estimated a single temperature of OLSEN(1973) have been used here to determine equilequilibration between 800” and 85O’C. Their electron ibration temperatures from both these thermometers. microprobe analytical technique was, by present stanBunch and Olsen reanalyzed 19 of the 2 1 H6, L6 and dards, of poor quality, and the accuracy of the temLL6 chondrites which had been analyzed by VAN perature determination was necessarily low. SCHMUSand KOFFMAN (1967). The electron microSince then the pyroxene thermometer has been reprobe analytical method and data reduction algorithm vised by a number of workers, principally BLANDER were superior to the older analyses, and of the same ( 1972), WOOD and BANNO( 1973), Ross and HUEBNER quality used today. The Bunch and Olsen analyses (1975), SAXENA(1976) and WELLS (1977). None of include only those which are within one relative percent them have been wholly satisfactory (BOHLENand Esof ideal sums and stoichiometty. Furthermore, analyses SENE, 1979). DODD (198 1, pp. 90-93) has reviewed have been rejected in which Si exceeds 2.000 or in the whole matter of mineral thermometry apphed to which there is insufficient Al to fulfill the tetrahedral ordinary chondrites. requirement of Si + Al = 2.000. Altogether 24 analyses Recently, KRETZ ( 1982) and LINDSLEY( 1983) have have been used from the original Bunch and Olsen produced thermometers based on the partitioning of 1973 data set of 48 analyses In addition to temperature calcium between high calcium clinopyroxene and low determinations from the two C&thermometers, temcalcium orthopyroxene, calibrated on both empirical peratures were also computed from Kretz’s recalibrated and experimental data. Katz, in addition, recalibrated Fe-Mg exchange thermometer. All temperatures are his Fe-Mg exchange thermometer. Lindsley finds presented in Table 1. Kretz’s ~-the~ometer agrees reasonably well with his own; however, he finds that Kretz’s Fe-Mg thermometer overestimates equilibration temperatures. RESULTS The calculations that lead to actual temperature values by these Ca-thermometers of Lindsley and Kretz &-eta’s ~-the~ometer gives values about 60 cenare very different. Kretz provides analytical equations tigrade degrees higher for the L and LL groups than that require measured values of only Ca, Mg and Fe Lindsley’s thermometer. For the H group, however, of the Ca-rich clinopyroxene. The Lindsley thermomboth thermometers agree that equilibration continued eter requires a lengthy calculation using an entire py- to a lower temperature, 820’~830°C, than for the L roxene analysis to compute Wo, En and Fs values that and LL groups. are adjusted to approximate the activities of these Kretz’s Fe-Mg exchange thermometer agrees reacomponents in his model; temperature is then obtained sonably well for the H group but gives even higher 1363

1364

F J. Olsen and T. E. Bunch Table

1.

Calculated equilibration for individual rhondrite

tempttratures pyroxenes,

Kretz h-T -_

H6 Croup Oakley

1

OakI& 2 Oakley 3*** Est.&do Estacado Estacado

1 7. 3

Guarena 2 Cedar 1 Kernouve 2 MC?Wl

84O'C 820 1000 850 820 820 800 820 850 830 t 20*

870-C 820 1080 800 81U 800 830 740 870 820 + 40*

780-C HO0 730 7SO X4(! YhCI R30 830 h90 Hi0 - irOt

820 810 890 900 860 900 900 890 850 890 900 870 k 35*

870 840 970 1000 890 940 900 970 960 920 950 930 + 5Q*

1000 990 900 990 850 880 870 ** ** 1090 1110 960

820 890 870 850 860 t 30'

940 920 900 900 920 f.20*

920 ** 960 1020 970 f 50"

L6 Group

Colby Kyushu 1 Kyushu 2 Langhalsen1 Langhalscn2 Lanxhalsen3

Bath Furnscc 1 58th Furnace 2 Bruderheim1 Bruderheim3 MeaIl

. us*

LL6 Groue Lake Labyrinth Dhurmsala 1 N& 1 NHs 2 nesn

*One standard **Cannot be orthapyrorene

deviation.

calculatedbecause no analysis of pat&es

analytical

requirements

(see

text).

***Excludedin computingH group mean.

values for the L and LL groups. KRETZ (I982), using a data summary of the BUNCH and OLSEN(1973) analysts as published in BUNCH and OLSEN (1974), obtained somewhat higher temperatures for both his thermometers (Kretx’s Table 4). That data summary included analyses which have been excluded here for exceeding one relative percent deviation from ideal stoichiometry. Liadsley’s thermometer is calibrated for several total pmssum~ I atm, 5 kb, 10 kb, 15 kb. Since metamorphic friction presumably took place at some modest depth within a parent body, it seemed worthwhile to determine temperatures from at least the 5 kb quadrilaterai. The mean group temperatures for the H6, L6 and LL6 groups differ moderately at 5 kb from the 1 atm values: 87O*C, 900”, 880°C, respectively. DODD (198 1), however, states that equilibration of the ordinary chondrites probably occurred at a pressure less than I kb. In some cases analyses of different individual meteorite pyroxenes within a single chondrite are present in the data set (Table 1). With the Lindsky thermometer the apparent spreads in temperature within a single meteorite have ranges of 10” to 40°, except for Oakley, which has two values within 20” of each other and one temperature (Oakley 3) 180” higher, at 1000°C.

Similarly, Kretz’s Ca-thermometer shows apparent spreads of 0” to 50°, except for the same Oakley 3 analysis, which is 1080°C. For both thermometers the OakIey 3 value is outside two standard deviatrons of this group. The cause of such an aberrant value may be an error in the CaO percentage of the original anaiysis fortuitously compensated by errors in other elements such that the anaiysis passed the tests of stotchiometry and summation. As discussed below. it would require an unusually large error to accomplish this. The high temperature value for this pyroxene could also be real, resulting from a shock event in the history of the parent body with resulting partial disequilibration. LINGNER ef al.(1984) report that all H group chondrites they have studied show evidence for some degree of shock. In terms of the KRETZ (1982) Fe-Mg thermometer this same grain also gives an apparent temperature significantly lower than the other two pyroxene grains in Oakley. The lack of agreement of the apparent temperature trends between the two thermometers for this grain must be related to differences in the mobility of Fe-Mg versus Ca under post shock conditions. Shock heating is remarkably inhomogeneous in its effects on a small volumetric scale. BUCHWALD ( 1975) notes evidence of extreme shock only a centimeter away from clearly unshocked material in iron meteorites. The relaxation period that followed shock heating in a small volume in Oakley may have lasted long enough to affect the calcium content of this clinopyroxene grain, Oakley 3.

ERRORS Because the ~-~e~ornete~ are obviously sensitive to the accuracy of CaO determinations, it is worthwhile to examine the effect of analytical error on the derived temperatures. The analyses used here are within one relative percent of ideal. As a worst possible case we have doubled #at value and assumed a 2% relative error entirely taken up in CaO values of two selected analyses, one each from the H and L groups. From the Lindsley thermometer we calculate 255” spread due to &2% relative error in CaO. From the Kretz Ca-thermometer we calculate a &45” spread. In addition, +2% relative errors in Fe, Mg and Si were examined for each thermometer. Ah spreads in derived temperatures are smalier than that from CaO error, although IQ-et& thermometer shows unexpected sensitivity to an MgO error of &2% relative, a spread of +50”. Thus, relative analytical errors of +2% or less can account for calculated temperature variations from pyroxene to pyroxene and from meteorite to meteorite within each group (Table 1). Analytical error cannot account for the calculated temperatures for Oakiey 3. The Oakley 3 values of iOOO”and 1080°C appear to be real effects of shock heating.

1365

Equilibration temperatures of chondriks

DISCUSSION Analytical error can affect absolute temperatures but not their relative values. Such errors can explain the occasional high or low values for individual pyroxene grains but cannot explain differences between mean values for whole groups. Both Kretz’s Ca-thermometer and Lindsley’s thermometer indicate the L group equilibrated at a higher temperature than the H group by approximately 100” and 40”, respectively. A t-test was applied (excluding the Oakley 3 datum), and in both cases the mean group differences in tqUilibration temperatures are significant at the 1% level. There is no significant difference between the L and LL groups in either case. Equilibration above the metal-sulfide eutectic, 988”C, is generally ruled out on petrographic grounds. At an elevated pressure, however, the eutectic temperature goes above 1000°C (KULLERUD and YODER, 1959).

To account for trace element differences between H and L chondrites, LINGNER et al. (1984) suggested two possibilities: higher nebular condensation temperature for the H group relative to the L group, or a short-term thermal pulse for the H group followed by extended cooling. The lower equilibration temperature observed here for the H group supports the latter possibility. Thermometry based on Fe-Mg equilibration of pyroxenes disagrees with the Ca-thermometer at the high temperature end. This may be due to the cooling rates that obtained in chondrite parent bodies. Fe-Mg thermometry based on olivine-orthopyroxene equilibration in chrondrites

(MUELLER, 1964;

MEDARIS, 1969)

proved impossible beccause of insensitivity to moderate temperature differences, although OLSEN and BUNCH ( 1970) suggested that large temperature differences (several hundred degrees) can be discerned by the shapes of curves that plot the partition of Fe and Mg between coexisting olivines and pyroxenes. CONCLUSIONS

Based on the two newly calibrated Ca-pyroxene thermometers, the H group chondrites equilibrated to a lower mean temperature, 820”-83O”C, than the L and LL groups, which equilibrated to approximately the same mean temperature. The two thermometers, unfortunately, disagree. on the mean value for the higher L and LL groups’ equilibration temperature (860” and 930°C). Both thermometers agree in signaling the presence of anomalous pyroxene grains (Oakley 3, in this case) that indicate shock history. It is hoped that as the Kretz and Lindsley Ca-thermometers are tested in numerous petrological cases their significant disagreement at the high temperature end will be resolved, and an accurate temperature of equilibration for the L and LL groups can be determined.

thank Dr. D. H. Lindsley for providing us with his computer program used to recalculate the

Acknowledgements-We

pyroxene analyses. We also thank an anonymous reviewer whwpointed lo conclusions that were more significant than originally observed by the authors. This work was supported by The National Aeronautics and Space Administration and by the ChalmersFund of the Field Museum of Natural History.

REFERENCES M. (1972) Thermodynamic properties of orthopyroxenes and clinopyroxenes based on the ideal two-site model. Geochim. Cosmochim. Acta 36.787-799. BUCHWALD V. F. (1975) Handbook of Iron Meteorites, Univ. of Calif. Press, Berkeley, CA. BOHLENS. R. and EOCENE E. J. (1979) A critical evaluation of two-pyroxene thermometry in Adirondack ganulites. Lithos 12, 335-345. BUNCH T. E. and OLSENE. (1973) Ortho- and clinopyroxene compositions in ordinary chondrites and related Blander model calculation procedures. NASA Technical Memo NASA TM X-62,259, 20 p. BUNCHT. E. and OLSENE. (1974) Restudy of pyroxenepymxene equilibration temperatures for ordinary chondrite meteorites. Contrib. Mineral. Petrol. 43, 83-90. DODD R. T. (198 I) Meteorites: A Petrologic-Chemical Synthesis. Cambridge University Press. KRETZR. ( 1963) Distribution of magnesium and iron between orthopyroxene and c&c pyroxene in natural mineral assemblages. J. Geol. 71, 773-785. KR!ZIZR. ( 1982) Transferand exchange equilibriain a portion of the pyroxene quadrilateral as deduced from natural and experimental data. Geochim. Cosmochim. Acta 46, 41 I421. KULLERUDG. and YODERH. S. (1959) Pyrite stability relations in the Fe-S system. Econ. Geol. 54, 533-572. LINDSLEY D. H. (1983) Pyroxene therrnometry. Amer. Mineral. 68, 477-493. LINGNERD. W., HUSTONT. J. and LIPXHUTZM. E. (1984) Volatile/mobile trace elements in H4-6 chondrites: A progress report. Abstracts of papers submitted lo the Fifteenth Lunar and PlanetaryScience Conference, Lunar and Planetary Sci. Institute, Houston, TX. MEDARISL. G., JR. (1969) Partitioning of Fe2+ and Mg2+ between coexisting synthetic olivine and orthopyroxene. Amer. J. Sci. 267, 945-968. MUELLER R. F. (1964) Phase equilibria and the crystallization of chondritic meteorites. Geochim. Cosmochim. Acta 28, 189-207. OLSENE. and BUNCHT. E. (1970) Empirical derivation of activity coefficients for the magnesium-rich portion of the olivine solid solution. Amer. Mineral. 55, 1829-1842. Ross M. and HUEBNERJ. S. (1975) A pyroxene geothermometer based on composition-temperature relationships of naturally occurring orthopyroxene, pigeonite, and au&e (abst.). In International Conference on Geothemrometry and Barometry. The Pennsylvania State University, University Park, PA, U.S.A. SAXENAS. K. (1976) Two-pyroxene geothermometry. Amer. Mineral. 61, 643-652. WELLSP. R. A. (197 I) Pyroxene thermometry in simple and complex systems. Contrib. Mineral. Petrol. 62, 129-I 39. WOODB. J. and BANNOS. ( 1973) Gamet-orthopyroxene and orthopyroxeneclinopyroxene relations in simple and complex systems. Contrib. Mineral. Petrol. 42, 109- 124. VAN SCHMUS W. R. and KOFFMAND. M. (1967) Equilibration temperatures of iron and magnesium in chondritic meteorites. Science 155, 1007- IO1 I.

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