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Sm-Nd systematics of a tonalitic augen gneiss and its constituent minerals from northern Michigan
0016-7037
RI 071245.05502.00 0 Pcrpmon Prcs Ltd
NOTEI Sm-Nd systematics of a tonalitic augen gneiss and its continent minerals from northern ~chiga~ KIYOTO FUTA U.S. Geological Survey. MS 963. Box 25046. Denver Federal Center. Denver. CO 80225. U.S.A. (Received
17 Sepren&-r
1980: accrpred
in ret+sedform
2 March
198 1)
Abstract-The Sm-Nd isotopic system of a tonalitic augen gneiss and its constituent minerals from northern Michigan was disturbed during metamorphism. Sm-Nd zircon apes are lower than the wholerock Sm-Nd model age. However. closely associated pairs of minerals (for example. sphene and biotite or apatite and plagioclase) retain their apparent metamorphic ages. The Sm-Nd model ape for the tonalitic augen 8neiss of 3919 + 30 myr. appears to reflect open system behavior during metamorphism. A mineralogically different gneiss from the same location has a Sm-Nd model age of 3520 + 70myr. The two whose rocks differ in their Sm-Nd and Rb-Sr systematics and in their chondr~te-normaIiz~ rare earth element (REE) patterns. The whose-r~k-normali~d mineral REE patterns show the contribution of the major and trace minerals to the REE content of the whole rock. The trace minerals contain a significant amount of the total REE.
INTRODUCTION THE Sm-Nd
method of dating has recently been applied to different rock types, including meteorites, as presented in the pioneering work by LUGMA~ (1974). and a variety of Precambrian terrestrial samples (HAMILTONet al., 1977; DEPAOLOand WASSERBURG.1978). This dating method is most appropriate for old rocks because of the long half life of ‘*‘Sm and the limited enrichment of Sm relative to Nd during petrogenetic processes. MCCULLCKHand WA~SERBURG(1980) obtained a Sm-Nd model age of 3600 Myer on a sample of gneiss from Watersmeet, Michigan. U-Th-Pb systematics for zircons from this rock indicate a minimum age of 3410 Myr and allow for the possibility of an age as great as 3800 Myr: whereas the Rb-Sr systems of both whole-rock and mineral samples were reset at 1750 Myr ago (PETERMAN rr (II. 1980). Sm and Nd are contained in significant amounts in zircons (NAGASAWA,1970). Zircons from the gneiss at Watersmeet. because of their old U-Th-Pb age, were selected to evaluate the Sm-Nd system of this mineral. Other minerals were also analyzed for to better understand the redistribution of Sm-Nd among mineral phases during metamorphism. The other minerals. separated from the tonalitic augen gneiss including alla&e. apatite. biotite. sphene, plagioclase. and quartz. were also analyzed for their rare earth elements IREE). To show the contribution of the mineral phases to the REE content of the whole rock, wholerock-normalized REP patterns of the minerals are presented.
The two whole-rock samples analyzed. M-83 and Dl395 are from an Archean gneiss dome mantled by
the Marquette Range Supergroup. Proterozoic X metavolcanic and met~i~ntary rocks in the Watersmeet area of the northern peninsula of Michigan. These samples belong to a complex informally called the gneiss at Watersmeet (SIMSand PETERMAN. 1976; PETERMAN et al.,1980). The samples were collected about 1 km apart (PETERMAN, oral communication, 1980). A description of the gneiss at Watersmeet and the geology of the area is given by SIMSand ~RMAN (1976). and F’ETERMAN er al., (1980). EXPERIMENTAL Sample
Sample M-83 is a tonahtic biotite augen gneiss with a groundmass of quartz, plagioclase and biotite. and augens composed of quartz and plagioclase (PETERMANer al.. 1980). The gneiss is thought to be of igneous origin. and a minimum age of 3,410 Myr is established (PETERMAN er ol.. 1980). Sample D-1395 is a medium-granted biotite-quartzfeldspar-plagioclase gneiss described by Sr~s and PETERMAN (t976). The Sm-Nd system was examined in the whole-rock M-83 and some of its constituent minerals. including allanite. apatite. biotite, plagioclase and quartz. sphene, and zircon. The separation of the quartz and plagioclase. biotite. apatite, and zircon was accomplished by conventional heavy liquids and magnetic methods. However. the allanite ranged from metamict to crystalline and could not be separated by conventional physical methods. The separation was accomplished by taking advantage of its solubility in HCI using the following procedure; in the heavy non-magnetic fraction, grains of allanite. sphene. and zircon were observed optically, and the X-ray fluorescence spectrum of this separate was recorded for the elements K. Ca, Ti. Fe, Hf. Pb. Th. Rb, Sr. Y. Nb. and Zr, using the Zr peak intensity as a reference. Then 6.0N HCI was added to the weighed mineral separate and heated and stirred for 1 hr. The HCI solution was removed and the washed resi-
1245
Notes
1246 'ACLE
Sample M-83 W.R. M-83 W.R. repeat Allanite Apatite Biotite Plag f Qtz Sphene
Zircon-100
Zircon 01395
1.
RARE
Ce
Nd
__
68.47
174.6 28,300 80.2 145.6
485
+270 72.99
64.65 9000 113.5
48.9 7.97 265 137.9 169.7 20.72
EARTH
Sm
ELEMENT
ABUNDANCE
REE Abundance Data (ppm) EU Gd
11.24 10.59 990 64.61
1.31 I.30 97.1 11.47
1.25 128 56.6 82.66 2.49
0.277 24.7 20.19 29.5 0.730
a.63
IATA
0.954
-8.08 T29.0 0:977 ---1.47
Dy
Er
Yb
-7.73 455 187.3 ~6.57 0.553 ---0.812
_.
_-
4.20 197 128.5 3.92 0.209
3.7: 144 107.1 3.18 0.146
__-
__-
0.390
0.353
-- Not determined.
was drted and weighed. The X-ray fluorescence spectrum recorded a marked decrease for the elements Fe, Hf, Pb. Th, Rb, Y, and some Nb. The peaks for elements in sphene. i.e. K. Ca, Ti. and Sr, remained unchanged using the Zr peak intensity for a reference. The weighed difference (+ S?,,) was used to calculate the amount of allanite present in the separate. Because the allanite was chemically and not physically separated from the sphene. and zircon, the acid may have leached some of the other materials, although the general REE pattern of the sphene agrees with published patterns (SIMMONS and HEDGE.1978). This mineral will be referred to as ‘allanite’. The residue left from the HCi leach was then heated and stirred for cu. I5 hr in concentrated HaSO, to dissolve the sphene from the separate. The X-ray fluorescence spectrum was recorded on the washed, dried, and weighed residue. Normalizing again to the Zr peak intensity, the spectrum had a marked decrease in K. Ca and Ti. and Sr. For the same due
as was used for the ‘alanite’. the sphene WIII be referred to as ‘sphene.’ Both the ‘allanite’ and ‘sphene’ solutions were analyzed fur the REE contents. reason
Anulyrical
methods
Sm was separated from Nd with a 26-cm gravity flow cation exchange (Bio-Rad AGSOW-8X) column in 3.0 N aqua regia. The separation among each of the REE is not as complete as those previously described (LUGMAIRer al., 1975). but isobaric interference was eliminated, and the required precision for radiometric dating was obtained. The ‘43Nd/“*Nd ratios were normalized to the ‘4hNd/‘44Nd ratio of 0.7219 (ONIONS er al.. 1977). They have an analytical uncertainty of 3 to 5 parts in 10’ at the 95”, confidence limit. Sm and Nd concentrations were determined by isotope dilution methods and precision of the Sm/Nd ratios were 0.9S;. The total procedural blank levels are in the order of 10-t” grams. The normalized
Fig. I. Whole-rock normalized mineral rare earth element pattern for M-83
Notes TABLE 2.
SAMARIUM AND NEODYMIUM DATA
1247
The Sm-Nd isotopic data for the two whole-rock samples and the minerals from sample M-83 are given *‘lbg,,d l147sm in Table 2. The Sm-Nd model ages for the wholeSample Sm-ppm Nd ppm 144Nd =w rock samples (calculated from a meteorite M-83 W. R. 11.24 68.47 .51021 + 2 0.100 M-83 W. R. 10.59 64.65 .510x T 4 0.0997 143Nd/144Nd ratio of 0.512636 and 147Sm/‘44Nd of Qtz + Plag 1.25 7.97 .51063 i: 5 0.0954 0.1936, LUGMAIR et al., 1975; NAKAMURA et al., 1976) Biotite 8.63 48.9 .51032 7 2 0.1074 "Allanite" 990 9000 .51006 7 3 0.0668 are 3919 + 30 and 3884 + 65 Myr for the two splits Apatite 64.61 113.46 .51367 + 4 0.3467 of M-83 and 3520 * 70 Myr for D1395. The Sm-Nd Zircon (-100) 56.60 137.91 .51131 7 2 0.2499 .51131 T 2 -. -. _model age for sample D1395 agrees with that Zircon (t270) 82.66 169.74 .51119 0.2965 obtained by MCCULLOCHand WASSERBURG (1980) for "Sphene" 127 265 .51238 + 3 0.291s D1395 R 2.49 20.72 .50983 T 5 0.0731 a sample of this same unit collected from a site 2.3 km BCR-1 6.50 28.27 .51267 7 5 east of D1395. Although the Rb-Sr systems in the 6.56 28.40 .51268 5 2 6.53 28.50 .51266 5 6 rocks were disturbed 1750Myr ago (PETERMANet al., *Normalized to 146Nd/144Nd = 0.7219. 1980), sample D1395 does give, perhaps fortuitously, a +147Sm/l““Nd + .9X, except "Allanite" and "Sphene" z +5%. Rb-Sr model age of 3679 + 107 Myr. The whole-rock sample M-83 gives a. much lower Rb-Sr age of ‘4JNd/‘44Nd ratio for the USGS Standard BCR-I is 2669 f 48 Myr. Presumably the whole-rock sample shown in Table 2: it is in agreement with published values D1395 has remained closed, in terms of both Sm-Nd (O’NIONS et al., 1977: DeP~oto and WFASERBURG,1976: and Rb-Sr systems, since 3600 Myr ago, whereas both HAWKESWORTH1979: ZINDLERet al.. 1979). systems in Sample M-83 were opened, presumably Rb and Sr concentrations were determined by isotope dilution. The *‘Sr/s6Sr ratios were normalized to during the 1750-Myr metamorphism. ans%r/ssSr ratio of 0.1194. The E and A standard Figure 2 illustrates the relationship between the s’Sr,@Sr ratio obtained was 0.70803 + 5. ‘43Nd/‘44Nd and 147Sm/144Nd ratios (data are given in Table 2). A reference line corresponding to an age of 3600 Myr is drawn through the data point for the RESULTS AND DISCU.SSION whole-rock sample D1395. Whole-rock sample M-83 plots slightly to the right of this line. and the mineral The REE concentrations for the two whole rocksamples and the mineral separates from the sample separates from M-83 scatter widely. Most of the mineral data can, however, be reasonM-83 are given in Table 1. The relative concentrations of REE in the various mineral phases are as ably interpreted. It is known from the Rb-Sr study expected (BUMAet al., 1971; NAGASAWA,1970; SIM- (PETERMAN er al., 1980) that these rocks were subject MONSand HEDGE, 1978) from the published distrito intense deformation and metamorphism about bution coefficients. The only exceptions are the two 1750Myr ago. The Rb-Sr data for the whole-rock zircons which have surprisingly high concentrations sample M-83 and its biotite are presented in Table 3. of Sm and Nd, and have lower than expected Sm/Nd The two points form a mineral isochron of 1718 Myr, which agrees with previous RbSr biotite age for this ratios. age (PETERMAN et al., 1980) and with the K-Ar biotite Figure 1 shows the REE abundances of the mineral age of 1710 Myr. It is. therefore, reasonable to expect separates plotted relative to the whole rock. It is imthat the Sm-Nd systems of the minerals were re-equiportant to note that the REE concentrations of the iibrated at the time. No single 1750-Myr line will fit trace minerals are so high that they actually contribthe mineral data, but pairs of minerals that are in ute more to the mass balance than do the major minerals. intimate contact in the rock do lie among approxi-
Fig. 2. Sm-Nd isochron diagram.
Notes
1248 TABLE
Sample M-B3 W. R. Biotite *
Normalized
3.
RUBIDIUM
AND
STRONTIUM
Rb-ppm
Sr-ppm
'87Sr/a6Sr
215.6 825
150.1 13.2
.8605 8.566
to 86Sr/BBSr
have an atIinity for REE, preferring the heavy REE; and the finer zircon fraction has a higher concentration of Sm and Nd. a higher ‘*‘Sm/144Nd ratio, and a lower 143Nd/‘44Nd ratio than the coarser zircon fraction, which can be explained by contamination by REE from the altered allanite. The two whole-rock samples differ considerably in their REE data. Sample M-83 has a higher concentration of REE than D1395. The chondrite-normalized REE patterns for the two whole-rock samples, as seen in Fig. 3, differ in that sample D1395 has a steeper slope in the light REE than M-83 and does not have the pronounced negative Eu anomaly of M-83. The origin of sample M-83 is clearly igneous PETERMAN et al., 1980) and the chondrite-normalized REE pattern is typical of granites (NAGASAWA, 1970).
DATA
e7Rb/86Sr 4.219 320.1
= 0.1194.
mately 1750-Myr isochrons. The rock is slightly banded with biotite concentrated in streaks between broader bands of plagioclase and quartz. The sphene occurs almost entirely within the biotite-rich zones. whereas the apatite occurs with the plagioclase. Sphene-biotite and apatite-plagioclase cords (Fig. 2) closely approximate the 17%Myr age (i.e. 1692 and 1839 Myr respectively), indicating that Sm-Nd only equilibrated between immediately adjacent minerals. Allanite occurs in the rock in a few large scattered grains. The above interpretation cannot explain the zircon data. however. The zircons also occur in the biotiterich zones. If they equilibrated with the biotite at 1750 Myr ago, any record of this equilibration has been obscured by later events. No unique interpretation of the zircon data is obvious, but a few observations may be significant. Apparent ages of the two zircon fractions are 1030 and 76OMyr. with the finer fraction giving the lower apparent age. These ages do not correspond to any known geological event, but they are similar to the lower intercept of the more U-rich zircons on the U-Pb concordia plot (Peterman et al., 1980). Both the zircons and the allanite crystals have suffered extensive radiation damage. Perhaps this damage has made these minerals susceptible to movements of Nd and Sm at later times under milder geologic conditions. The allanite has a great affinity for the REE. However it has been extensively altered and. thus can readily lose REE. Combining this loss of REE with its low 143Nd/144Nd ratio, the allanite may be the source of REE for the zircons. The zircons also
’ “c.
, Na
on
I l”
Fig. 3. Chondrite-normalized
CONCLUSIONS The Sm-Nd system for the whole-rock samples M-83 and its constituent minerals were reset during metamorphism. The Sm-Nd systematics for zircons with a metamorphic history do not retain their original age or reliable metamorphic ages. Although zircons do not give interpretable ages. other pairs of spatially related minerals (i.e. sphene-~ biotite and apatite-plagioclase) yield approximate metamorphic ages. The two whole rocks, M-83 and D1395. are different in their mineralogy and trace elements. The Sm-Nd and Rb-Sr systems and the chondrite-normalized REE patterns differ between. the two rocks. Model ages by the Sm-Nd system for the whole-rock M-83 and D1395 samples were 3919 + 30 and 3520 k 70 Myr, respectively. Acknowledgements-1 wish to thank Zell E. Peterman not only for providing the samples, but also for the hours spent helping and advising me on the mineral separation and X-ray texchniques. Thanks also is extended to ZELL E. PETERMAN and CARL E. HEDGEfor advice and assistance
/ w
or
ET
rare earth element patterns
“b
1249
Notes on this manuscript. The computer program is due to the efforts of ERNESTE. WILSON,and the K-Ar age was determined by RICHARD F. MARVIN.The author is also grateful to GL’NTER W. LUGMAIR. WILLIAM M. WHITE. and RICHARDW. CARLSONfor review of this manuscript. REFERENCES
Search for extinct ‘%m 1. The isotopic abundance of 14’Nd in the Juvinas meteorite. Earrh and Planet. Sci. Len. 21, 79-84. MCCULLOCHM. T. and WASSERBURG G. J. (1980) Sm-Nd model ages form an early Archean tonalitic gneiss northern Michigan. Geol. Sot. Am. Spec. Pap. 182, 135. NAGASAWA H. (1970) Rare earth concentration in zircons and apatites and their host dacites and granites. Earth Planet
BUMA G.. FREY F. A. and WONES D. R. (1971) New England granites: Trace element evidence regarding their origin and differentiation. Conrrib. Mineral Petrol. 31, 300-320. DEPAOLOD. J. and WASSERBURG G. J. (1976) Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249. DEPAOLOD. J. and WASSERBURG G. J. (1978) Sm-Nd age of the Stillwater Complex and the mantle evolution curve for neodymium. Geochim. Cosmochim. Acra 43, 999.
HAMILTONP. J.. O’NIONSR. K. and EVENSENN. M. (1977) Sm-Nd dating of Archean basic and ultra basic volcanits. Earth Planer. Sci. Lert. 36, 263-268. HAWKESWORTH C. J., O’NIONS R. K. and ARCULUSR. J. (1979) Nd and Sr isotope geochemistry of island arc volcanics. Grenada. Lesser Antilles. Earth Planet. Sci. Lerl. 45. 237-248. LUGMAIRG. W. (1974) Sm-Nd: A new dating method (Abstract). Meteoritics 9, 369. LUGUAIR G. M., SCHEININN. B. and MARTI K. (1975)
Sci. Lert. 9, 359-364.
NAKAMURA N., TATSUMOTO M., NUNESP. D., UN&H D. M., SCHWABA. P. and WILDEMANT. R. (1976) Proc. 71h Lunar Planet. Sci. ConJ p. 2309. O’NIONSp. K., HAMILTONP. J. and EVENSENN. M. (1977) Variations in ‘43Nd/‘44Nd and “Sr/‘%r ratios in oceanic basalts. Earrh Planet. Sci. Lett. 34, 13-22. PETERMAN Z. E., ZARTMANR. E. and SIMS P. K. (1980) Tonalitic gneiss of early Archean age from northern Michigan. Geol. Soci. Ame. Spec. Pap. 182, 125. SIMMONS E. G. and HEDGEC. E. (1978) Minor-element and Sr-isotope geochemistry of Tertiary stocks, Colorado Mineral Belt. Contrib. Mineral. Perrol. 67. 379-396. SIMSP. K. and PETERMAN Z. E. (1976) Geology and Rb-Sr ages of reactivated Precambrian gneisses and granite in the MerenisceWatersmeet area. northern Michigan. U.S. Geol. Surr. .I. Res. 4, 405-414. ZINDLERA., HART S. R.. FREY F. A. and JAKOBSSON S. P. (1979) Nd and Sr isotope ratios and rare earth element abundance in Reykjanes Peninsula basalts: Evidence for mantle heterogeniety beneath Iceland. Earth Planet. Sci. Lerr. 45, 249-262.
RI 071245.05502.00 0 Pcrpmon Prcs Ltd
NOTEI Sm-Nd systematics of a tonalitic augen gneiss and its continent minerals from northern ~chiga~ KIYOTO FUTA U.S. Geological Survey. MS 963. Box 25046. Denver Federal Center. Denver. CO 80225. U.S.A. (Received
17 Sepren&-r
1980: accrpred
in ret+sedform
2 March
198 1)
Abstract-The Sm-Nd isotopic system of a tonalitic augen gneiss and its constituent minerals from northern Michigan was disturbed during metamorphism. Sm-Nd zircon apes are lower than the wholerock Sm-Nd model age. However. closely associated pairs of minerals (for example. sphene and biotite or apatite and plagioclase) retain their apparent metamorphic ages. The Sm-Nd model ape for the tonalitic augen 8neiss of 3919 + 30 myr. appears to reflect open system behavior during metamorphism. A mineralogically different gneiss from the same location has a Sm-Nd model age of 3520 + 70myr. The two whose rocks differ in their Sm-Nd and Rb-Sr systematics and in their chondr~te-normaIiz~ rare earth element (REE) patterns. The whose-r~k-normali~d mineral REE patterns show the contribution of the major and trace minerals to the REE content of the whole rock. The trace minerals contain a significant amount of the total REE.
INTRODUCTION THE Sm-Nd
method of dating has recently been applied to different rock types, including meteorites, as presented in the pioneering work by LUGMA~ (1974). and a variety of Precambrian terrestrial samples (HAMILTONet al., 1977; DEPAOLOand WASSERBURG.1978). This dating method is most appropriate for old rocks because of the long half life of ‘*‘Sm and the limited enrichment of Sm relative to Nd during petrogenetic processes. MCCULLCKHand WA~SERBURG(1980) obtained a Sm-Nd model age of 3600 Myer on a sample of gneiss from Watersmeet, Michigan. U-Th-Pb systematics for zircons from this rock indicate a minimum age of 3410 Myr and allow for the possibility of an age as great as 3800 Myr: whereas the Rb-Sr systems of both whole-rock and mineral samples were reset at 1750 Myr ago (PETERMAN rr (II. 1980). Sm and Nd are contained in significant amounts in zircons (NAGASAWA,1970). Zircons from the gneiss at Watersmeet. because of their old U-Th-Pb age, were selected to evaluate the Sm-Nd system of this mineral. Other minerals were also analyzed for to better understand the redistribution of Sm-Nd among mineral phases during metamorphism. The other minerals. separated from the tonalitic augen gneiss including alla&e. apatite. biotite. sphene, plagioclase. and quartz. were also analyzed for their rare earth elements IREE). To show the contribution of the mineral phases to the REE content of the whole rock, wholerock-normalized REP patterns of the minerals are presented.
The two whole-rock samples analyzed. M-83 and Dl395 are from an Archean gneiss dome mantled by
the Marquette Range Supergroup. Proterozoic X metavolcanic and met~i~ntary rocks in the Watersmeet area of the northern peninsula of Michigan. These samples belong to a complex informally called the gneiss at Watersmeet (SIMSand PETERMAN. 1976; PETERMAN et al.,1980). The samples were collected about 1 km apart (PETERMAN, oral communication, 1980). A description of the gneiss at Watersmeet and the geology of the area is given by SIMSand ~RMAN (1976). and F’ETERMAN er al., (1980). EXPERIMENTAL Sample
Sample M-83 is a tonahtic biotite augen gneiss with a groundmass of quartz, plagioclase and biotite. and augens composed of quartz and plagioclase (PETERMANer al.. 1980). The gneiss is thought to be of igneous origin. and a minimum age of 3,410 Myr is established (PETERMAN er ol.. 1980). Sample D-1395 is a medium-granted biotite-quartzfeldspar-plagioclase gneiss described by Sr~s and PETERMAN (t976). The Sm-Nd system was examined in the whole-rock M-83 and some of its constituent minerals. including allanite. apatite. biotite, plagioclase and quartz. sphene, and zircon. The separation of the quartz and plagioclase. biotite. apatite, and zircon was accomplished by conventional heavy liquids and magnetic methods. However. the allanite ranged from metamict to crystalline and could not be separated by conventional physical methods. The separation was accomplished by taking advantage of its solubility in HCI using the following procedure; in the heavy non-magnetic fraction, grains of allanite. sphene. and zircon were observed optically, and the X-ray fluorescence spectrum of this separate was recorded for the elements K. Ca, Ti. Fe, Hf. Pb. Th. Rb, Sr. Y. Nb. and Zr, using the Zr peak intensity as a reference. Then 6.0N HCI was added to the weighed mineral separate and heated and stirred for 1 hr. The HCI solution was removed and the washed resi-
1245
Notes
1246 'ACLE
Sample M-83 W.R. M-83 W.R. repeat Allanite Apatite Biotite Plag f Qtz Sphene
Zircon-100
Zircon 01395
1.
RARE
Ce
Nd
__
68.47
174.6 28,300 80.2 145.6
485
+270 72.99
64.65 9000 113.5
48.9 7.97 265 137.9 169.7 20.72
EARTH
Sm
ELEMENT
ABUNDANCE
REE Abundance Data (ppm) EU Gd
11.24 10.59 990 64.61
1.31 I.30 97.1 11.47
1.25 128 56.6 82.66 2.49
0.277 24.7 20.19 29.5 0.730
a.63
IATA
0.954
-8.08 T29.0 0:977 ---1.47
Dy
Er
Yb
-7.73 455 187.3 ~6.57 0.553 ---0.812
_.
_-
4.20 197 128.5 3.92 0.209
3.7: 144 107.1 3.18 0.146
__-
__-
0.390
0.353
-- Not determined.
was drted and weighed. The X-ray fluorescence spectrum recorded a marked decrease for the elements Fe, Hf, Pb. Th, Rb, Y, and some Nb. The peaks for elements in sphene. i.e. K. Ca, Ti. and Sr, remained unchanged using the Zr peak intensity for a reference. The weighed difference (+ S?,,) was used to calculate the amount of allanite present in the separate. Because the allanite was chemically and not physically separated from the sphene. and zircon, the acid may have leached some of the other materials, although the general REE pattern of the sphene agrees with published patterns (SIMMONS and HEDGE.1978). This mineral will be referred to as ‘allanite’. The residue left from the HCi leach was then heated and stirred for cu. I5 hr in concentrated HaSO, to dissolve the sphene from the separate. The X-ray fluorescence spectrum was recorded on the washed, dried, and weighed residue. Normalizing again to the Zr peak intensity, the spectrum had a marked decrease in K. Ca and Ti. and Sr. For the same due
as was used for the ‘alanite’. the sphene WIII be referred to as ‘sphene.’ Both the ‘allanite’ and ‘sphene’ solutions were analyzed fur the REE contents. reason
Anulyrical
methods
Sm was separated from Nd with a 26-cm gravity flow cation exchange (Bio-Rad AGSOW-8X) column in 3.0 N aqua regia. The separation among each of the REE is not as complete as those previously described (LUGMAIRer al., 1975). but isobaric interference was eliminated, and the required precision for radiometric dating was obtained. The ‘43Nd/“*Nd ratios were normalized to the ‘4hNd/‘44Nd ratio of 0.7219 (ONIONS er al.. 1977). They have an analytical uncertainty of 3 to 5 parts in 10’ at the 95”, confidence limit. Sm and Nd concentrations were determined by isotope dilution methods and precision of the Sm/Nd ratios were 0.9S;. The total procedural blank levels are in the order of 10-t” grams. The normalized
Fig. I. Whole-rock normalized mineral rare earth element pattern for M-83
Notes TABLE 2.
SAMARIUM AND NEODYMIUM DATA
1247
The Sm-Nd isotopic data for the two whole-rock samples and the minerals from sample M-83 are given *‘lbg,,d l147sm in Table 2. The Sm-Nd model ages for the wholeSample Sm-ppm Nd ppm 144Nd =w rock samples (calculated from a meteorite M-83 W. R. 11.24 68.47 .51021 + 2 0.100 M-83 W. R. 10.59 64.65 .510x T 4 0.0997 143Nd/144Nd ratio of 0.512636 and 147Sm/‘44Nd of Qtz + Plag 1.25 7.97 .51063 i: 5 0.0954 0.1936, LUGMAIR et al., 1975; NAKAMURA et al., 1976) Biotite 8.63 48.9 .51032 7 2 0.1074 "Allanite" 990 9000 .51006 7 3 0.0668 are 3919 + 30 and 3884 + 65 Myr for the two splits Apatite 64.61 113.46 .51367 + 4 0.3467 of M-83 and 3520 * 70 Myr for D1395. The Sm-Nd Zircon (-100) 56.60 137.91 .51131 7 2 0.2499 .51131 T 2 -. -. _model age for sample D1395 agrees with that Zircon (t270) 82.66 169.74 .51119 0.2965 obtained by MCCULLOCHand WASSERBURG (1980) for "Sphene" 127 265 .51238 + 3 0.291s D1395 R 2.49 20.72 .50983 T 5 0.0731 a sample of this same unit collected from a site 2.3 km BCR-1 6.50 28.27 .51267 7 5 east of D1395. Although the Rb-Sr systems in the 6.56 28.40 .51268 5 2 6.53 28.50 .51266 5 6 rocks were disturbed 1750Myr ago (PETERMANet al., *Normalized to 146Nd/144Nd = 0.7219. 1980), sample D1395 does give, perhaps fortuitously, a +147Sm/l““Nd + .9X, except "Allanite" and "Sphene" z +5%. Rb-Sr model age of 3679 + 107 Myr. The whole-rock sample M-83 gives a. much lower Rb-Sr age of ‘4JNd/‘44Nd ratio for the USGS Standard BCR-I is 2669 f 48 Myr. Presumably the whole-rock sample shown in Table 2: it is in agreement with published values D1395 has remained closed, in terms of both Sm-Nd (O’NIONS et al., 1977: DeP~oto and WFASERBURG,1976: and Rb-Sr systems, since 3600 Myr ago, whereas both HAWKESWORTH1979: ZINDLERet al.. 1979). systems in Sample M-83 were opened, presumably Rb and Sr concentrations were determined by isotope dilution. The *‘Sr/s6Sr ratios were normalized to during the 1750-Myr metamorphism. ans%r/ssSr ratio of 0.1194. The E and A standard Figure 2 illustrates the relationship between the s’Sr,@Sr ratio obtained was 0.70803 + 5. ‘43Nd/‘44Nd and 147Sm/144Nd ratios (data are given in Table 2). A reference line corresponding to an age of 3600 Myr is drawn through the data point for the RESULTS AND DISCU.SSION whole-rock sample D1395. Whole-rock sample M-83 plots slightly to the right of this line. and the mineral The REE concentrations for the two whole rocksamples and the mineral separates from the sample separates from M-83 scatter widely. Most of the mineral data can, however, be reasonM-83 are given in Table 1. The relative concentrations of REE in the various mineral phases are as ably interpreted. It is known from the Rb-Sr study expected (BUMAet al., 1971; NAGASAWA,1970; SIM- (PETERMAN er al., 1980) that these rocks were subject MONSand HEDGE, 1978) from the published distrito intense deformation and metamorphism about bution coefficients. The only exceptions are the two 1750Myr ago. The Rb-Sr data for the whole-rock zircons which have surprisingly high concentrations sample M-83 and its biotite are presented in Table 3. of Sm and Nd, and have lower than expected Sm/Nd The two points form a mineral isochron of 1718 Myr, which agrees with previous RbSr biotite age for this ratios. age (PETERMAN et al., 1980) and with the K-Ar biotite Figure 1 shows the REE abundances of the mineral age of 1710 Myr. It is. therefore, reasonable to expect separates plotted relative to the whole rock. It is imthat the Sm-Nd systems of the minerals were re-equiportant to note that the REE concentrations of the iibrated at the time. No single 1750-Myr line will fit trace minerals are so high that they actually contribthe mineral data, but pairs of minerals that are in ute more to the mass balance than do the major minerals. intimate contact in the rock do lie among approxi-
Fig. 2. Sm-Nd isochron diagram.
Notes
1248 TABLE
Sample M-B3 W. R. Biotite *
Normalized
3.
RUBIDIUM
AND
STRONTIUM
Rb-ppm
Sr-ppm
'87Sr/a6Sr
215.6 825
150.1 13.2
.8605 8.566
to 86Sr/BBSr
have an atIinity for REE, preferring the heavy REE; and the finer zircon fraction has a higher concentration of Sm and Nd. a higher ‘*‘Sm/144Nd ratio, and a lower 143Nd/‘44Nd ratio than the coarser zircon fraction, which can be explained by contamination by REE from the altered allanite. The two whole-rock samples differ considerably in their REE data. Sample M-83 has a higher concentration of REE than D1395. The chondrite-normalized REE patterns for the two whole-rock samples, as seen in Fig. 3, differ in that sample D1395 has a steeper slope in the light REE than M-83 and does not have the pronounced negative Eu anomaly of M-83. The origin of sample M-83 is clearly igneous PETERMAN et al., 1980) and the chondrite-normalized REE pattern is typical of granites (NAGASAWA, 1970).
DATA
e7Rb/86Sr 4.219 320.1
= 0.1194.
mately 1750-Myr isochrons. The rock is slightly banded with biotite concentrated in streaks between broader bands of plagioclase and quartz. The sphene occurs almost entirely within the biotite-rich zones. whereas the apatite occurs with the plagioclase. Sphene-biotite and apatite-plagioclase cords (Fig. 2) closely approximate the 17%Myr age (i.e. 1692 and 1839 Myr respectively), indicating that Sm-Nd only equilibrated between immediately adjacent minerals. Allanite occurs in the rock in a few large scattered grains. The above interpretation cannot explain the zircon data. however. The zircons also occur in the biotiterich zones. If they equilibrated with the biotite at 1750 Myr ago, any record of this equilibration has been obscured by later events. No unique interpretation of the zircon data is obvious, but a few observations may be significant. Apparent ages of the two zircon fractions are 1030 and 76OMyr. with the finer fraction giving the lower apparent age. These ages do not correspond to any known geological event, but they are similar to the lower intercept of the more U-rich zircons on the U-Pb concordia plot (Peterman et al., 1980). Both the zircons and the allanite crystals have suffered extensive radiation damage. Perhaps this damage has made these minerals susceptible to movements of Nd and Sm at later times under milder geologic conditions. The allanite has a great affinity for the REE. However it has been extensively altered and. thus can readily lose REE. Combining this loss of REE with its low 143Nd/144Nd ratio, the allanite may be the source of REE for the zircons. The zircons also
’ “c.
, Na
on
I l”
Fig. 3. Chondrite-normalized
CONCLUSIONS The Sm-Nd system for the whole-rock samples M-83 and its constituent minerals were reset during metamorphism. The Sm-Nd systematics for zircons with a metamorphic history do not retain their original age or reliable metamorphic ages. Although zircons do not give interpretable ages. other pairs of spatially related minerals (i.e. sphene-~ biotite and apatite-plagioclase) yield approximate metamorphic ages. The two whole rocks, M-83 and D1395. are different in their mineralogy and trace elements. The Sm-Nd and Rb-Sr systems and the chondrite-normalized REE patterns differ between. the two rocks. Model ages by the Sm-Nd system for the whole-rock M-83 and D1395 samples were 3919 + 30 and 3520 k 70 Myr, respectively. Acknowledgements-1 wish to thank Zell E. Peterman not only for providing the samples, but also for the hours spent helping and advising me on the mineral separation and X-ray texchniques. Thanks also is extended to ZELL E. PETERMAN and CARL E. HEDGEfor advice and assistance
/ w
or
ET
rare earth element patterns
“b
1249
Notes on this manuscript. The computer program is due to the efforts of ERNESTE. WILSON,and the K-Ar age was determined by RICHARD F. MARVIN.The author is also grateful to GL’NTER W. LUGMAIR. WILLIAM M. WHITE. and RICHARDW. CARLSONfor review of this manuscript. REFERENCES
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NAKAMURA N., TATSUMOTO M., NUNESP. D., UN&H D. M., SCHWABA. P. and WILDEMANT. R. (1976) Proc. 71h Lunar Planet. Sci. ConJ p. 2309. O’NIONSp. K., HAMILTONP. J. and EVENSENN. M. (1977) Variations in ‘43Nd/‘44Nd and “Sr/‘%r ratios in oceanic basalts. Earrh Planet. Sci. Lett. 34, 13-22. PETERMAN Z. E., ZARTMANR. E. and SIMS P. K. (1980) Tonalitic gneiss of early Archean age from northern Michigan. Geol. Soci. Ame. Spec. Pap. 182, 125. SIMMONS E. G. and HEDGEC. E. (1978) Minor-element and Sr-isotope geochemistry of Tertiary stocks, Colorado Mineral Belt. Contrib. Mineral. Perrol. 67. 379-396. SIMSP. K. and PETERMAN Z. E. (1976) Geology and Rb-Sr ages of reactivated Precambrian gneisses and granite in the MerenisceWatersmeet area. northern Michigan. U.S. Geol. Surr. .I. Res. 4, 405-414. ZINDLERA., HART S. R.. FREY F. A. and JAKOBSSON S. P. (1979) Nd and Sr isotope ratios and rare earth element abundance in Reykjanes Peninsula basalts: Evidence for mantle heterogeniety beneath Iceland. Earth Planet. Sci. Lerr. 45, 249-262.