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Resistance of quartz silt to isotopic exchange under burial and intense weathering conditions
I5
Geochimica et Cosmochimica Acta. Vol. 42. pp. I7 to I522 0 Pergamon Press Ltd. 1978. Printed in Great Britain
0016-7037/711/1001-I 517SO2.00/0
Resistance of quartz silt to isotopic exchange under burial and intense weathering conditions ROBERT N. CUYTON Enrico Fermi Institute, and Departments of Chemistry and the Geophysical Sciences, University of Chicago, Chicago. IL 60637, U.S.A.
and M. L. JACKSON and K. SRIDHAR* Department of Soil Science, University of Wisconsin, Madison, WI 53706. U.S.A.
(Received 16 May 1977; accepted in revised form 5 June 1978) Abstract-Oxygen isotopic compositions of quartz from silty sediments buried to 5400m depth from two wells in the Gulf of Mexico each showed 6’*0 variations of less than 1.7’&,. Clay diagenesis has been reported within this depth. The observed variations in the quartz do not appear to be primarly diagenetic effects, but rather are mainly depositional features reflecting variations in the sources of the sediments. Sources may be influenced by the variation of distance from the shore at different depths in a given sampling location and by sediment production by continental glaciations. Stability of the oxygen isotopic composition of quartz in the lO-20pm size range under long-time humid, temperate weathering conditions was studied by analysis of saprolites formed from Pennsylvanian to Precambrian crystalline rocks. In four of the five cases, the lO-20pm fraction was found to have only O.l-O.S’& greater 6’*0 than the corresponding 20-50pm fraction. This increase may be attributable either to a slight oxygen isotopic exchange with ambient ground waters or original differences within the rock since the saprolites were sufficiently coherent to make an influx of extraneous detrital silt unlikely. The amount of oxygen isotopic exchange in silt size quartz over periods of many million years of shallow burial or weathering appears to be small enough to permit the use of the oxygen isotopic ratio of quartz in tracing the origin of eohan and fluvial additions of minerals to continental soils and pelagic sediments.
INTRODUCTION
of this paper are to assess the stability of the oxygen isotope ratio of silt-size quartz under conditions of (a) burial at depths up to 5400 meters in marine sediments of the Gulf of Mexico, and (b) intense humid temperate weathering. Quartz is exceedingly resistant to ‘oxygen isotopic exchange with water in geothermal systems at temperatures up to 350°C (CLAYTONet al., 1968), and during pyrosulfate fusion in the laboratory at about 600°C (SYERS et al., 1968; SRIDHAR et al., 1975). Based on the assumption that tine silt-size quartz (l-10 pm) is resistant to isotopic exchange in weathering and sedimentary environments, the oxygen isotopic ratio of quartz in soils and sediments has been used to trace the eolian addition of minerals (REX et al., 1969; SYER~ et al., 1969; JACKSON et a/., 1971; MOKMAet al., 1972). Quartz in sediments of the Precambrian Belt Supergroup has undergone isotopic exchange during low-grade metamorphism upon deep burial (XKtO4WOm) @LINGER and SAWN, 1973). The amount of oxygen isotopic exchange in sandand silt-size quartz of Pliocene-Pleistocene sediments has not been determined, although little post-deposiTHE
OBJE~~VES
* Present address: Sridhar Komameni, Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802, U.S.A.
tional change was inferred for quartz in shales throughout the Phanerozoic (CHURCHMAN et al., 1973, 1976). In this study, we determined the oxygen isotopic composition of quartz from various depths of burial, up to 5400 m, in sediments in the Gulf of Mexico. Quartz added to modem soils and sediments has #*O values intermediate between those of igneous rocks and low-temperature chemical sediments. This has been interpreted as reflecting mixtures of quartz of high- and low-temperature origin (CLAYTONet al., 1972). To verify this, it is necessary to show that hightemperature quartz retains its characteristic low 6**0 value during weathering and disintegration. Millimeter-size quartz grains from beach sands have undergone no significant isotopic change in the weathering process (SAWN and EPSTEIN,1970). Likewise, the
6’aO of whole-rock quartz varies little (< ly&,) through soil profiles developed on crystalline rocks (LAWRENCE and TAYLOR, 1972). EXPERIMENTAL
PROCEDURES
The shale samples were drill-cuttings taken during the boring of two wells in the Gulf of Mexico. The depth assignments were made from the depth of the drill bit at the time of sampling. We studied saprolites formed by the weathering of crystalline rocks to a depth of many meters, namely: a Pennsylvanian granitic gneiss, north-central and south part of
1517
R. N.
1518
CLAYTON.
M. L. JACK~CJN and K.
SRIDHAR
Table 1. Quartz content and oxygen isotopic ratio from shale at various depths of burial, Gulf of Mexico
3009 3418 3865 4299
-
3019 3427 3875 4308
3090 ,644 4522 4957 5396
-
3126 3612 4549 4985 5424
si /
19.4 16.2 22.4 21.6 20.4 ?8.6
5833 5834 S835 5836 S837 5838
20.1
I
19.4 19.2 19.* 19.1 19.4
5
20.7 19.7-0.6 19.210.8
Atianta, GA; a Precambrian hornblende gneiss, just east of Forsyth. GA: a Precambrian oligoclase-biotite schist. 8 km east of Griffin, GA. The interiors of boulders were sampled to avoid possible admixture of transported fine particles. The time of exposure to weathering is not known, but the great thickness of the saprolite attests to the long action of ground waters. perhaps on the order of 50 Myr or more. The study of saprolites concentrated on the finest grains present, since these contribute appreciably to the eolian dust. Samples from the I@-20pm and 20-50,um ranges were analyzed. Negligible amounts of quartz with diameter i IOpm were found. Quartz was isolated from the samples by 6N HCI treatment, Na*S,O, fusion (KIELY and JACKSON.1965). treatment with H$iF, at 18 + 1°C to remove amorphous material and feldspars (SRIDHARet ai., 1975) and washings with 0.1 N HF, HsBOs. and H,O to remove fluorides (JACKSONet al.. 1974). Particle size fractionation of quartz isolates was determined by centrifugation, sedimentation and decantation (JACKSON.1975) before the samples were dried. Oxygen isotopic compositions were determined by analytical procedures described by CLAYTONand MAYEDA (1963). Isotopic analyses were made with a 15-cm 6CY-sector double~oll~ting mass spectrometer and are reported in the usual 6 notation as per mil deviations from Standard Mean Ocean Water (SNOW).
,
i
,\,
I8
20
SO'8 rel. SMOW Fig. 1. Whole quartz oxygen isotopic ratio (&‘*O) from two drill holes in the Gulf of Mexico (0) = hole HMSl; (0) = hole RCD 2. RESULTS AND DISCUSSION
Gurf of Mexico sediments The &I80 values of whole-quartz (excluding the >O.S mm particles which were assumed to be extraneous to the shales) from the Gulf of Mexico shales vary by less than 1.79&,within each of two Pleistocene-Upper Pliocene wells (Table 1 and Fig. 1). In general, the isotopic compo~tions resemble those of quartz in shales from mid-continental U.S.A. (CHURCHMANet al., 1976), from North Pacific pelagic sediments (CILAYTONet al., 1972), and from many Northern Hemisphere soils, dusts and stream loads (SYERS er al., 1969; REX ef al., 1969; JACKSON et al., 1971). All of these have 6180 values lying between those typical of quartz from igenous and metamorphic rocks (S-1433, and those found for low-tempera-
Table 2. Quartz contents (“/,) and oxygen isotopic ratios (S”O) in various grain size fractions at four burial depths in ICICAL Well RCD 2 Depth interval in meters
Size fraction,
Pm 20 Total, mean whole**
2438-2465 (S891-5a)*
3090-3126 (S896-900a)
4522-4549 (S901"5.a)
Weight 6l*O
Weight 6'*0
Weight E'S0
%
%,
%
%.
3.8 7.7 18.4 14.4 16.2 39.5
21.7 21.7 20.9 19.7 18.0 13.2
3.6 6.9 7.4 12.7 15.5 53.9
23.1 21.9 20.8 19.6 17.5 13.7
17.3t 20.1
100
16.5t 19.2
100
%
%,
4.0 13.3 7.1 27.4 25.2 23.0
23.0 22.9 22.0 21.1 18.9 lfi?2_
100
19.m 19.1
X396-5424 (S906-10a) "eight 6'b
%
%.
5.2 5.0 20.8 23.4 24.6 21.0
25.0 24.0 23.2 21.5 19.1 16.2
100
20.P 20.7
* Isolate numbers are given in parentheses. 7 For whole quartz, calculated as the weighted mean for the different size fractions. ** For whole quartz, measured on samples, excluding >0.5 mm sand grains.
Resistance of quartz silt to isotopic exchange
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26SMtwLW 9396-6424m 24 _
m-m_
16
-Well
Pierre
shale
No. MD2
i I .-----r-
t
Fig. 2. General correspondence of oxygen isotopic ratio to quartz grain size of Cretaceous shales (Pierre) of Montana and South Dakota to two depths of shale in the Gulf of Mexico. ture chemically precipitated quartz (20-W&). All show increasing ??sO with decreasing grain size, as do the Gulf of Mexico samples (Table 2). They have been interpreted as mechanical mixtures with a higher proportion of low-temperature quartz in the finer size fractions. The small variations which are observed in the Gulf of Mexico sediments may be primary effects, reflecting different provenances of sedimentary materials, or they might result from diagenetic processes such as precipitation of small amounts of secondary quartz. There is a remarkable similarity in the pattern of variation of isotopic composition with particle size between the Gulf Coast samples and samples of the Pierre shale (SRIDHAR et aI., 1975). In Fig. 2, RCD2(2438-2465) is seen to be indistinguishable from Pierre sample T159783 (Claggett Shale) collected in Montana, and RCD2(5396-5424) is identical to Pierre sample T259547 (Sharon Springs Shale), collected in South Dakota. The difference between the two Pierre Shale samples has been attributed to paleogeographic factors, the Montana sample having been deposited close to the source of sediments in the west, and the South Dakota sample coming from the central basin of the Pierre sea, far from the source of sediments (CHURCHMAN et al., 1976). The increased influence of low-temperature quartz (chert produced in situ, or quartz derived by long-range eolian transport) for the central basin samples has provided a greater proportion of higher 6’sO quartz in all size fractions. Clusters of fine chert quartz can occur in all size fractions (SAYINand JACKSON,1975). The same kind of paleogeographic effect may have caused the trend toward higher al80 values for deeper samples in the Gulf Coast sediments, since the position of well RCD2 has become progressively closer to the continent as the shoreline migrated southward during Tertiary time. Clay mineral diagenesis takes place at depths of 300&4000 m in the regions sampled (BURST,1969) but there is no evidence in the whole-quartz depth profile O.C.A. 42/10--o
(Fig. 1) to indicate any observable effect in the isotopic composition at that depth. Some evidence of diagenetic effects may be seen in the data for individual size fractions. First, it should be noted that there is a discrepancy of almost 3%, between the wholequartz analysis and the weighted mean of the data on size-fractions for the two shallower samples. This results from retention of the >0.5mm grains in the >2O/rm fraction, and their removal from the ‘wholequartz’ sample. More than half of the quartz in this size fraction consisted of l-2mm sand grains, presumably with #so values -. lOy&,. Excluding the coarse fraction, it can be seen that in any given grainsize interval, there is an increase on the order of l-20/, from the shallowest to the deepest samples, which may be due to the paleogeographic effects described above, or to the addition of secondary authigenic quartz. The al80 value of authigenic quartz depends both on the temperature of precipitation and on the isotopic composition of the water from which it precipitates. The effect of temperature can be estimated from the semi-empirical quartz-water fractionation of BECKERand CLAYTON(1976). The isotopic composition of interstitial water is more difficult to estimate. Measurements of oil-field brines of the Gulf Coast area (mostly Mississippi and East Texas) range from +27&, to +Y/&,, increasing with increasing depth, temperature and salinity (CLAYTONet al., 1966). These waters are predominantly or meteoric origin, and have increased in salinity and 6’*0 values by interaction with sediments at progressively higher temperatures. Figure 3 shows the combinations of temperature and water composition required to produce quartz of various 6’*0 values. It can be seen that for the temperatures in the deeper parts of the wells sampled (lOO--15o”C), and for likely #so values of water at those depths (44&J, secondary quartz would have #*O in the range of 19-297&,.Thus precipitation of some authigenic quartz would lead to a
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R. N. CLAYTON, M. L. JACKSONand K. SRIDHAR
after the HF stripping (which removed 54-65% of the quartz) the difference in isotopic composition between the near-shore and central basin samples remained substantial (1S-2.87&,) when corresponding size-fractions are compared. Less than half of the difference in isotopic compositions of samples from the two p4 locations is attributable to post-depositional effects. That is, even after removal of the apparent secondary p quartz, a difference in the primary detrital minerals remains, presumably due to the paleogeographic factors discussed above. We have not carried out HF-stripping experiments on the Gulf Coast samples, but their close similarity Temp “C to the samples of Pierre Shale suggests that they too have significant differences in primary isotopic comFig. 3. Oxygen isotopic composition of quartz as a funo position, which are modified somewhat (on the order tion of temperature of crystallization and of the isotopic size range) by deposition of composition of water from which it is precipitated. The of I%, in the 1-10~ cross-hatched area covers the range of conditions prevalent secondary quartz. Larger isotopic effects due to in deeper parts of the Gulf Coast wells: temperatures secondary quartz are seen in the data of YEH and 10&15O”C, and 6’EO of water +4 to +87&,. Since the SAVIN(1977) on other Gulf Coast well samples, since temperature and 6’*0 of water are correlated and increase together with depth, the isotopic composition of diageneti- their work deals primarily with the < 2 pm size fraction. Although larger effects are seen in the finest catly precipitated quartz should lie close to the line 6”O (quartz) = 24yW grain sizes, they contribute little to the bulk composition of the sample, due to their low abundance (Table 2). modest increase in the 1a0/‘60 ratios of the quartz While secondary quartz occurs in the deeply buried in each grain-size fraction, with the largest effects Gulf Coast and Pierre shales, the previous observaexpected in the finest size fraction. These observations suggest that modification of the tion of absence of such overgrowths in Pacific pelagic sediments and in Hawaiian soils (CLAYTON et al., isotopic composition of quartz in the l-1Opm dia1972) remains valid. The pelagic sediments were taken meter range due to post-depositional processes has been small but not entirely negligible. One test for from the upper two meters of the sediment where temperatures are too low for diagenetic quartz formaauthigenic overgrowths of quartz is a partial dissolution with hydrofluoric acid, to observe differences in tion to have proceeded significantly. Deeply buried sediments deposited in epicontinental seas or on conisotopic composition between the original sample and the residues, which are expected to consist of the tinental margins, such as the Gulf Coast sediments cores of the original grains. This test, applied to and the Pierre shale, are subject to diagenetic chemiquartz (I-10 pm diameter) from Pacific Ocean pelagic cal and isotopic modification as a result of the combination of higher temperatures and exposure to movsediments showed no measurable amount of authigenie quartz (CLAYTONet al., 1972). However, YEH ing water. The 6’*0 values of the three shallowest samples and SAVIN(1977), in a similar experiment on a sample from well HMSl (Table l), all of Pleistocene age, are of 1-2~ quartz from a Gulf Coast well sample, found evidence for a considerable degree of over- identical to those of quartz in recent pelagic sedithe removed having ments throughout the North Pacific, implying that growth, with quartz 6”O > 30%,. It may be significant that their sample the well samples have undergone little or no diageneis considerably older (Upper Oligocene) than those tic alteration. However, their 6180 values are systematically 1-1.5ym lower than those of samples deeper analyzed in the present study (Pliocene and Pleistocene). The logged temperature for their sample was in the well. Even lower 6’sO values (16.5ym-17.8’&,) were found for quartz (1-10pm) from 11 loessial silt 68”C, which is consistent with the very high 180/160 strata from the several Pleistocene substages in the ratio in the secondary quartz. The same two Pierre Shale quartz samples shown State of Mississippi (JACKSONet al., 1974). It may be in Fig. 2 were also subjected to an HF-stripping ex- that the higher &values for greater depths in the wells periment (SRIDHAR et al., 1975). The l-3.5pm size are more typical of non-glacial periods because glacial fraction showed small changes ( _ -0.87&,), whereas erosion would be expected to introduce a higher prothe larger fractions (3.5-7 and 7-10 pm) showed larger portion of low-‘*0 quartz of the crystalline rocks of changes (-0.8 to -2.6?&,). By material balance the the Precambrian shield in Canada and the northern isotopic compositions of the dissolved quartz was cal- U.S.A. culated to be about 22X for the near-shore sample, Quartz in saproiites and 24”/, for the central basin sample. These values During the weathering of crystalline rocks to saproare very close to the compositions of the finestgrained fractions (Table 2). However, in the residues lites under humid, temperate conditions, quartz is
Resistance of quartz silt to isotopic exchange Table 3. Oxygen isotopic ratios of two size fractions of quartz from saprolites weathered under humid temperate climate from old rocks
20-50ym
Field sample80.
80.
Granitic-gneiss Q-1 Q-2
Weight* %
intnlsive. and south part s925 S926
26.4 26.4
Oli~+~lase-biotice schist, Precambrian, east Avh-l Avh-2 Gneiss, **r-1
s927 9928
26.9 39.2
metamorphosed 5924
10-20Ym 6’80 z.
Uelght* %
61% x.
Pennsylvania”. north-central of Arlanta, GA 9.6 8.5
5931 s923
6.9 10.5
9.7 9.0
highly metamorphosed of Griffin, GA 11.6 10.5
Precambrian,
22.6
NO.
s930 s929 east
13.0
of Forayth,
SO48
13.0 10.8
6.6 17.2
3.6
GA 13.3
* Weight (%) is of quartz in the respective silt fraction after its size fractionation from the gravelly saprolite.
released into sand and finer particles. If there is a significant amount of oxygen isotopic exchange between quartz and the weathering solutions, it should be most evident in the finest grain sizes because of their higher specific surface area. For each of the saprolites studied, two size fractions were analyzed to seek evidence for change in isotopic composition (Table 3). The finer quartz size fractions of four of the sapr6lites had al80 values from 0.1 to OS?&, higher than those of the coarser fractions, suggesting the possibility that slight oxygen exchange had occurred under humid temperate weathering conditions in old landscapes. Quartz overgrowths are not expected in humid climates although they occur commonly in arid climates (FLACH et al., 1969). In the case of Awh-1, derived from oligoclase-biotite schist, the change was 1.4”&, indicating a greater degree of exchange on weathering. With Awh-1, as well as the other four saprolites, there is a possibility of inheritance of small differences in aI80 with grain size in the original rock. However, the amount of isotopic exchange is small enough so as not to preclude the use of oxygen isotope ratios in tracing the provenance of quartz in soils and sediments. CONCLUSIONS (1) The range of variation of 8i80 values of whole quartz from Gulf of Mexico shales is 18.0-20.7y&,.The variations are believed to be predominantly of primary origin, reflecting different particle size distributions and different provenances of detritus, resulting from paleogeographic and glacial/non-glacial climatic changes during the Tertiary and Quatemary. (2) Increases of 6180 with decreasing particle size are very marked (up to 6ym), and are similar to those in many shales, loesses, stream loads and soils. (3) Fine grain-size sub-fractions (in the 1-20~ range) from late Tertiary and younger samples, buried to a depth of several thousand meters in the Gulf Coast sediments, may increase in 6t80 by l-2’&, as
1521
a result of precipitation of authigenic quartz under increased temperature and 6t80 of water. (4) The al80 value of quartz in the lO-20pm size range, occurring in saprolites developed on crystalline igeous and metamorphic rocks in a humid, temperate climate, is greater by only O.lIO.S%, than that in quartz in the 20-50~ range (in 4 of 5 samples analyzed). (5) Because changes in isotopic composition of quartz in the 1-1.0~ size range (typical of longrange aerosol particles) are small, both in the weathering processes and in shallow burial in sediments, quartz appears to be sufficiently stable isotopically to be used as a tracer for provenance of eolian and fluvial additions of minerals to soils and sediments. AcknowledgementsThis research was supported in part by the Enrico Fermi Institute, University of Chicago; in part by the School of Natural Resources, College of Agriculture and Life Sciences, University of Wisconsin, Madison, WI 53706, under projects 1123 and 1336; in part by the Ecological Sciences Branch, Division of Biomedical and Environmental Research, U.S. Department of Energy Contract EY-76-S-02-1515 (paper COO-1515-66); and in part by the National Science Foundation, EAR-7619783Jackson, EAR-74-19038Clayton; through an International Consortium for Interinstitutional Cooperation in the Advancement of Learning (ICICAL). We thank T~~HIKOK. MAYEDAfor assistance with the isotopic analyses.
REFERENCES BECKERR. H. and CLAYTONR. N. (1976) Oxygen isotope study of a Precambrian banded iron-formation, Hammersley Range, Western Australia. Geochint. Cosmochim. Acta 40, 11531165. BURSTJ. F., JR. (1969) Diagenesis of Gulf Coast clayey sediments and its possible relation to petroleum migration. Am. Ass. Petrol. Geologists Bull. 53, 73-93. CHURCHMAN G. J., CLAYTONR. N., SRIDHARK. and JACKSON M. L. (1973) Oxygen isotope abundance in quartz from shales (abstract). EOS Trans. AGU 54, 489. CHURCHMAN G. J., CLAYTONR. N., SRIDHARK. and JACKSONM. L. (1976) Oxygen isotopic composition of aerosol-sized eolian quartz in shales. J. Geophys. Res. 81, 381-386. CLAYTONR. N. and MAYEDAT. K. (1963) The use of bromine pentalIuoride in the extraction of oxygen from oxides and silicates for isotopic analysis. Geochim. Cosmochim. Acta 27, 43-52. CLAYTONR. N., FRIEDMANI., GRAF D. L., MAYEDAT. K., MEENTSW. F. and SHIMP N. F. (1966) The origin of saline formation waters--I. Isotopic compositions. J. Geophys. Res. 71, 3869-3882. CLAYTONR. N., MUFFLERL. J. P. and WHITE D. E. (1968) Oxygen isotope study of calcite and silicates of the River Ranch No. 1 well, Salton Sea geothermal field, California. Am. J. Sci. 266, 968-979. CLAYTONR. N., Rat R. W., SYERSJ. K. and JACKX~N M. L. (1972) Oxygen isotope abundance in quartz from Pacific pelagic sediments. J. Geophys. Res. 77, 3907-391s. ESLINGERE. V. and SAVIN S. M. (1973) Oxygen isotope geothermometry of the burial metamorphic rocks of the Precambrian Belt Supergroup, Glacier National Park, Montana. Geol. Sot. Am. Bull. 84, 2549-2560. FLACH K. W., NE~LETON W. D., GILE L. H. and CADY J. G. (1969) Pedocementation: induration by silica, car-
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bonates and sesquioxides in the Quaternary. Soil Sci. 107.442Xi3. JACKSONM. L. (1975) Soil Chemical Analysis-Advanced Course (second edition). (Published by the author.) JACKSONM. L.. LEVELTT. W. M.. SYERSJ. K.. REX R. W.. CLAYTONR. N.. SHERMANG. D. and UEHARAG. (1971j Geomorphological relationships of tropospherically derived quartz in the soils of the Hawaiian Islands, Soif Sci. Sot. Am. Proc. 35, 515-525. JACKSONM. L., SAY~NM. and CLAYTONR. N. (1974) Quartz isolation for oxygen isotopic abundance by the pyrosulfate-hexafluorosilicic acid method applied to several locsses. Agronomy Abstracts, Am. Sot. Agronomy, Madison, WI 53711. pp. 184-185. KIELY P. V. and JACKSONM. L. (1965) Quartz, feldspar and mica determination for soils by sodium pyrosulfate fusion. Soif Sci. Sot. Am. Proe. 29, 159-163. LAWRENCEJ. R. and TAYLORH. P. (1972) Hydrogen and oxygen isotope systematics in weathering profiles. Geochim. Cosmochim.
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Hawaiian Isknds and in Pacific pelagic sediments. Science 163, 277-279. SAVIN S. M. and EPSTEINS. (1970) The oxygen isotopic compositions of coarse grained sedimentary rocks and minerals. Geochim. Cosmochim. Acta 34, 323-329. SAV~NM. and JACKSONM. L. (1975) Scanning electron microscopy of cherts in relation to the oxygen isotopic variation of soil quartz. Clays Clay Minerals 23. 365-368.
SRIDHARK., JACKSONM. L. and CLAYTONR. N. (197.5) Quartz oxygen isotopic stability in relation to isolation from sediments and diversity of source. Soil Sci. Sot. Am. Proc. 39, 1209-1213.
SYERSJ. K., CHAPMANS. L., JACKSONM. L.. REX R. W. and CLAYTONR. N. (1968) Quartz isolation from rocks, sediments and soils for determination of oxygen isotopic composition. Geochim. Cosm~him. Acta 32. 1022-1025. SYERSJ. K., JACKSONM. L., BERKHE~SER V. E., CLAYTON R. N. and REX R. W. (1969) Eolian sediment influence on pedogenesis during. the Quaternary. Soil Sci. 107,
MOKMAD. L., SYERSJ. K., CLAYTONR. N. and REX R. W. 421-427. (19n) Aeolian addiilbns to soils and sediments in the YEH H.-W. and SAVINS. M. (1977) Mechanism of burial South Pacific area. J. Soil Sci. 23. 147-162. metamorphism of argiflaceous sediments--III. O-isotope Rar R. W., SYERSJ. K., JACKSONM. L. and CLAYTON evidence. GeoL SK Am. Buff. 88, 1321-1330. R. N. (1969) Eotian origin of quartz in soils of the
Geochimica et Cosmochimica Acta. Vol. 42. pp. I7 to I522 0 Pergamon Press Ltd. 1978. Printed in Great Britain
0016-7037/711/1001-I 517SO2.00/0
Resistance of quartz silt to isotopic exchange under burial and intense weathering conditions ROBERT N. CUYTON Enrico Fermi Institute, and Departments of Chemistry and the Geophysical Sciences, University of Chicago, Chicago. IL 60637, U.S.A.
and M. L. JACKSON and K. SRIDHAR* Department of Soil Science, University of Wisconsin, Madison, WI 53706. U.S.A.
(Received 16 May 1977; accepted in revised form 5 June 1978) Abstract-Oxygen isotopic compositions of quartz from silty sediments buried to 5400m depth from two wells in the Gulf of Mexico each showed 6’*0 variations of less than 1.7’&,. Clay diagenesis has been reported within this depth. The observed variations in the quartz do not appear to be primarly diagenetic effects, but rather are mainly depositional features reflecting variations in the sources of the sediments. Sources may be influenced by the variation of distance from the shore at different depths in a given sampling location and by sediment production by continental glaciations. Stability of the oxygen isotopic composition of quartz in the lO-20pm size range under long-time humid, temperate weathering conditions was studied by analysis of saprolites formed from Pennsylvanian to Precambrian crystalline rocks. In four of the five cases, the lO-20pm fraction was found to have only O.l-O.S’& greater 6’*0 than the corresponding 20-50pm fraction. This increase may be attributable either to a slight oxygen isotopic exchange with ambient ground waters or original differences within the rock since the saprolites were sufficiently coherent to make an influx of extraneous detrital silt unlikely. The amount of oxygen isotopic exchange in silt size quartz over periods of many million years of shallow burial or weathering appears to be small enough to permit the use of the oxygen isotopic ratio of quartz in tracing the origin of eohan and fluvial additions of minerals to continental soils and pelagic sediments.
INTRODUCTION
of this paper are to assess the stability of the oxygen isotope ratio of silt-size quartz under conditions of (a) burial at depths up to 5400 meters in marine sediments of the Gulf of Mexico, and (b) intense humid temperate weathering. Quartz is exceedingly resistant to ‘oxygen isotopic exchange with water in geothermal systems at temperatures up to 350°C (CLAYTONet al., 1968), and during pyrosulfate fusion in the laboratory at about 600°C (SYERS et al., 1968; SRIDHAR et al., 1975). Based on the assumption that tine silt-size quartz (l-10 pm) is resistant to isotopic exchange in weathering and sedimentary environments, the oxygen isotopic ratio of quartz in soils and sediments has been used to trace the eolian addition of minerals (REX et al., 1969; SYER~ et al., 1969; JACKSON et a/., 1971; MOKMAet al., 1972). Quartz in sediments of the Precambrian Belt Supergroup has undergone isotopic exchange during low-grade metamorphism upon deep burial (XKtO4WOm) @LINGER and SAWN, 1973). The amount of oxygen isotopic exchange in sandand silt-size quartz of Pliocene-Pleistocene sediments has not been determined, although little post-deposiTHE
OBJE~~VES
* Present address: Sridhar Komameni, Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802, U.S.A.
tional change was inferred for quartz in shales throughout the Phanerozoic (CHURCHMAN et al., 1973, 1976). In this study, we determined the oxygen isotopic composition of quartz from various depths of burial, up to 5400 m, in sediments in the Gulf of Mexico. Quartz added to modem soils and sediments has #*O values intermediate between those of igneous rocks and low-temperature chemical sediments. This has been interpreted as reflecting mixtures of quartz of high- and low-temperature origin (CLAYTONet al., 1972). To verify this, it is necessary to show that hightemperature quartz retains its characteristic low 6**0 value during weathering and disintegration. Millimeter-size quartz grains from beach sands have undergone no significant isotopic change in the weathering process (SAWN and EPSTEIN,1970). Likewise, the
6’aO of whole-rock quartz varies little (< ly&,) through soil profiles developed on crystalline rocks (LAWRENCE and TAYLOR, 1972). EXPERIMENTAL
PROCEDURES
The shale samples were drill-cuttings taken during the boring of two wells in the Gulf of Mexico. The depth assignments were made from the depth of the drill bit at the time of sampling. We studied saprolites formed by the weathering of crystalline rocks to a depth of many meters, namely: a Pennsylvanian granitic gneiss, north-central and south part of
1517
R. N.
1518
CLAYTON.
M. L. JACK~CJN and K.
SRIDHAR
Table 1. Quartz content and oxygen isotopic ratio from shale at various depths of burial, Gulf of Mexico
3009 3418 3865 4299
-
3019 3427 3875 4308
3090 ,644 4522 4957 5396
-
3126 3612 4549 4985 5424
si /
19.4 16.2 22.4 21.6 20.4 ?8.6
5833 5834 S835 5836 S837 5838
20.1
I
19.4 19.2 19.* 19.1 19.4
5
20.7 19.7-0.6 19.210.8
Atianta, GA; a Precambrian hornblende gneiss, just east of Forsyth. GA: a Precambrian oligoclase-biotite schist. 8 km east of Griffin, GA. The interiors of boulders were sampled to avoid possible admixture of transported fine particles. The time of exposure to weathering is not known, but the great thickness of the saprolite attests to the long action of ground waters. perhaps on the order of 50 Myr or more. The study of saprolites concentrated on the finest grains present, since these contribute appreciably to the eolian dust. Samples from the I@-20pm and 20-50,um ranges were analyzed. Negligible amounts of quartz with diameter i IOpm were found. Quartz was isolated from the samples by 6N HCI treatment, Na*S,O, fusion (KIELY and JACKSON.1965). treatment with H$iF, at 18 + 1°C to remove amorphous material and feldspars (SRIDHARet ai., 1975) and washings with 0.1 N HF, HsBOs. and H,O to remove fluorides (JACKSONet al.. 1974). Particle size fractionation of quartz isolates was determined by centrifugation, sedimentation and decantation (JACKSON.1975) before the samples were dried. Oxygen isotopic compositions were determined by analytical procedures described by CLAYTONand MAYEDA (1963). Isotopic analyses were made with a 15-cm 6CY-sector double~oll~ting mass spectrometer and are reported in the usual 6 notation as per mil deviations from Standard Mean Ocean Water (SNOW).
,
i
,\,
I8
20
SO'8 rel. SMOW Fig. 1. Whole quartz oxygen isotopic ratio (&‘*O) from two drill holes in the Gulf of Mexico (0) = hole HMSl; (0) = hole RCD 2. RESULTS AND DISCUSSION
Gurf of Mexico sediments The &I80 values of whole-quartz (excluding the >O.S mm particles which were assumed to be extraneous to the shales) from the Gulf of Mexico shales vary by less than 1.79&,within each of two Pleistocene-Upper Pliocene wells (Table 1 and Fig. 1). In general, the isotopic compo~tions resemble those of quartz in shales from mid-continental U.S.A. (CHURCHMANet al., 1976), from North Pacific pelagic sediments (CILAYTONet al., 1972), and from many Northern Hemisphere soils, dusts and stream loads (SYERS er al., 1969; REX ef al., 1969; JACKSON et al., 1971). All of these have 6180 values lying between those typical of quartz from igenous and metamorphic rocks (S-1433, and those found for low-tempera-
Table 2. Quartz contents (“/,) and oxygen isotopic ratios (S”O) in various grain size fractions at four burial depths in ICICAL Well RCD 2 Depth interval in meters
Size fraction,
Pm 20 Total, mean whole**
2438-2465 (S891-5a)*
3090-3126 (S896-900a)
4522-4549 (S901"5.a)
Weight 6l*O
Weight 6'*0
Weight E'S0
%
%,
%
%.
3.8 7.7 18.4 14.4 16.2 39.5
21.7 21.7 20.9 19.7 18.0 13.2
3.6 6.9 7.4 12.7 15.5 53.9
23.1 21.9 20.8 19.6 17.5 13.7
17.3t 20.1
100
16.5t 19.2
100
%
%,
4.0 13.3 7.1 27.4 25.2 23.0
23.0 22.9 22.0 21.1 18.9 lfi?2_
100
19.m 19.1
X396-5424 (S906-10a) "eight 6'b
%
%.
5.2 5.0 20.8 23.4 24.6 21.0
25.0 24.0 23.2 21.5 19.1 16.2
100
20.P 20.7
* Isolate numbers are given in parentheses. 7 For whole quartz, calculated as the weighted mean for the different size fractions. ** For whole quartz, measured on samples, excluding >0.5 mm sand grains.
Resistance of quartz silt to isotopic exchange
1519
26SMtwLW 9396-6424m 24 _
m-m_
16
-Well
Pierre
shale
No. MD2
i I .-----r-
t
Fig. 2. General correspondence of oxygen isotopic ratio to quartz grain size of Cretaceous shales (Pierre) of Montana and South Dakota to two depths of shale in the Gulf of Mexico. ture chemically precipitated quartz (20-W&). All show increasing ??sO with decreasing grain size, as do the Gulf of Mexico samples (Table 2). They have been interpreted as mechanical mixtures with a higher proportion of low-temperature quartz in the finer size fractions. The small variations which are observed in the Gulf of Mexico sediments may be primary effects, reflecting different provenances of sedimentary materials, or they might result from diagenetic processes such as precipitation of small amounts of secondary quartz. There is a remarkable similarity in the pattern of variation of isotopic composition with particle size between the Gulf Coast samples and samples of the Pierre shale (SRIDHAR et aI., 1975). In Fig. 2, RCD2(2438-2465) is seen to be indistinguishable from Pierre sample T159783 (Claggett Shale) collected in Montana, and RCD2(5396-5424) is identical to Pierre sample T259547 (Sharon Springs Shale), collected in South Dakota. The difference between the two Pierre Shale samples has been attributed to paleogeographic factors, the Montana sample having been deposited close to the source of sediments in the west, and the South Dakota sample coming from the central basin of the Pierre sea, far from the source of sediments (CHURCHMAN et al., 1976). The increased influence of low-temperature quartz (chert produced in situ, or quartz derived by long-range eolian transport) for the central basin samples has provided a greater proportion of higher 6’sO quartz in all size fractions. Clusters of fine chert quartz can occur in all size fractions (SAYINand JACKSON,1975). The same kind of paleogeographic effect may have caused the trend toward higher al80 values for deeper samples in the Gulf Coast sediments, since the position of well RCD2 has become progressively closer to the continent as the shoreline migrated southward during Tertiary time. Clay mineral diagenesis takes place at depths of 300&4000 m in the regions sampled (BURST,1969) but there is no evidence in the whole-quartz depth profile O.C.A. 42/10--o
(Fig. 1) to indicate any observable effect in the isotopic composition at that depth. Some evidence of diagenetic effects may be seen in the data for individual size fractions. First, it should be noted that there is a discrepancy of almost 3%, between the wholequartz analysis and the weighted mean of the data on size-fractions for the two shallower samples. This results from retention of the >0.5mm grains in the >2O/rm fraction, and their removal from the ‘wholequartz’ sample. More than half of the quartz in this size fraction consisted of l-2mm sand grains, presumably with #so values -. lOy&,. Excluding the coarse fraction, it can be seen that in any given grainsize interval, there is an increase on the order of l-20/, from the shallowest to the deepest samples, which may be due to the paleogeographic effects described above, or to the addition of secondary authigenic quartz. The al80 value of authigenic quartz depends both on the temperature of precipitation and on the isotopic composition of the water from which it precipitates. The effect of temperature can be estimated from the semi-empirical quartz-water fractionation of BECKERand CLAYTON(1976). The isotopic composition of interstitial water is more difficult to estimate. Measurements of oil-field brines of the Gulf Coast area (mostly Mississippi and East Texas) range from +27&, to +Y/&,, increasing with increasing depth, temperature and salinity (CLAYTONet al., 1966). These waters are predominantly or meteoric origin, and have increased in salinity and 6’*0 values by interaction with sediments at progressively higher temperatures. Figure 3 shows the combinations of temperature and water composition required to produce quartz of various 6’*0 values. It can be seen that for the temperatures in the deeper parts of the wells sampled (lOO--15o”C), and for likely #so values of water at those depths (44&J, secondary quartz would have #*O in the range of 19-297&,.Thus precipitation of some authigenic quartz would lead to a
1520
R. N. CLAYTON, M. L. JACKSONand K. SRIDHAR
after the HF stripping (which removed 54-65% of the quartz) the difference in isotopic composition between the near-shore and central basin samples remained substantial (1S-2.87&,) when corresponding size-fractions are compared. Less than half of the difference in isotopic compositions of samples from the two p4 locations is attributable to post-depositional effects. That is, even after removal of the apparent secondary p quartz, a difference in the primary detrital minerals remains, presumably due to the paleogeographic factors discussed above. We have not carried out HF-stripping experiments on the Gulf Coast samples, but their close similarity Temp “C to the samples of Pierre Shale suggests that they too have significant differences in primary isotopic comFig. 3. Oxygen isotopic composition of quartz as a funo position, which are modified somewhat (on the order tion of temperature of crystallization and of the isotopic size range) by deposition of composition of water from which it is precipitated. The of I%, in the 1-10~ cross-hatched area covers the range of conditions prevalent secondary quartz. Larger isotopic effects due to in deeper parts of the Gulf Coast wells: temperatures secondary quartz are seen in the data of YEH and 10&15O”C, and 6’EO of water +4 to +87&,. Since the SAVIN(1977) on other Gulf Coast well samples, since temperature and 6’*0 of water are correlated and increase together with depth, the isotopic composition of diageneti- their work deals primarily with the < 2 pm size fraction. Although larger effects are seen in the finest catly precipitated quartz should lie close to the line 6”O (quartz) = 24yW grain sizes, they contribute little to the bulk composition of the sample, due to their low abundance (Table 2). modest increase in the 1a0/‘60 ratios of the quartz While secondary quartz occurs in the deeply buried in each grain-size fraction, with the largest effects Gulf Coast and Pierre shales, the previous observaexpected in the finest size fraction. These observations suggest that modification of the tion of absence of such overgrowths in Pacific pelagic sediments and in Hawaiian soils (CLAYTON et al., isotopic composition of quartz in the l-1Opm dia1972) remains valid. The pelagic sediments were taken meter range due to post-depositional processes has been small but not entirely negligible. One test for from the upper two meters of the sediment where temperatures are too low for diagenetic quartz formaauthigenic overgrowths of quartz is a partial dissolution with hydrofluoric acid, to observe differences in tion to have proceeded significantly. Deeply buried sediments deposited in epicontinental seas or on conisotopic composition between the original sample and the residues, which are expected to consist of the tinental margins, such as the Gulf Coast sediments cores of the original grains. This test, applied to and the Pierre shale, are subject to diagenetic chemiquartz (I-10 pm diameter) from Pacific Ocean pelagic cal and isotopic modification as a result of the combination of higher temperatures and exposure to movsediments showed no measurable amount of authigenie quartz (CLAYTONet al., 1972). However, YEH ing water. The 6’*0 values of the three shallowest samples and SAVIN(1977), in a similar experiment on a sample from well HMSl (Table l), all of Pleistocene age, are of 1-2~ quartz from a Gulf Coast well sample, found evidence for a considerable degree of over- identical to those of quartz in recent pelagic sedithe removed having ments throughout the North Pacific, implying that growth, with quartz 6”O > 30%,. It may be significant that their sample the well samples have undergone little or no diageneis considerably older (Upper Oligocene) than those tic alteration. However, their 6180 values are systematically 1-1.5ym lower than those of samples deeper analyzed in the present study (Pliocene and Pleistocene). The logged temperature for their sample was in the well. Even lower 6’sO values (16.5ym-17.8’&,) were found for quartz (1-10pm) from 11 loessial silt 68”C, which is consistent with the very high 180/160 strata from the several Pleistocene substages in the ratio in the secondary quartz. The same two Pierre Shale quartz samples shown State of Mississippi (JACKSONet al., 1974). It may be in Fig. 2 were also subjected to an HF-stripping ex- that the higher &values for greater depths in the wells periment (SRIDHAR et al., 1975). The l-3.5pm size are more typical of non-glacial periods because glacial fraction showed small changes ( _ -0.87&,), whereas erosion would be expected to introduce a higher prothe larger fractions (3.5-7 and 7-10 pm) showed larger portion of low-‘*0 quartz of the crystalline rocks of changes (-0.8 to -2.6?&,). By material balance the the Precambrian shield in Canada and the northern isotopic compositions of the dissolved quartz was cal- U.S.A. culated to be about 22X for the near-shore sample, Quartz in saproiites and 24”/, for the central basin sample. These values During the weathering of crystalline rocks to saproare very close to the compositions of the finestgrained fractions (Table 2). However, in the residues lites under humid, temperate conditions, quartz is
Resistance of quartz silt to isotopic exchange Table 3. Oxygen isotopic ratios of two size fractions of quartz from saprolites weathered under humid temperate climate from old rocks
20-50ym
Field sample80.
80.
Granitic-gneiss Q-1 Q-2
Weight* %
intnlsive. and south part s925 S926
26.4 26.4
Oli~+~lase-biotice schist, Precambrian, east Avh-l Avh-2 Gneiss, **r-1
s927 9928
26.9 39.2
metamorphosed 5924
10-20Ym 6’80 z.
Uelght* %
61% x.
Pennsylvania”. north-central of Arlanta, GA 9.6 8.5
5931 s923
6.9 10.5
9.7 9.0
highly metamorphosed of Griffin, GA 11.6 10.5
Precambrian,
22.6
NO.
s930 s929 east
13.0
of Forayth,
SO48
13.0 10.8
6.6 17.2
3.6
GA 13.3
* Weight (%) is of quartz in the respective silt fraction after its size fractionation from the gravelly saprolite.
released into sand and finer particles. If there is a significant amount of oxygen isotopic exchange between quartz and the weathering solutions, it should be most evident in the finest grain sizes because of their higher specific surface area. For each of the saprolites studied, two size fractions were analyzed to seek evidence for change in isotopic composition (Table 3). The finer quartz size fractions of four of the sapr6lites had al80 values from 0.1 to OS?&, higher than those of the coarser fractions, suggesting the possibility that slight oxygen exchange had occurred under humid temperate weathering conditions in old landscapes. Quartz overgrowths are not expected in humid climates although they occur commonly in arid climates (FLACH et al., 1969). In the case of Awh-1, derived from oligoclase-biotite schist, the change was 1.4”&, indicating a greater degree of exchange on weathering. With Awh-1, as well as the other four saprolites, there is a possibility of inheritance of small differences in aI80 with grain size in the original rock. However, the amount of isotopic exchange is small enough so as not to preclude the use of oxygen isotope ratios in tracing the provenance of quartz in soils and sediments. CONCLUSIONS (1) The range of variation of 8i80 values of whole quartz from Gulf of Mexico shales is 18.0-20.7y&,.The variations are believed to be predominantly of primary origin, reflecting different particle size distributions and different provenances of detritus, resulting from paleogeographic and glacial/non-glacial climatic changes during the Tertiary and Quatemary. (2) Increases of 6180 with decreasing particle size are very marked (up to 6ym), and are similar to those in many shales, loesses, stream loads and soils. (3) Fine grain-size sub-fractions (in the 1-20~ range) from late Tertiary and younger samples, buried to a depth of several thousand meters in the Gulf Coast sediments, may increase in 6t80 by l-2’&, as
1521
a result of precipitation of authigenic quartz under increased temperature and 6t80 of water. (4) The al80 value of quartz in the lO-20pm size range, occurring in saprolites developed on crystalline igeous and metamorphic rocks in a humid, temperate climate, is greater by only O.lIO.S%, than that in quartz in the 20-50~ range (in 4 of 5 samples analyzed). (5) Because changes in isotopic composition of quartz in the 1-1.0~ size range (typical of longrange aerosol particles) are small, both in the weathering processes and in shallow burial in sediments, quartz appears to be sufficiently stable isotopically to be used as a tracer for provenance of eolian and fluvial additions of minerals to soils and sediments. AcknowledgementsThis research was supported in part by the Enrico Fermi Institute, University of Chicago; in part by the School of Natural Resources, College of Agriculture and Life Sciences, University of Wisconsin, Madison, WI 53706, under projects 1123 and 1336; in part by the Ecological Sciences Branch, Division of Biomedical and Environmental Research, U.S. Department of Energy Contract EY-76-S-02-1515 (paper COO-1515-66); and in part by the National Science Foundation, EAR-7619783Jackson, EAR-74-19038Clayton; through an International Consortium for Interinstitutional Cooperation in the Advancement of Learning (ICICAL). We thank T~~HIKOK. MAYEDAfor assistance with the isotopic analyses.
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MOKMAD. L., SYERSJ. K., CLAYTONR. N. and REX R. W. 421-427. (19n) Aeolian addiilbns to soils and sediments in the YEH H.-W. and SAVINS. M. (1977) Mechanism of burial South Pacific area. J. Soil Sci. 23. 147-162. metamorphism of argiflaceous sediments--III. O-isotope Rar R. W., SYERSJ. K., JACKSONM. L. and CLAYTON evidence. GeoL SK Am. Buff. 88, 1321-1330. R. N. (1969) Eotian origin of quartz in soils of the