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Compositional characteristics of olivines from Apollo 12 samples
GTeochimicaet Coamochimica Acta, 1972, Vcd. 36, pp, 773 to 785. Pergamon Press. Printed in Northern Ireland
Compositional characteristics of olivines from Apollo 12 samples PATRICKBUTLER,JR. TL4, Manned Spacecraft Center, NASA, Houston, Texas 77058 (Received31 May 1971; acceptedim revisedform 7 Fe&uary 1972) Abstr&--The olivine phenocrysts of four basalts (12004, 12008, 12009 and 12022) are concentrically zoned and have core compositions about as magnesian as experimentally produced liquidus olivines, features which suggest fractional crystallization and absence of Fe-Mg reequilibration. In the magnesium- and olivine-rich granular basalt 12035, the olivines are either unzoned OPare zoned toward adjacent grains and have compositions more iron-rich than either cumulus olivines or liquidus olivines (should the rock represent the composition of a melt), features which suggest extensive Fe-Mg re-equilibration. If one assumes that concentrations of minor elements at crystallization have also been preserved in the Mg-rich centers of olivines in the basalt porphyries, identification of two possible comagmatic pairs can be made: 12004 and 12009 (lower Ti and higher Cr in olivines), and 12008 and 12022 (higher Ti and lower Cr in olivines). Correlationsare observed between the Fe-Mg proportions and the concentrations of minor elements in olivine (Ti, Mn and Ca all follow Fe, Cr follows Mg).
~0~~1~~~x0~ of olivine data from several studies of Apollo 11 and Apollo 12 samples shows compositional populations of lunar olivines that are distinctive of lithologic type and of collection site (MUON et al., 1971; TAYLOR and MARVIN,1971; SMITH, 1971). The olivine populations may reflect different magma compositions and may indicate possible co~elations among basal& (GIBB and ZOSSB~~N,1971). Varying degrees of partial re-equilibration or subsequent metamorphism will produce changes in the compositional populations and complicate such correlations. This study examines the compositional populations of olivines from basalts with similar and apparently uncomplica~d thermal histories in order to establish a baseline of inter- and intra-sample variations. These variations are then compared with the compositional variations of olivines from a sample that shows evidence of reequilibration.
PETROGRAPHY Four of the basalts studied (12004,12008,12009 and 12022) are olivine porphyries. The olivine phenocrysts are subhedral, stoutly columnar, average about O-3mm in size, and form 9 to 16 per cent of the rocks. Samples 12008 and 12009 are vitrophyres of very similar appearance. Olivine and chromite form compact grains, skeletal overgrowths, and separate skeletal grains, and plagioclase and troilite are not microscopically recognizable. Samples 12004 and 12022 are holocrystalline with subvariolitic intergrowths of the pyroxene and plagioolase forming the groundmass material. The vitrophyres obviously were the most rapidly cooled of the porphyries Smaller grain size of the groundmass minerals during the final stages of so~~cation. in 12022 shows it to have cooled more rapidly than 12004. The fifth basalticrrock studied (12035) is granular and contains 35 per cent olivine, 773 5
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PATRICK BUTLER, JR.
and about equal portions of clinopyroxene (both high and low Ca) and plagioclase, all of which range in size up to 2 mm. The pyroxenes and olivines are subhedral to euhedral, and are set in a matrix of anhedral plagioclase grains. The three analyzed porphyritic rocks (12004, 12009 and 12022) are chemically similar and have intermediate MgO contents (11-12 per cent) in the range shown by Apollo 12 basaltic rocks (6-16 per cent MgO; Fig. 4 in WARNER, 1971). The granular basaltic rock, 12035, is at the MgO-rich end of this range. Further petrographic information on these samples can be found in BRETT et al. (1971). METHODS Analyses were made on an ABL-EMX electron microprobe X-ray analyzer at 15 kV and sample currents ranging from 0.02 to 0.1 ,uA. For all measurements, the beam was focused to give as small a spot as possible, approximately 1 to 4 p. All values were normalized to beam current counts, and values for Fe, Mg and Si were taken simultaneously. The minor elements Ti, Cr, Mn, Ca were individually analyzed on two spectrometers simultaneously with Mg on the remaining spectrometer. The analyses were made on rectangular areas, 30 to 50 p on a side, in the centers of the olivine grains. Each analysis consisted of measuring 6 to 20 spots 2 to 5 p apart along one or two traverses. The Mg values were used as a monitor for the homogeneity of the olivine cores and for the quality of the surface at each spot. About 10 per cent of the spots measured in both the Cr and Ti runs gave high values, by 15 to 3 times, usually concurrently with a slight decrease in Mg intensity. These values were rejected, as were the values for the adjacent spots if they were the next highest values, as being due to inclusions. Although the olivine grams show various concentrations of bubble-like and dusty inclusions, microscopic examination has failed to relate them to any of the locations of high Cr or Ti intensity. Values from the two spectrometers were treated as independent analyses. One of the values was rejected on occasion because of excessive drift (more than 3 per cent&r on intensities normalized to beam current counts). Agreement between spectrometers was generally good despite systematic interspectrometer differences of 10 to 20 per cent for peak intensities and up to 50 per cent for backgrounds. This background divergence resulted from a difference in electronic noise level between the two circuits. Since the replicate runs were at different places within the central areas, compositional variation may have contributed significantly to the size of standard deviations. Estimates of the degree of this variability were obtained through repeated analysis runs. The standard deviation for olivine No. 16 in 12009, Table 1, after 6 and 10 replicates of TiO, and CrsO, respectively (of 10 and 15 total replicates) reached minimum values (11 and 2 per cent) that are probably a good measure of the homogeneity of the area analyzed. Analyses were made at least 50 ,a from other minerals to avoid possible effects from secondary fluorescence of Ti and Ca (SMITH, 1971). An analysis made 60 p from an ilmenite inclusion gave no greater Ti concentration than others up to 110 p from the inclusion. Since background intensity depended on the olivine composition, the major variation of which can be specified with one parameter, a considerable advantage was gained in the precision of the background values for the minor elements through correction of the measured background intensities for individual grains to a second degree regression on the Mg intensities for the grams. The standards used were: Fe 8.98 per cent, Mg 29.0 per cent, Si 18.78 per cent, all in olivine from the Marjalahti meteorite (BUSECK and GOLDSTEIN, 1969); Ca 12.97 per cent in augite (Australian National University ANU 6-44-6); Mn 3.11 per cent in orthopyroxene (OPX-205, BUTLER, 1969); Cr 30.48 per cent in chromite (52-NL-11 DINNIN, 1959); Ti 29.66 per cent in ilmenite (Australian National University ANU 5-44-3). Corrected values for the standards and the ohvines were processed by computer for atomic number, absorption, and fluorescence corrections using the parameters and iterative procedure of GOLDSTEIN and COMELLA (1969).
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0.325 @28% 0.294 0*275 0304 @302 o-283 0.283 o-274 0.274 0347
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0.037 0.033 0*035 0*04s 0.037 0.036 0.039 0.042 0.044 0.043 O.O%f. 0.04% 0.036
0.235 0.242 iKEii3 0.304 0*281 0*261 0.264 @293 0.277 0.298 0*3i5 0.288 0*239
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0.289 0*3fB 0.318 @309 0.290 0~302 0.291 0*293 0.30% 0.248 0.252 0~300 0.292
&312 *32x e337 0.339 0.293 0.330 0298 0302 0.31% 0*298 0.293 0*303 0.297
37-O 36-7 36.4 38-7 37.1 3&Z 364 36.6 36.7 37.6 37.8 36.9 36-9
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PATRICEBUTLISR,JR.
776
MAJOR ELEMENTS The zoning patterns shown in Fig. 1 for olivines in 12009 and 12022 are typical of the porphyritic rocks studied. As Mg is selective partitioned into olivine over Fe from basaltic liquids (ROEDER and EMSLIE, 1970), fractional crystallization causes Fe/Mg to increase in both liquid and olivine as crystallization progresses. Low compositional gradients in the cores of grains becoming steeper near their margins probably result from the increase of the ratio of solid to liquid with increasing grain radius. Unzoned cores, as shown by some of the olivines in 12009 (grain 16 in Fig. 1) may represent early re-equilibration with the remain~g liquid. However, no extensive re-equilibration between olivine and liquid could have occurred in the porph~iti~ rocks because the most Fe-poor olivines observed have about the same compositions as the liquidus olivines produced from melts of these rocks. The observed (this paper) vs the experimental values (GREEN et al., 1971) are Fo77 vs Fo75 for 12009, and Fo73 vs Fo77 for 12022.
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Fig. 1. Typica major element zoning in olivinesas chamcterized by Fe0 contents. 12009 and 12022 are porphyritic b&salts, 12035 is 8 granular basalt. The numbers and arrows distinguish individual grains. In 12035 grains 6 and 7 show distinctly different orientation (their c axes are nearly pe~endie~~); the edge of the section truncates the right edge of grain 7; the left side of 6 abuts ilmenite. The outer edges of the grains of 12022 and 12009 adjoin groundmass material.
Compositional ehraracteristics of olivines from Apollo 12 samples
777
The symmetry of the profiles (Fig. 1) suggests that olivine compositions have been little altered by subsolidus re-equilibration with any adjacent minerals. The sharp gradients in olivines 42 and 44 of 12022 (Fig. 1) on both sides of their mutual contact must have formed by crystallization from a liquid before the grams joined; had there been subsequent Fe-Mg diffusion, these sharp gradients would have been considerably reduced. Ranges of Fe content in the interiors of olivines in the porph~itic basalts {Fig. 2) can be accounted for by assuming either that the more Fe-rich olivinea nucleated later than the others or that few of the true cores, which would have the least Fe content, are transected by the sections. Significant inverse correlations between grain widths and central Fe contents in sections 12008,17, 12009,8 and 12022,12 conform with both expirations, although lack of correlation for the olivines of 12004,8 indicates some additional complexity. Under either assumption the progress of fractional crystallization is marked by increasing Fe contents of the olivines 4
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Fig. 2. Distribution of Fe0 values for the interiors of olivines from Apollo 12 rooks (this study), and for olivines from Apollo 11 rocks, brew&s and soils (the sources are all contained in the caption of Fig. 3, with the exception of Woon et aZ., 1970). The fields of successively more Fe-rich olivines are designated I,2 and 3.
778
PATRICEBUTLER,JR.
In contrast to the case of the olivines in the porphyritic rocks, there are several indications that the olivines of 12035 have undergone considerable Fe-Mg reequilibration. Zoning is strong in some grains, but it is not concentric and seems to relate to the adjoining grains. There is little zoning against plagioclase but pronounced zoning toward mafic silicates. Figure 1 shows two adjoining olivines with a continuous compositional gradient between them, which suggests that there has been Fe-Mg diffusion in response to differences in their compositions. The major compositions of most Apollo 12 basaltic rocks can be derived from any intermediate composition through the subtraction of olivine to produce the low-Mg basalts, and the addition of olivine to produce the Mg-rich basalts like 12035. The range of olivine ‘control’ compositions that have been derived through relating groups of Apollo 12 rocks in this way is Fo67 to Fo79 (KUSHIRO and HARAMURA, COMSTONet al., BI~QAR et al., HASKIN et al., all 1971), which is about the range shown by the cores of olivines in the basalt porphyries studied (Fig. 2). Since the compositions of this range are all more forsteritic than any observed in 12035 (the maximum is Fo64, Fig. 2), the accumulate hypothesis requires significant re-equilibration to account for the Fe-rich compositions of its olivines. If, however, the bulk composition of 12035 represents a liquid composition, * the liquidus olivines would have been Mg-rich (12040, with a similar composition, shows Fo82 liquidus olivine, GREEN et al., 1971), with the result that even more Fe-Mg re-equilibration of the olivines of 12035 than that required by olivine accumulation would have been necessary. Further evidence of re-equilibration in 12035 was found by SCHNETZLERand PHILPOTTS (197 1) in the distribution of trace elements among pyroxenes and plagioclase, and by REID (1971) in the compositions of spinels. A broad range of compositions for the interiors of olivine grains in 12035 (Fig. 2) is a natural consequence of partial re-equilibration, and, therefore, increase in Fe-content in olivines cannot be directly related to the progress of fractional crystallization, as is assumed to be the case for the porphyritic rocks. MINOR ELEMENTS Table 1 gives the analyses of the central areas of 60 olivine grains, chosen to cover the ranges in their Fe content for each rock. The 12 or 13 olivines analyzed in each of three porphyritic rocks show significant correlation at the 95 per cent level for each of the minor elements (Ti, Cr, Mn, Ca) with Fe content. For the five olivines analyzed in each sample, correlation with Fe is significant for Ti, Cr, Mn in 12035 and only for Mn in 12008. The relationships of the minor elements to Fe in olivines are plotted in Figs. 3-6, with boxes to show the range of two-thirds Standard Error of Estimate (ALDER and ROESSLER, 1968; the “region within which about twothirds of the points of the sample fall.“). The linear regression for each set of data would bisect its box, paralleling the top and bottom sides. Since increase in Fe content in the olivines of the porphyritic rocks marks the progress of fractional crystallization, changes in minor element content may also relate to fractional crystallization. Textural relations (BRETT et al., 1971) show that titanian chromite, although present in accessory amounts, was the most abundant * Intrinsic evidence against this origin, such as concentrically zoned Mg-rich absent in 12035, as was pointed out to be the case for 12040 by GREENet al., 1971.
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Compositional characteristicsof olivines from Apollo 12 samples
779
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Fig. 3. Summary of the TiO, vs Fe0 contents of lunar olivines. The date points for 12004 (04), 12009 (09) and 12022 (22) are not plotted but are summarized by boxes, the upper and lower sides of which are parallel to and spaced one Standard Error of Estimate from a linear regression on the points. A are the values for the cores of olivines from 12008 (OS), 12013 (13), 12035 (35) and 12041 (41). These values are either joined by lines or rareassociated with a f two-thirds Standard Error of Estimate box for the rock in question. For 12013 (13) the middle value is from this study, the left value is from DRAKEet al. (1970), and the right is from LUNATICASYLUM (19’70). 0 are olivine core values from Apollo 11 basalts (10020, 10022, and 10045 from: BRO~KNet al. (1970); HAUGERTYet al., (1970); KEIL et al., (1970) KUSHIROand NAKAXCJR A (1970); WEILL et al. (1970)). q are analyses of olivines from anorthositic fragments from Apollo 11 fines or breccias reported by: AURELLet al., 1970; EEIL et al., 1970; REID et al., 1970. . are values for olivines from Apollo 11 hnes and breccias that are not in anorthositic fragments given by: AGRELLet al., 1970; KEIL et al., 1970; LOVERWO and WARE, 1970. The diagonal lines have the slope of the average of the linear regressionsand they divide the diagram into compositional fields. phase throughout most of olivine crystallization. Only olivine and chromite need be considered as affecting differentiation during crystallization of the olivine cores. Partition coefficients for the minor elements between olivine and liquid may be approximated by using bulk rock composition (COMPSTON et al., 1971; KUSHIRO and HARAMURA, 1971) for the liquid that was in equilibrium with the Mg-rich olivines. Owing to strong partitioning into the liquid of Ti (by about 75 times) and Ca (by about 32 times), olivine crystallization alone appears to be nearly sufficient to account for the increased Ti and Ca concentrations in olivine with increase in Fe coprecipitating
PA~ICK
780
BUTLER,
JR.
0.10 -
Fig. 4. Summary of the CraOs vs Fe0 contents of olivines. The explanations and references are as given for Fig. 3. The downward arrow indicates a value below detection, the limit of which is shown by the associated point.
conoentration, as can be shown by Rayleigh depletion calculations using approximate initial and final olivine compositions and modes. Some change in the partition coefficients in favor of olivine as it became richer in Fe would more amply account for the Ti increases. Small fractionation between liquid and Mg-rich olivines for both Mn (by about 1-l times) and Cr (by about 1*4 times), however, makes olivine crystallization ineffective in changing their concentrations in the ~main~g liquid. On the basis of modes and compositions of chromite (BRETTet al., 1971), its copre&pit&ion with olivine would have been sufficient to account for the anticorrelation of Cr and Fe in the olivine. Increase of Mn in olivine with Fe, however, must result from an increased acceptance of Mn into olivine with the increase in Fe content, because Mn enters chromite in approximately the same concentrations as it is present in the liquids (for example see spine1 analyses in HAWERTY and MEYER, 1970). The scattered relationships for &-Fe (Fig. 4) and &,-Fe (Fig. 6) of the 12035 olivines are what would be expected for partial re-equilibration involving independent components. If the olivines of 12035 formed with minor element contents like
Compositiontalcharacteristicsof olivines from Apollo 12 samples
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The explanations and Fig. 5. Summary of the MnO vs Fe0 contents of o&&es. referencesare as given for Fig. 3, the arrow is as given for Fig. 4.
those of the porphyritic olivines, reequilibration would have involved an increase of Ca (Fig. 6) and a decrease in Cr (Fig. 4) accompanying the increases in Fe. Ti-Fe (Fig. 3) and Mn-Fe (Fig, 5) relationships in the olivines of 12035 that are like those of the porphyritic basalts are difficult to explain in view of partial Fe-Mg reequilibration. The concentrations of Ti and Mn appear to have been strongly governed by the major composition of olivme during all stages of re-equilibration. DISWJSSION All olivines crystallizing from a homogeneous melt at a given stage of differentiation should have the same concentrations of major and minor elements, provided the physical differences among the sites of crystallization, such as cooling rate or pressure, are not so great as to affect partitioning coefficients. Thus, the composition of olivines, where not si~ificantly altered through re-eq~libration, may provide criteria for possible comagmatic origins of samples. The cores of the olivines from the four porphyritic rocks studied lie within a restricted range of Fe concentrations (Fig. 2) and therefore represent the same stage of differentiation,
PATRICKBUTLER,JR.
782
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Fig. 6. Summary of the CeO vs Fe0 contents of olivines. The explanationsmd referencesm-e as given for Fig. 3.
The Ca-Fe relations of their olivines (Fig. 6) appear to group the porphyries according to texture. The vitrophyres (12008 and 12009) have olivines with higher Ca contents than those of the two holocrystalline rocks (12004 and 12022). These differences in the Ca may depend on cooling rate as evidenced by textures, or may be a fine scale example of the inverse relation between Ca content in olivine and depth of crystallization discovered by SIMKIN and WITH (1970) for terrestrial rocks. In contrast to the Ca relations in olivines, the relative contents of the other minor elements appear to be unrelated to the textural differences among the four porphyries studied. The two vitrophyres (12008 and 12009) are clearly distinct from one another in terms of Ti, Cr and Mn in their olivines (Figs. 3-5). Holocrystalline rock 12022, moreover, resembles 12008 on the basis of similar Ti, Cr and Mn contents in their olivines (Figs. 3-5). The Ti-Fe and particularly the Cr-Fe relations for the olivines of 12004 show more scatter than those for the other rocks, possibly indicating some re-equilibration. The most primitive olivines of 12004 (those with the lowest Fe
Compositional oharacteristicsof olivines from Apollo 12 samples
783
con~nts), however, have Ti and Cr contents most like those of 12009. Thus, on the basis of the olivine data, the porphyritic rocks form two possibly correlative pairs: 12004 and 12009, with lower Ti and higher Cr olivines; 12008 and 12022, with higher Ti and lower Cr olivines. Rocks 12004 and 12009 are similar to each other in many chemical characteristics, although COJYIPSTONet al. (1971) point out several chemical and isotopic hindrances to a comagmatic origin. 12022 differs considerably in its chemical characteristics from most Apollo 12 basalts, as first shown by KUSHIR~ and HARAMUBA(1971). only trace element analyses have been made on 12008 (PAPANASTASSIOU and WASSERBURQ, 1971; ANDERSet a,!., 1971). The Ti content of 12022 is greater than that of any other analyzed Apollo 12 basalt, but less than that of any of the Apollo I1 basalts. Ti concentrations in olivines (Fig. 3) appear to reflect accurately these relative bulk Ti concentrations. From initial Sr isotopic ratios, PAPANASTASSIOU and WASSERB~R~ (1971) show 12008 to be from a different magma than the texturally nearly identical 12009, in conformity with the evidence from the olivine compositions. An analysis for major elements in 12008 would not only further test the possibility of its correlation with 12022, but would closely define the liquid composition from which it formed since only chromite and olivine are possible cumulus phases. The higher Ti and lower Cr contents in the olivines of the Apollo 11 basalts relative to the values for olivines in the Apollo 12 porphyries (Figs. 3 and 4) mimic the relative bulk rock contents of Ti and Cr between the two sites (for example see KWSHIROand HARAMURA,1971; or GOLESet al., 1971). These relations suggest that the initial concentrations in olivine, determined by pa~ition~g from the melts, were subsequently preserved. Where fractional crystallization has had a significant role in the formation of a rock, re-equilibration produces an increase in Fe and a decrease in Mg in the cores of olivines. No clear effect on either the Ti or Ca contents of olivine accompanies this re-equilibration. It does seem apparent, however, that reduction in Cr concentration is an effect of re-equilibration, since in the Cr-Fe diagrams (Fig. 4) the area of high Cr content is without data points for the more Fe-rich olivines. Decrease in the concentration of Cr in olivines may occur even more readily than decrease in Mg, and so may account for the low Cr contents of relatively Mg-rich olivines in some of the nob-basaltic rocks (Fig. 4). Re-equilibration of olivines in 12035 renders their compositions unsuitable for testing oorrelations. The same may be true for sample 12040, a similar Mg- and olivine-rich basalt studied by GIBB and ZUSSMANN(1971). These authors cited non-colinearity of Cr-Fe and C&-Fe relations of olivines in 12040 with the more Mg-rich olivines of the po~hy~ti~ basalts 12052, 12075 and 12076 as evidence against possible comagmatic relations. As in the case of 12035, however, the noncolinearity in these olivine relationships may be a consequence of re-equilibration, even if the rocks were comagmatic. ~~~~~gs~~~~-~ thank ROBIN BRETT, ABCEI Ed. REID, &.~~JEILBASS, and MICH~~EL B. DUKE all of MSC for help at various stages in this study, which was done during tenure of an NRC-NASA Resident lClesearch Associateship.
784
PATRICKBUTLER, JR. REFERENCES
S. O., SCOON, J. H., Mum I. D., LONGJ. V. P., MCCONNELL J. D. and PECKETTA. (1970) Observations on the chemistry, mineralogy, and petrology of some Apollo 11 lunar samples. Proc. First LunurSci.Conf., Geochim. Cosmochim. Acta, Supp. 1,Vol. 1, pp. 93-128, Pergamon. ALDERH. L. and ROESSLER E. B. (1968) Introduction to probab&ty and statistics. 4th Ed, H. W. Freeman, San Francisco. ANDERSE., GANAPATHYR., KEAYS R. R., LAUL J. C. and MORGANJ. W. (1971) Volatile and siderophile elements in lunar rocks: Comparison with terrestrial and meteoritic basalts. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 2, pp. 1021-1036, M.I.T. BIUUARG. M., OHARA M. J., PECEETTA. and HUMPERIESD. J. (1971) Lunar lava lakes and the achondrites: petrogenesis of protohypersthene basalts in the maria lava lakes. Proc. Second Lunar Sci. Conf,, Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 617-643, M.I.T. BRET~Y R., BUTLERP. JR., MEYER C. JR., REID A. M., TAKEDAH. and WILLIAMSR. (1971) Apollo 12 igneous rocks 12004, 12008, 12009 and 12022: a mineralogical and petrological study. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 301-317, M.I.T. BROWNG. M., E~LEUS C. H., HOLLANDJ. G. and PHILLIPSR. (1970) Mineralogical, chemical and petrological features of Apollo 11 rocks and their relationship to igneous processes. Proc. First Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 1, Vol. 1, pp. 195-219, Pergamon. BUSECKP. R. and GOLDSTEIN J. I. (1969) Olivine compositions and cooling rates of pallasitic meteorites. B. GeoZ. Sot. Amer. 80, 2141-2158. BUTLERP. JR. (1969) Mineral compositions and equilibriain the metamorphosed iron formation of the Gagnon Region, Quebec, Canada. J. Petrol. 10,66-101. COMPSTON W., BERRY H., VERNONM. J., CHAPPELL B. W. and KAYE M. J. (1971) Rubidiumstrontium chronology and chemistry of lunar material from the Ocean of Storms. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 2, pp. 1471-1486, M.I.T. DINNIN J. I. (1959) Rapid analysis of chromite and chrome ore. U. S. &oZ. Sure. BUZZlo@B, 31-68. DRAKEM. J., McCALL~~~I. S., MCKAY G. A. and WELLLD. F. (1970) Mineralogy and petrology of Apollo 12 sample No. 12013: a progress report. Earth Planet. Sci. Lett. 9, 103-123. GOB F. G. F. and ZUSS~UNJ. (1971) Zoned olivine in four Apollo 12 samples. Earth Planet. Sci. Lett. 11,161-167. GOLESG. G., DUNCANA. R., LINDSTROM D. J., MARTINM. R., BEYERR. L., OSAWAM., RANDLE K., MEEK L. T., STEINBORN T. L. and MCKAY S. (1971) Analyses of Apollo 12 specimens: compositional variations, differentiation processes, and lunar soil mixing models. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acts, Supp. 2, Vol. 2, pp. 1063-1081, M.I.T. GOLDSTEINJ. I. and COMELLAP. A. (1969). A computer program for electron probe microanalysis in the fields of metallurgy and geology. Goddard Spaceflight Center X-642-69-115. GREEND. H., RINUWOODA. E., WARE N. G., HIBBERSON W. O., MAJORA. and KISS E. (1971) Experimental petrology and petrogenesis of Apollo 12 basalts. Proc. Second Lulaar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 601-616, M.I.T. HAUUERTYS. E., BOYD F. R., BELL P. M., FINUERL. W. and BRYAN W. B. (1970) Opaque minerals and olivine in lavas and breccias from Mare Tranquillitatis. Proc. First Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 1, Vol. 1, pp. 513-538. Pergamon. HAGGERTYS. E. and MEYER H. 0. A. (1970) Apollo 12: Opaque oxides. Earth Planet. SC& Lett. 9, 379-387. HASKIN L. A., HELMKEP. A., ALLEN R. O., ANDERSONM. R., KOROTEVR. L. and Z~EIFEL K. A. (1971) Rare-earth elements in Apollo 12 lunar materials. Proc. SecondLunar Sci. Cmf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 2, pp. 1307-1317, M.I.T. KEIL K., BUNCH T. E. and PRINZ M. (1970) Mineralogy and composition of Apollo 11 lunar samples. Proc. First Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 1, Vol. 1, pp. 561-598, Pergamon. AGRELI,
Compositional characteristicsof olivines from Apollo 12 samples
785
Y. (1970) Petrology of some lunar crystalline rocks. Proc. First KUSHIROI. and NAKAMXJRA hnar Xci. Co$., Geochim. Cosmochim. Acta, Supp. 1, Vol. 1, pp. 607-626, Pergamon. KUSH~ROI., and IIARAMXJRA H. (1971) Major element variation and possible source materials of Apollo 12 crystalline rocks. Science 171,12351237. LUNATICASYLUM (1970) Mineralogic and isotopic investigations on lunar rock 12013. Earth Planet. Sci. Lett. 9, 137-163. LOVERINGJ. F. and WEE N. G. (1970) Electron probe microanalyses of minerals and glasses in Apollo 11 lunar samples. Proc. Pirst Lunar &a’. Conf., Geochim. Cosmochim. Acta, Supp. 1, Vol. 1, pp. 633-654, Pergamon. MARVINU. B., WOOD J. A., TAYLORG. J., REID J. B. JR., POWELLB. N., DICKEYJ. S. JR. and BOXER J. F. (1971) Relative proportions and probable sources of rock fragments in the Apollo 12 samples. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 679-699, M.I.T. MEYER C. JR., BRETT R., HUBBARDN. J., MORRISON,D. A., MCKAY, D. S. AITKEN, F. K., TAKEDA H., and SCHONFELD E. (1971) Mineralogy, chemistry and origin of the KREEP component in soil samples from the Ocean of Storms. Proc. Secondhnar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 393-411, M.I.T. PAPANASTASSIOU D. A. and WASSERBURG G. J. (1971) Lunar chronology and evolution from Rb-Sr studios of Apollo 11 and 12 samples. Earth Planet. Sci. Lett. 11,37-62. REID A. M., FRASERJ. Z., FUJITAH. and EVERSONJ. E. (1970) Chemical compositions of the major phases in Apollo 11 lunar samples. Scripps Institution of Oceanography Ref. 70-4. REID J. B. Jr. (1971) Apollo 12 spinels as petrogenetic indicators. Ean?h PZanet.Sci. Lett. 10, 351-356. ROEDERP. L. and EMSLIER. F. (1970) Olivine-liquid equilibrium. Contrib. Mineral. Petrol. 29, 275-289. SCHNETZLER C. C. and PHILPOTTS J. A. (1971) Alkali, alkaline earth and rare-earth element concentrations in some Apollo 12 soils, rocks, and separated phases. Proc. Second hnar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 2, pp. 1101-1122, M.I.T. SIMKINT. and SMITHJ. V. (1970) Minor-element distribution in olivine. J. Cfeol. 78, 304325. SMITH,J. V. (1971) Minor elements in Apollo 11 and Apollo 12 olivine and plagioclase. Proc. Second tinar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 143-150, M.I.T. TAYLOR G. J. and MARVINU. B. (1971) A dunite-norite lunar microbreccia. Meteoritice 6, 173-179. WARNER J. L. (1971) Lunar crystalline rocks: petrology and geology. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 2, Vol. 1, pp. 469-480, M.I.T. WEILL D. F., MCCALLUM I. S. BOTTINGAY., DRAKEM. J. and MCKAY G. A. (1970) Mineralogy and petrology of some Apollo 11 igneous rocks. Proc. l%st Lunar Sci. Conf., Geochim. Cosmochim. Acta, Supp. 1, Vol. 1, pp. 937-956, Pergamon. WOODJ. A., Marvin U. B., POWELLB. N. and DICKEYJ. S. JR. (1970) Mineralogy and petrology of the Apollo 11 lunar sample. Smithsonian Institution Astrophysical Observatory SAO Special Report No. 307.
Compositional characteristics of olivines from Apollo 12 samples PATRICKBUTLER,JR. TL4, Manned Spacecraft Center, NASA, Houston, Texas 77058 (Received31 May 1971; acceptedim revisedform 7 Fe&uary 1972) Abstr&--The olivine phenocrysts of four basalts (12004, 12008, 12009 and 12022) are concentrically zoned and have core compositions about as magnesian as experimentally produced liquidus olivines, features which suggest fractional crystallization and absence of Fe-Mg reequilibration. In the magnesium- and olivine-rich granular basalt 12035, the olivines are either unzoned OPare zoned toward adjacent grains and have compositions more iron-rich than either cumulus olivines or liquidus olivines (should the rock represent the composition of a melt), features which suggest extensive Fe-Mg re-equilibration. If one assumes that concentrations of minor elements at crystallization have also been preserved in the Mg-rich centers of olivines in the basalt porphyries, identification of two possible comagmatic pairs can be made: 12004 and 12009 (lower Ti and higher Cr in olivines), and 12008 and 12022 (higher Ti and lower Cr in olivines). Correlationsare observed between the Fe-Mg proportions and the concentrations of minor elements in olivine (Ti, Mn and Ca all follow Fe, Cr follows Mg).
~0~~1~~~x0~ of olivine data from several studies of Apollo 11 and Apollo 12 samples shows compositional populations of lunar olivines that are distinctive of lithologic type and of collection site (MUON et al., 1971; TAYLOR and MARVIN,1971; SMITH, 1971). The olivine populations may reflect different magma compositions and may indicate possible co~elations among basal& (GIBB and ZOSSB~~N,1971). Varying degrees of partial re-equilibration or subsequent metamorphism will produce changes in the compositional populations and complicate such correlations. This study examines the compositional populations of olivines from basalts with similar and apparently uncomplica~d thermal histories in order to establish a baseline of inter- and intra-sample variations. These variations are then compared with the compositional variations of olivines from a sample that shows evidence of reequilibration.
PETROGRAPHY Four of the basalts studied (12004,12008,12009 and 12022) are olivine porphyries. The olivine phenocrysts are subhedral, stoutly columnar, average about O-3mm in size, and form 9 to 16 per cent of the rocks. Samples 12008 and 12009 are vitrophyres of very similar appearance. Olivine and chromite form compact grains, skeletal overgrowths, and separate skeletal grains, and plagioclase and troilite are not microscopically recognizable. Samples 12004 and 12022 are holocrystalline with subvariolitic intergrowths of the pyroxene and plagioolase forming the groundmass material. The vitrophyres obviously were the most rapidly cooled of the porphyries Smaller grain size of the groundmass minerals during the final stages of so~~cation. in 12022 shows it to have cooled more rapidly than 12004. The fifth basalticrrock studied (12035) is granular and contains 35 per cent olivine, 773 5
774
PATRICK BUTLER, JR.
and about equal portions of clinopyroxene (both high and low Ca) and plagioclase, all of which range in size up to 2 mm. The pyroxenes and olivines are subhedral to euhedral, and are set in a matrix of anhedral plagioclase grains. The three analyzed porphyritic rocks (12004, 12009 and 12022) are chemically similar and have intermediate MgO contents (11-12 per cent) in the range shown by Apollo 12 basaltic rocks (6-16 per cent MgO; Fig. 4 in WARNER, 1971). The granular basaltic rock, 12035, is at the MgO-rich end of this range. Further petrographic information on these samples can be found in BRETT et al. (1971). METHODS Analyses were made on an ABL-EMX electron microprobe X-ray analyzer at 15 kV and sample currents ranging from 0.02 to 0.1 ,uA. For all measurements, the beam was focused to give as small a spot as possible, approximately 1 to 4 p. All values were normalized to beam current counts, and values for Fe, Mg and Si were taken simultaneously. The minor elements Ti, Cr, Mn, Ca were individually analyzed on two spectrometers simultaneously with Mg on the remaining spectrometer. The analyses were made on rectangular areas, 30 to 50 p on a side, in the centers of the olivine grains. Each analysis consisted of measuring 6 to 20 spots 2 to 5 p apart along one or two traverses. The Mg values were used as a monitor for the homogeneity of the olivine cores and for the quality of the surface at each spot. About 10 per cent of the spots measured in both the Cr and Ti runs gave high values, by 15 to 3 times, usually concurrently with a slight decrease in Mg intensity. These values were rejected, as were the values for the adjacent spots if they were the next highest values, as being due to inclusions. Although the olivine grams show various concentrations of bubble-like and dusty inclusions, microscopic examination has failed to relate them to any of the locations of high Cr or Ti intensity. Values from the two spectrometers were treated as independent analyses. One of the values was rejected on occasion because of excessive drift (more than 3 per cent&r on intensities normalized to beam current counts). Agreement between spectrometers was generally good despite systematic interspectrometer differences of 10 to 20 per cent for peak intensities and up to 50 per cent for backgrounds. This background divergence resulted from a difference in electronic noise level between the two circuits. Since the replicate runs were at different places within the central areas, compositional variation may have contributed significantly to the size of standard deviations. Estimates of the degree of this variability were obtained through repeated analysis runs. The standard deviation for olivine No. 16 in 12009, Table 1, after 6 and 10 replicates of TiO, and CrsO, respectively (of 10 and 15 total replicates) reached minimum values (11 and 2 per cent) that are probably a good measure of the homogeneity of the area analyzed. Analyses were made at least 50 ,a from other minerals to avoid possible effects from secondary fluorescence of Ti and Ca (SMITH, 1971). An analysis made 60 p from an ilmenite inclusion gave no greater Ti concentration than others up to 110 p from the inclusion. Since background intensity depended on the olivine composition, the major variation of which can be specified with one parameter, a considerable advantage was gained in the precision of the background values for the minor elements through correction of the measured background intensities for individual grains to a second degree regression on the Mg intensities for the grams. The standards used were: Fe 8.98 per cent, Mg 29.0 per cent, Si 18.78 per cent, all in olivine from the Marjalahti meteorite (BUSECK and GOLDSTEIN, 1969); Ca 12.97 per cent in augite (Australian National University ANU 6-44-6); Mn 3.11 per cent in orthopyroxene (OPX-205, BUTLER, 1969); Cr 30.48 per cent in chromite (52-NL-11 DINNIN, 1959); Ti 29.66 per cent in ilmenite (Australian National University ANU 5-44-3). Corrected values for the standards and the ohvines were processed by computer for atomic number, absorption, and fluorescence corrections using the parameters and iterative procedure of GOLDSTEIN and COMELLA (1969).
37*7
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38.1 364 35.0 38.0 35.9 36.2 3&T 38.7 39.0 36.7 33.2
0.098 0.034 0.061 0.035 0.042 Q-G39 0.038 0.035 0.039 0.040 0.070
31.7 37-3 37.6 36.6 36.8
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37.6 36.2 87.6 34.2 34*%
37*% 37.9 37-8 37.0 37.2 38-O 37.9 37.1 37.4 37.4 36.9 37.3 37-8
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27s’i 38.8 38.8 34.3 38.6 S&6 38*‘7 35.6 57.1 36.0 34.8 3%*3 39.2
388
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O+?%ja~ 0.318 0.300 0.312 o-249 0.294 tb306 a*250 0.249 O+?& O-285 0.339
0.325 @28% 0.294 0*275 0304 @302 o-283 0.283 o-274 0.274 0347
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0.235 0.242 iKEii3 0.304 0*281 0*261 0.264 @293 0.277 0.298 0*3i5 0.288 0*239
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&312 *32x e337 0.339 0.293 0.330 0298 0302 0.31% 0*298 0.293 0*303 0.297
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PATRICEBUTLISR,JR.
776
MAJOR ELEMENTS The zoning patterns shown in Fig. 1 for olivines in 12009 and 12022 are typical of the porphyritic rocks studied. As Mg is selective partitioned into olivine over Fe from basaltic liquids (ROEDER and EMSLIE, 1970), fractional crystallization causes Fe/Mg to increase in both liquid and olivine as crystallization progresses. Low compositional gradients in the cores of grains becoming steeper near their margins probably result from the increase of the ratio of solid to liquid with increasing grain radius. Unzoned cores, as shown by some of the olivines in 12009 (grain 16 in Fig. 1) may represent early re-equilibration with the remain~g liquid. However, no extensive re-equilibration between olivine and liquid could have occurred in the porph~iti~ rocks because the most Fe-poor olivines observed have about the same compositions as the liquidus olivines produced from melts of these rocks. The observed (this paper) vs the experimental values (GREEN et al., 1971) are Fo77 vs Fo75 for 12009, and Fo73 vs Fo77 for 12022.
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Fig. 1. Typica major element zoning in olivinesas chamcterized by Fe0 contents. 12009 and 12022 are porphyritic b&salts, 12035 is 8 granular basalt. The numbers and arrows distinguish individual grains. In 12035 grains 6 and 7 show distinctly different orientation (their c axes are nearly pe~endie~~); the edge of the section truncates the right edge of grain 7; the left side of 6 abuts ilmenite. The outer edges of the grains of 12022 and 12009 adjoin groundmass material.
Compositional ehraracteristics of olivines from Apollo 12 samples
777
The symmetry of the profiles (Fig. 1) suggests that olivine compositions have been little altered by subsolidus re-equilibration with any adjacent minerals. The sharp gradients in olivines 42 and 44 of 12022 (Fig. 1) on both sides of their mutual contact must have formed by crystallization from a liquid before the grams joined; had there been subsequent Fe-Mg diffusion, these sharp gradients would have been considerably reduced. Ranges of Fe content in the interiors of olivines in the porph~itic basalts {Fig. 2) can be accounted for by assuming either that the more Fe-rich olivinea nucleated later than the others or that few of the true cores, which would have the least Fe content, are transected by the sections. Significant inverse correlations between grain widths and central Fe contents in sections 12008,17, 12009,8 and 12022,12 conform with both expirations, although lack of correlation for the olivines of 12004,8 indicates some additional complexity. Under either assumption the progress of fractional crystallization is marked by increasing Fe contents of the olivines 4
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Fig. 2. Distribution of Fe0 values for the interiors of olivines from Apollo 12 rooks (this study), and for olivines from Apollo 11 rocks, brew&s and soils (the sources are all contained in the caption of Fig. 3, with the exception of Woon et aZ., 1970). The fields of successively more Fe-rich olivines are designated I,2 and 3.
778
PATRICEBUTLER,JR.
In contrast to the case of the olivines in the porphyritic rocks, there are several indications that the olivines of 12035 have undergone considerable Fe-Mg reequilibration. Zoning is strong in some grains, but it is not concentric and seems to relate to the adjoining grains. There is little zoning against plagioclase but pronounced zoning toward mafic silicates. Figure 1 shows two adjoining olivines with a continuous compositional gradient between them, which suggests that there has been Fe-Mg diffusion in response to differences in their compositions. The major compositions of most Apollo 12 basaltic rocks can be derived from any intermediate composition through the subtraction of olivine to produce the low-Mg basalts, and the addition of olivine to produce the Mg-rich basalts like 12035. The range of olivine ‘control’ compositions that have been derived through relating groups of Apollo 12 rocks in this way is Fo67 to Fo79 (KUSHIRO and HARAMURA, COMSTONet al., BI~QAR et al., HASKIN et al., all 1971), which is about the range shown by the cores of olivines in the basalt porphyries studied (Fig. 2). Since the compositions of this range are all more forsteritic than any observed in 12035 (the maximum is Fo64, Fig. 2), the accumulate hypothesis requires significant re-equilibration to account for the Fe-rich compositions of its olivines. If, however, the bulk composition of 12035 represents a liquid composition, * the liquidus olivines would have been Mg-rich (12040, with a similar composition, shows Fo82 liquidus olivine, GREEN et al., 1971), with the result that even more Fe-Mg re-equilibration of the olivines of 12035 than that required by olivine accumulation would have been necessary. Further evidence of re-equilibration in 12035 was found by SCHNETZLERand PHILPOTTS (197 1) in the distribution of trace elements among pyroxenes and plagioclase, and by REID (1971) in the compositions of spinels. A broad range of compositions for the interiors of olivine grains in 12035 (Fig. 2) is a natural consequence of partial re-equilibration, and, therefore, increase in Fe-content in olivines cannot be directly related to the progress of fractional crystallization, as is assumed to be the case for the porphyritic rocks. MINOR ELEMENTS Table 1 gives the analyses of the central areas of 60 olivine grains, chosen to cover the ranges in their Fe content for each rock. The 12 or 13 olivines analyzed in each of three porphyritic rocks show significant correlation at the 95 per cent level for each of the minor elements (Ti, Cr, Mn, Ca) with Fe content. For the five olivines analyzed in each sample, correlation with Fe is significant for Ti, Cr, Mn in 12035 and only for Mn in 12008. The relationships of the minor elements to Fe in olivines are plotted in Figs. 3-6, with boxes to show the range of two-thirds Standard Error of Estimate (ALDER and ROESSLER, 1968; the “region within which about twothirds of the points of the sample fall.“). The linear regression for each set of data would bisect its box, paralleling the top and bottom sides. Since increase in Fe content in the olivines of the porphyritic rocks marks the progress of fractional crystallization, changes in minor element content may also relate to fractional crystallization. Textural relations (BRETT et al., 1971) show that titanian chromite, although present in accessory amounts, was the most abundant * Intrinsic evidence against this origin, such as concentrically zoned Mg-rich absent in 12035, as was pointed out to be the case for 12040 by GREENet al., 1971.
olivine,
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Compositional characteristicsof olivines from Apollo 12 samples
779
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Fig. 3. Summary of the TiO, vs Fe0 contents of lunar olivines. The date points for 12004 (04), 12009 (09) and 12022 (22) are not plotted but are summarized by boxes, the upper and lower sides of which are parallel to and spaced one Standard Error of Estimate from a linear regression on the points. A are the values for the cores of olivines from 12008 (OS), 12013 (13), 12035 (35) and 12041 (41). These values are either joined by lines or rareassociated with a f two-thirds Standard Error of Estimate box for the rock in question. For 12013 (13) the middle value is from this study, the left value is from DRAKEet al. (1970), and the right is from LUNATICASYLUM (19’70). 0 are olivine core values from Apollo 11 basalts (10020, 10022, and 10045 from: BRO~KNet al. (1970); HAUGERTYet al., (1970); KEIL et al., (1970) KUSHIROand NAKAXCJR A (1970); WEILL et al. (1970)). q are analyses of olivines from anorthositic fragments from Apollo 11 fines or breccias reported by: AURELLet al., 1970; EEIL et al., 1970; REID et al., 1970. . are values for olivines from Apollo 11 hnes and breccias that are not in anorthositic fragments given by: AGRELLet al., 1970; KEIL et al., 1970; LOVERWO and WARE, 1970. The diagonal lines have the slope of the average of the linear regressionsand they divide the diagram into compositional fields. phase throughout most of olivine crystallization. Only olivine and chromite need be considered as affecting differentiation during crystallization of the olivine cores. Partition coefficients for the minor elements between olivine and liquid may be approximated by using bulk rock composition (COMPSTON et al., 1971; KUSHIRO and HARAMURA, 1971) for the liquid that was in equilibrium with the Mg-rich olivines. Owing to strong partitioning into the liquid of Ti (by about 75 times) and Ca (by about 32 times), olivine crystallization alone appears to be nearly sufficient to account for the increased Ti and Ca concentrations in olivine with increase in Fe coprecipitating
PA~ICK
780
BUTLER,
JR.
0.10 -
Fig. 4. Summary of the CraOs vs Fe0 contents of olivines. The explanations and references are as given for Fig. 3. The downward arrow indicates a value below detection, the limit of which is shown by the associated point.
conoentration, as can be shown by Rayleigh depletion calculations using approximate initial and final olivine compositions and modes. Some change in the partition coefficients in favor of olivine as it became richer in Fe would more amply account for the Ti increases. Small fractionation between liquid and Mg-rich olivines for both Mn (by about 1-l times) and Cr (by about 1*4 times), however, makes olivine crystallization ineffective in changing their concentrations in the ~main~g liquid. On the basis of modes and compositions of chromite (BRETTet al., 1971), its copre&pit&ion with olivine would have been sufficient to account for the anticorrelation of Cr and Fe in the olivine. Increase of Mn in olivine with Fe, however, must result from an increased acceptance of Mn into olivine with the increase in Fe content, because Mn enters chromite in approximately the same concentrations as it is present in the liquids (for example see spine1 analyses in HAWERTY and MEYER, 1970). The scattered relationships for &-Fe (Fig. 4) and &,-Fe (Fig. 6) of the 12035 olivines are what would be expected for partial re-equilibration involving independent components. If the olivines of 12035 formed with minor element contents like
Compositiontalcharacteristicsof olivines from Apollo 12 samples
0
I
I
1s
20
, 25
i
Fe0
R’i
t
I
30
35
I 40
781
1
4:
% tNOLlVINE
The explanations and Fig. 5. Summary of the MnO vs Fe0 contents of o&&es. referencesare as given for Fig. 3, the arrow is as given for Fig. 4.
those of the porphyritic olivines, reequilibration would have involved an increase of Ca (Fig. 6) and a decrease in Cr (Fig. 4) accompanying the increases in Fe. Ti-Fe (Fig. 3) and Mn-Fe (Fig, 5) relationships in the olivines of 12035 that are like those of the porphyritic basalts are difficult to explain in view of partial Fe-Mg reequilibration. The concentrations of Ti and Mn appear to have been strongly governed by the major composition of olivme during all stages of re-equilibration. DISWJSSION All olivines crystallizing from a homogeneous melt at a given stage of differentiation should have the same concentrations of major and minor elements, provided the physical differences among the sites of crystallization, such as cooling rate or pressure, are not so great as to affect partitioning coefficients. Thus, the composition of olivines, where not si~ificantly altered through re-eq~libration, may provide criteria for possible comagmatic origins of samples. The cores of the olivines from the four porphyritic rocks studied lie within a restricted range of Fe concentrations (Fig. 2) and therefore represent the same stage of differentiation,
PATRICKBUTLER,JR.
782
I
0.25
0.10 -
0.05 -
0
I
IS
I
20
I
I
I
#
2s
30
35
40
F&l WT X IN OLIVINE
Fig. 6. Summary of the CeO vs Fe0 contents of olivines. The explanationsmd referencesm-e as given for Fig. 3.
The Ca-Fe relations of their olivines (Fig. 6) appear to group the porphyries according to texture. The vitrophyres (12008 and 12009) have olivines with higher Ca contents than those of the two holocrystalline rocks (12004 and 12022). These differences in the Ca may depend on cooling rate as evidenced by textures, or may be a fine scale example of the inverse relation between Ca content in olivine and depth of crystallization discovered by SIMKIN and WITH (1970) for terrestrial rocks. In contrast to the Ca relations in olivines, the relative contents of the other minor elements appear to be unrelated to the textural differences among the four porphyries studied. The two vitrophyres (12008 and 12009) are clearly distinct from one another in terms of Ti, Cr and Mn in their olivines (Figs. 3-5). Holocrystalline rock 12022, moreover, resembles 12008 on the basis of similar Ti, Cr and Mn contents in their olivines (Figs. 3-5). The Ti-Fe and particularly the Cr-Fe relations for the olivines of 12004 show more scatter than those for the other rocks, possibly indicating some re-equilibration. The most primitive olivines of 12004 (those with the lowest Fe
Compositional oharacteristicsof olivines from Apollo 12 samples
783
con~nts), however, have Ti and Cr contents most like those of 12009. Thus, on the basis of the olivine data, the porphyritic rocks form two possibly correlative pairs: 12004 and 12009, with lower Ti and higher Cr olivines; 12008 and 12022, with higher Ti and lower Cr olivines. Rocks 12004 and 12009 are similar to each other in many chemical characteristics, although COJYIPSTONet al. (1971) point out several chemical and isotopic hindrances to a comagmatic origin. 12022 differs considerably in its chemical characteristics from most Apollo 12 basalts, as first shown by KUSHIR~ and HARAMUBA(1971). only trace element analyses have been made on 12008 (PAPANASTASSIOU and WASSERBURQ, 1971; ANDERSet a,!., 1971). The Ti content of 12022 is greater than that of any other analyzed Apollo 12 basalt, but less than that of any of the Apollo I1 basalts. Ti concentrations in olivines (Fig. 3) appear to reflect accurately these relative bulk Ti concentrations. From initial Sr isotopic ratios, PAPANASTASSIOU and WASSERB~R~ (1971) show 12008 to be from a different magma than the texturally nearly identical 12009, in conformity with the evidence from the olivine compositions. An analysis for major elements in 12008 would not only further test the possibility of its correlation with 12022, but would closely define the liquid composition from which it formed since only chromite and olivine are possible cumulus phases. The higher Ti and lower Cr contents in the olivines of the Apollo 11 basalts relative to the values for olivines in the Apollo 12 porphyries (Figs. 3 and 4) mimic the relative bulk rock contents of Ti and Cr between the two sites (for example see KWSHIROand HARAMURA,1971; or GOLESet al., 1971). These relations suggest that the initial concentrations in olivine, determined by pa~ition~g from the melts, were subsequently preserved. Where fractional crystallization has had a significant role in the formation of a rock, re-equilibration produces an increase in Fe and a decrease in Mg in the cores of olivines. No clear effect on either the Ti or Ca contents of olivine accompanies this re-equilibration. It does seem apparent, however, that reduction in Cr concentration is an effect of re-equilibration, since in the Cr-Fe diagrams (Fig. 4) the area of high Cr content is without data points for the more Fe-rich olivines. Decrease in the concentration of Cr in olivines may occur even more readily than decrease in Mg, and so may account for the low Cr contents of relatively Mg-rich olivines in some of the nob-basaltic rocks (Fig. 4). Re-equilibration of olivines in 12035 renders their compositions unsuitable for testing oorrelations. The same may be true for sample 12040, a similar Mg- and olivine-rich basalt studied by GIBB and ZUSSMANN(1971). These authors cited non-colinearity of Cr-Fe and C&-Fe relations of olivines in 12040 with the more Mg-rich olivines of the po~hy~ti~ basalts 12052, 12075 and 12076 as evidence against possible comagmatic relations. As in the case of 12035, however, the noncolinearity in these olivine relationships may be a consequence of re-equilibration, even if the rocks were comagmatic. ~~~~~gs~~~~-~ thank ROBIN BRETT, ABCEI Ed. REID, &.~~JEILBASS, and MICH~~EL B. DUKE all of MSC for help at various stages in this study, which was done during tenure of an NRC-NASA Resident lClesearch Associateship.
784
PATRICKBUTLER, JR. REFERENCES
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