Boron isotope geochemistry during diagenesis. Part II. Applications to organic-richsediments
LYNDA B. WILLIAMS ,1 RICHARD L. HERVIG,1 and IAN HUTCHEON2
1Center for Solid State Science and Department of Geology, Arizona State University, Tempe, Arizona 84287-1404, USA2Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada
(Received March20, 2000;accepted in revised form December29, 2000)
Abstract—The measured clay-water isotope fractionation for boron was applied to natural organic-richsediments undergoing illitization. Two field areas were chosen that show illitization occurring over a range oftemperatures (80–500°C). Samples representing diagenetic temperatures of illitization (80–200°C) are fromthe Gulf of Mexico sedimentary basin at 4 to 6-km depth in the Eocene Wilcox Fm and Jurassic Norphlet Fm.The higher temperatures of illitization (200–500°C) occur in a contact metamorphic aureole of the CretaceousPierre shale near Walsenburg, Colorado. Here the kinetics of the illitization reaction are more rapid than ina slowly subsiding sedimentary basin, but the chemical and mineralogical variations are minimized ascomplete illitization occurs over a small lateral distance in a single bentonite layer. These studies indicate thatB-isotopes provide a more sensitive indicator of fluid variations in sedimentary basins thanO-isotopes, andthat B-isotope analyses of authigenic illite can be a valuable geochemical tracer of fluid/rock interactions.
Boron isotope ratios in authigenic illite (pore filling) and muscovite (stylolites) from reservoir sandstonesin the Gulf of Mexico are distinct from adjacent illitic mudstones, whereas the oxygen isotopic ratios showlittle variation. Fluids in equilibrium with the mudstones cannot precipitate the authigenic clays with higherd11B values measured in the hydrocarbon reservoirs. This suggests that the reservoir fluids were not incommunication with the adjacent mudstone pore fluids but were introduced from another source area, perhapscarrying a B-isotopic label derived from the hydrocarbon source region.
Boron is enriched in shales and marine sediments (.100ppm) (Goldschmidt and Peters, 1932) because of preferentialuptake of B by clay minerals. Recent studies of B-isotopesystematics related to subduction of marine sediments showsthat fluids derived from the subducted sediment have lightd11Bvalues compared with surficial waters (Bebout et al., 1993; Youet al., 1995; Peacock and Hervig, 1999). The large differencesin d11B of fluids have made B-isotopes useful as an indicator offluid sources. Although B-isotopes have been explored on atectonic scale in subduction zones, we believe that they can beuseful for monitoring fluid flow in sedimentary basins as wellbecause B is highly mobile in the aqueous phase (Levinson,1980).
Studies of B in surficial sedimentary environments havefocused primarily on adsorbed-B on clay surfaces (Schwarcz etal., 1969; Brumsack and Zulegar, 1992). Adsorption studiesshow a greater affinity of B for smectite and illite than for otherclay minerals (Hingston, 1964; Couch and Grim, 1968; Kerenand Mezuman, 1981). The enrichment of B in clay mineralsdoes not occur during weathering of igneous rocks nor duringexposure to seawater (Spivack et al., 1987). Clay-rich marinesediments (,4-mm fraction) have an average adsorbed-B con-tent ,20 ppm with d11B near 115‰, but the adsorbed-Baccounts for only'10 to 20% of the total-B in shales (Spivacket al., 1987). As temperatures approach 120°C, B-adsorption
becomes negligible (You et al., 1995). During diagenesis, sub-stitution of B for Si occurs as smectite reacts to illite (Perry,1972). This tetrahedrally substituted-B in clay minerals ac-counts for the greater portion of B in marine sediments (.100ppm B), and it has an averaged11B of 25‰ (Palmer et al.,1987). As clay minerals concentrate10B during illitization thepore fluids should become progressively11B-enriched, unlessthere is an additional source of B from hydrothermal fluids,organic material, or breakdown of other detrital minerals withinthe sedimentary basin.
Many oilfields contain B-enriched brines associated withhydrocarbon accumulations (Sivan, 1972; Collins, 1975; Ven-gosh et al., 1994; Moldovanyi et al., 1994). Some organicmatter contains several hundred ppm B (e.g., Gulyayeva et al.,1966; Goodarzi and Swain, 1994), which is a potential sourceof B in sedimentary basins (Gulyayeva et al., 1966; Williams etal., 1997, Williams et al., 2001). Because metasedimentarygraphite contains negligible amounts of B (Douthitt, 1985), wededuce that B is released from organic matter during thermalmaturation, similar to the release of H and O as organiccompounds thermally degrade (Williams et al., 2001). Boron isnot adsorbed by clay minerals at common temperatures ofhydrocarbon generation (.120°C) (You et al., 1995), but par-titions preferentially into the aqueous phase and may be trappedalong with hydrocarbons in sandstone reservoirs.
Combining our knowledge of the B-isotope fractionation in
illite/smectite (Williams et al., Part I) withO-isotope fraction-ation (Savin and Lee, 1988), the two isotopic systems can beapplied in tandem to evaluate the interaction of migrating fluidswith illitic sediments undergoing burial diagenesis. The neo-formation of illite at depths common to hydrocarbon generationmake it a potential tracer of fluids related to organic maturation.Thus, authigenic illite may record a distinctive B-isotope ratiorelated to the presence of hydrocarbons in potential reservoirs,and could be used to identify hydrocarbon migration paths.
In this article we have applied the B-isotope fractionationresults from hydrothermal experiments (Williams et al., Part I,2001), to a variety of natural geologic samples displayingillitization over a wide range of temperatures. Measurements ofd11B andd18O were used to evaluate the mechanism and timingof isotope exchange. Knowledge of the fractionation factors forB-isotopes at a known temperature allows us to predict themagnitude of B-isotope variations in illite/smectite and to eval-uate variations in the formation fluid chemistry. A comparisonof predicted and observed B-isotope ratios in natural illitesupports the experimental measurements and utility of B-iso-topes as a geothermometer.
2. SAMPLES
Samples were collected from two distinctly different geo-logic environments to test the temperature dependence of theB-isotope fractionation during illitization of smectite. We stud-ied a slowly subsiding sedimentary basin and a contact-meta-morphosed shale, each displaying illitization of smectite underdifferent temperature regimes.
The Gulf of Mexico sedimentary basin shows a classicillitization pattern extending from 20% illite in I/S to 80% illitein I/S (Hower et al., 1976) over burial depths from 1 to 5 km.Illitization of smectite occurs over 50 to 55 Ma at 80 to 200°C.Our samples were collected from south central Louisiana corespenetrating the Wilcox Fm at'4 km ('120°C) where illitiza-tion is at'70% (Williams et al., 1995) and at'6.7 km in theNorphlet Fm presently at'200°C. The burial history of thebasin shows a linear trend over time (Boles and Franks, 1979)with a constant thermal gradient approximating 30°C/km basedon corrected bottom-hole temperatures (Williams et al., 1995).
Meta-bentonite samples were collected from an organic-richportion of the Cretaceous Pierre shale near Walsenburg, Colo-rado, where it is intruded normal to bedding by a compositelamprophyre dike of Eocene–Oligocene age (Johnson, 1964).The Pierre shale samples show complete illitization of smectitein a single bentonite layer over a distance of 25 m in,100 yr(Pytte, 1982) at 200 to 500°C (Bostick and Pawlewicz, 1984).
3. ANALYTIC METHODS
3.1. Mineralogy
Bulk powder X-ray diffraction (XRD) was used to determine themineralogy of the sediment. The,2-mm size fraction of the sedimentwas analyzed to identify the clay minerals (Moore and Reynolds, 1997)and degree of illitization of mixed-layered illite/smectite (I/S).
3.2. Isotope Geochemistry
Boron isotope analyses and B-content of the clay fraction weredetermined by secondary ion mass spectrometry (SIMS). Adsorbed-Bwas removed from these samples by multiple washings (five or more)
in deionized water to remove the marine pore fluid salts and allow claydisaggregation. Samples were then washed in mannitol, a B-complex-ing agent that helps to remove adsorbed-B contaminants (Hingston,1964). A concentrated slurry of the clay fraction was dropped on a1-inch round, B-free glass slide. After drying at 60°C, the samples weregold coated for charge compensation during SIMS analysis.
We used a Cameca IMS 3f SIMS equipped with a standard duo-plasmatron for generating a primary beam of O2 ions. To calibrate theSIMS for measuring the B-isotopic compositions of clay minerals, wefirst conducted tests to evaluate the matrix effect of the mineral on theinstrumental mass fractionation. We used two natural samples that hadbeen analyzed previously (You et al., 1995) by thermal ionization massspectrometry (TIMS). The samples were mineralogically similar to thebentonite, comprised of mainly illite/smectite with minor kaolinite andchlorite. The initial sample had a bulkd11B of 25‰. An aliquot washeated at 350°C and 80 MPa for 2 months producing complete recrys-tallization of the I/S to illite (You et al., 1995) and a change ind11B to210‰. SIMS analyses of the two samples also gave11B/10B ratios thatchanged by the same magnitude (25‰), indicating that the instrumentcalibration is independent of mineral matrix changes due to recrystal-lization. Other measurements of clay mineral repository standards ofillite (IMt-1) and smectite (SWy-1) were made by both SIMS andTIMS methods (Williams, 2000) showing the same instrumental frac-tionation for each phase. Chaussidon et al. (1997) also found matrixeffects for analyses of B-isotopes in a variety of minerals to beunnoticeable. Further details of the analytical technique can be found inHervig (1996) and Chaussidon et al. (1997). The instrumental massfractionation (IMF) is dependent on the analytical parameters of theinstrument (primary voltage, mass resolution, and detector) and re-mains constant (within error) throughout an analysis session. There-fore, it can be used to correct the measured isotope ratio to give anaccurate value of the nominal mineral isotope ratio. The isotope ratiosare reported as delta values (d) relative to boric acid standard NBSSRM 951 (11B/10B 5 4.0437).
The IMF is measured on the standard before each analytical session andis rechecked after each sample. Precision ind11B was 1 to 2‰ for eachanalysis, and each sample was analyzed.10 times, producing analyt-ical errors, 1‰.
The B-content was determined from a calibration curve constructedfrom SIMS measurements of B-glass standards prepared with variousB-concentrations. By measuring the ratio of11B1 to 30Si1 (bothmeasured with an electron multiplier) this curve can be used to deter-mine the B-content of the sample if the SiO2 content is known.Precision in measurement of B-concentration was,5%. Measurementsof B-content of clay minerals standard SWy-1 and IMt-1 by SIMScompared within 1% to the B concentration measured on digestedaliquots of the clay minerals by ICP-AES.
3.2. Organic Geochemistry
Vitrinite reflectance of organic matter and bottom-well temperatureswere used to establish the temperature of samples in the Wilcox Fmsamples (Williams et al., 1995) and Norphlet Fm (Thomas et al., 1993).Total organic carbon contents of all samples were determined by usinga CHN Elemental Analyzer (Perkin–Elmer Model 240C) on bulk rockpowders (Williams and Ferrell, 1991; Williams et al., 1995). Rock–Eval pyrolysis was used to determine the organic maturity of sedimentsin the Wilcox and Norphlet Fms (Williams et al., 1992; Thomas et al.,1993).
The vitrinite reflectance data of Bostick and Pawlewicz (1984) wereused to establish the temperatures across the contact aureole in thePierre shalemeta-bentonite. The contact metamorphic temperatureswere also modeled by using Jaeger thermal models based on thecomposition and thickness of the intrusive (Pytte, 1982). These datashowed approximately 50°C higher temperatures than indicated by thevitrinite reflectance. Rock–Eval pyrolysis was also performed as anindication of organic maturity (Williams and Ferrell, 1991).
1784 L. B. Williams et al.
4. BORON ISOTOPES IN THE GULF COASTSEDIMENTARY BASIN
4.1. Background
The use of isotopes as a geothermometer requires analyses of coex-isting minerals containing the same isotope. Precipitation temperaturescan be calculated if the temperature-dependent isotopic fractionationfor the minerals is significantly different and if they have coprecipi-tated. This technique is not simple to use in a diagenetic environmentwhere coprecipitation and equilibrium conditions are difficult to deter-mine, and most authigenic phases have fractionation curves with sim-ilar slopes (Kyser, 1987).
It may be useful, however, to use two isotopic systems, i.e., B and O,that are geochemically linked by the prograde mineral reaction ofsmectite to illite to predict equilibrium conditions. The fractionationcurves for the two different isotopic systems have significantly differ-ent slopes (Fig. 1) (Williams et al., Part I). In a closed system domi-nated by I/S (common in mudstones in many sedimentary basins), thereshould be a unique temperature and water composition that satisfies theequilibrium of both B andO-isotopes in the I/S. If other mineralreactions compete to influence eitherO- or B-isotopes in the fluids, thena unique temperature cannot be interpreted. However, mudstones in theGulf Coast have shown a remarkably closed system behavior withrespect toO-isotopes (Yeh and Savin, 1977) and thus provide a usefultest for the B-isotope behavior in a natural setting. The following is anexample of the integrated use of B andO-isotopes for evaluating thetiming and conditions of illitization in sediments from the Gulf ofMexico sedimentary basin.
5. RESULTS
5.1. Mineralogy
The Eocene Wilcox Fm is dominantly a thick deltaic mud-stone that encloses several sandstone lenses that contain hy-drocarbons. XRD results from whole rock powders indicatethat the Wilcox mudstone is 65% quartz, 25% clay, 5% feld-spar, and 5% carbonate. The,2-mm clay fraction consistsprimarily of mixed-layered illite/smectite (I/S), with minoramounts of kaolinite and chlorite. At the depth of the Wilcoxreservoirs studied, all of the mixed-layered I/S is'70% illite.Three stacked sandstone reservoirs were examined that areconnected by a normal growth fault thought to be a conduit forhydrocarbon migration into the reservoirs (Williams et al.,1995). The Wilcox sandstones consist of 75 to 80% quartz,
with 5 to 10% feldspar and carbonate, and pore-filling clayminerals comprising,10% of the reservoir rock (Williams etal., 1995).
The Norphlet sandstone is a subarkosic dune sand underlyingthe Jurassic Smackover carbonates. Thomas et al. (1993) de-scribed three muscovite morphologies within this sandstone.Large stylolites are comprised of 1 Md muscovite. In addition,muscovite laths occur as pore filling, and muscovite pods arefound in the insoluble residue intermixed with kerogen. Agedating (Thomas et al., 1993) has shown the three morpholog-ically distinct groups of muscovite to be formed at differenttimes during burial.
5.2. Boron and Oxygen
Table 1 shows the results of B- andO-isotope measurementsmade on clay minerals from the three stacked sandstone reser-voirs in the Wilcox Fm and the mudstone above and belowthese reservoirs. Data for the Norphlet Fm sandstone, presentlyat a depth of 6.7 km, is shown for comparison. B-isotopeanalyses are presented for the three morphologically distinctmuscovites found in the Norphlet gas reservoir.
5.3. Formation Waters
Formation waters were collected from the three stackedhydrocarbon reservoirs in the Wilcox Fm. The temperatures ofthe sandstones range from'90 to 125°C. Thed11B value of theformation water determined by thermal ionization mass spec-trometry ranges from128 to136‰ and the B-content rangesfrom 20 to 80 ppm (Williams et al., 2001). The isotopic ratiosfit the regional trend of formation waters from Cenozoic res-ervoirs in the Gulf Coast basin (Land and Macpherson, 1992),which show a general increase in B-content and decrease ind11B with depth (Fig. 2).The regional trend also indicates an
Fig. 1. B-isotope fractionation curve derived from experimental data(Williams et al., Part 1) compared withO-isotope fractionation (Yehand Savin; 1977) for I/S.
Table 1. Results of SIMS analyses on authigenic I/S and muscovitein sandstones and mudstones from the Eocene Wilcox Fm and JurassicNorphlet Fm.
n 5 number of analyses. Each analysis is comprised of 49 isotoperatio measurements.
1785Boron geochemistry during diagenesis: Application
increase inO-isotope composition of the formation waters withdepth.
Information on the fluid chemistry of mudstones is difficultto obtain because of difficulty in extracting water from aqui-cludes. However, theO-isotopic composition of waters in equi-librium with I/S in Gulf Coast mudstones have been modeledon the basis of the degree of illitization and thermal gradient(Suchecki and Land (1983). These calculations indicate that theO-isotopic composition of water in equilibrium with I/S(.70% illite) at 4 km, under a geothermal gradient of 30°C/km,should be approximately18 ‰. By combining the informationon B andO-isotopes in the mudstone and using the knowledgeof isotope fractionation expected in a closed system, it ispossible to deduce the isotopic composition of pore fluids inequilibrium with the mudstone.
6. DISCUSSION
6.1. Wilcox Fm
6.1.1. Mudstones
According to Yeh and Savin (1977), the change ind18O ofI/S within mudstones of the Eocene Wilcox Fm is nearly linearover a depth range of 2.5 to 5 km (68–155°C). It is believedthat the mudstones represent a closed system with respect tofluids and that complete isotopic exchange resulted during theillitization of smectite (Savin and Lee, 1988). Bloch et al.(1998) suggested that volcanic ash is a significant source of thedetritus in the Gulf Coast basin. Therefore, the smectite wasprobably of volcanic origin, with ad18O representing the vol-canic source. The pore fluid was initially seawater with ad18Oof '0‰.
Coinciding with the oxygen trend, there is an increase in thefixed-B content of I/S as temperatures exceed 60°C (Perry,1972). The B increases from 100 to 200 ppm over depths from1.5 to 6 km. Smectite from altered volcanic ash or weatheredcontinental rocks would be expected to have an initial B-isotopic composition around 0‰ (Leeman and Sisson, 1996;Palmer and Swihart, 1996). Marine pore fluids generally havean initial d11B near139.5‰ (seawater), with some variationsdue to preferential adsorption of10B onto clay surfaces. It isimportant to realize, however, that the detrital minerals andfluid are not in isotopic equilibrium at surface temperatures;therefore, the isotopic fractionation curve cannot be applied todetrital marine sediments. However, if one assumes that B-isotopic changes (toward equilibrium) accompany the observedincrease in fixed-B with burial and that the B-isotopic compo-sition of the I/S is linked to isotopic changes in oxygen, then thetwo isotopic systems could be used as a predictive tool.
Measurements by Yeh and Savin (1977) on illitic clays fromthe Gulf Coast basin showed isotope ratios of 20‰ at 4 km.The SIMS measurements on Wilcox mudstones gave values of23 6 1.7‰ (1s), which is within error of the conventionalanalyses. By using the estimated18‰ d18O for the diageneti-cally altered formation water in the mudstones at 4 km(Sucheki and Land, 1983) and a value of 20‰ for thed18O ofthe mudstone, theDmineral-waterfor oxygen (112‰) reflects atemperature of 116°C (Fig. 1). This interpreted temperature iswithin the range of bottom-hole temperatures measured in thecores examined (Williams et al., 1995), indicating that theO-isotopes are near equilibrium.
By applying the new B-isotope fractionation equation, aDmineral-waterfor boron of 223.5‰ is expected at 116°C. Byusing the measurements ofd11B on the illitic mudstone(218.8 6 0.8‰) we would predict that pore fluid in equilib-rium with this mudstone should have ad11B of '15‰. Thisvalue is lower than reported measurements of formation watersfrom the Gulf Coast (Fig. 2) showing a minimumd11B of112‰ (Macpherson and Land, 1989; Land and Macpherson,1992; Moldovanyi et al., 1994), but those measurements wereonly made on sandstone reservoir waters, not mudstones. Thepredictedd11B for mudstone water is not unreasonable in thecontext of waters associated with sedimentary rocks. Palmerand Sturchio (1990) reportedd11B values for continental hy-drothermal waters as low as28‰.
Fig. 2. (A) B-isotope compositions of formation waters from car-bonate and clastic reservoirs in the Gulf of Mexico sedimentary basin.(B) Changes ind11B andd18O of waters from clastic reservoirs relativeto B-content. Data compiled from Land and Macpherson (1992) andMoldovanyi et al. (1994).
1786 L. B. Williams et al.
6.1.2. Sandstones
Sandstones are usually not closed systems in sedimentarybasins. They are open to migration of hydrocarbons and relatedfluids, and they are not dominated by clay minerals. Formationwaters from sandstone reservoirs will be influenced by otherdiagenetic sources of oxygen and boron; therefore, it is notpossible to use the two isotopic systems to predict fluid com-positions. However, understanding the general diageneticframework for B- andO-isotopes related to I/S, one can beginto appreciate the mobility of B in the sedimentary basin. Withsome careful consideration of the various sources of B it maybe possible to use B-isotopes as a tracer of fluid migration.
The d11B value of the formation water in the Wilcox reser-voirs is quite variable (28–36‰), whereas the pore-filling(authigenic) clay minerals in the sandstones (mostly illite) havea consistentd11B value of22 6 2‰. This fact alone suggeststhat the sandstone clay minerals formed in equilibrium with adifferent fluid. If the pore-filling clay minerals precipitatedunder current reservoir conditions ('100°C), the B-isotopefractionation curve (Fig. 1), predicts that waters in equilibriumwith the I/S should have an isotopic composition of122‰.This value fits well with the regional trend for formation waters(Fig. 2). However, because the water analyses show higherd11B values than predicted, this is an indication that the clayminerals either precipitated at a much lower temperature(,60°C), or the isotopic composition of the reservoir fluid hasbeen altered since the clay cements precipitated.
The I/S in the Wilcox reservoirs consists of'70% authi-genic illite (Williams et al., 1995), and it would be difficult toachieve this degree of illitization at temperatures,80°C in thetime since Late Eocene deposition. It is more reasonable toassume that the oil field brines represent influx of fluids con-taining B from a new source introduced after formation of theauthigenic illite. A similar argument was made for these res-ervoirs on the basis ofN-isotopes in authigenic illite (Williamset al., 1995). The enrichment of11B in the present waters,relative to that predicted, may result from mixing with seawa-ter, dissolution of evaporites containing positived11B boratesor a gas phase separation that concentrates11B in a morevolatile fraction (Palmer and Sturchio, 1990). Certainly, theB-isotopic composition of the sandstone water is not at allsimilar to the mudstone pore fluids. The lowd11B of themudstones indicates that they are not a source of the presentday reservoir water, but water released from the mudstoneduring earlier compaction and smectite dehydration could havesupplied10B to waters that precipitated the I/S.
The mass balance considerations for the sedimentary basinas a whole indicate that there must be a significant source of10B at depth. Figure 3shows estimates of the B-content andd11B of sediment and water in the Gulf of Mexico, from thesurface (Spivack et al., 1987; Perry, 1972) to deep in the basin(this study). The Mississippi River sediment (,4-mm fraction)contains '100 ppm fixed-B with ad11B of 1.9 6 0.4‰(Spivack et al., 1987). The marine sediments contain seawaterin the pore space on deposition, which would provide'5 ppmB. Modern seawater has ad11B averaging 39.5‰, with varia-tions of 6 10‰ due to adsorption and surficial processes(Vengosh et al., 1991).
At 4-km depth, the depositionally equivalent marine sand-
stones contain I/S that has increased in B-content ('150 ppmB) but decreased ind11B by as much as 6‰ (fixed-B) from theoriginally deposited sediment. Similarly, the formation waterhas increased in B-content by'50 ppm with a decrease ind11Bof '12‰. This requires addition of'100 ppm of isotopicallylight-B over the course of 4-km burial. More deeply buriedsediments (Norphlet Fm, discussed below) indicate a continuedincrease in B-content of authigenic clay minerals and decreasein the d11B with depth (and age).
Boron that adsorbs to clay at surface conditions will bereleased gradually as temperatures approach'120°C (You etal., 1996) where the partition coefficient for adsorbed-B ap-proaches zero. The adsorbed fraction of the near-surface sedi-ment averages115‰ (Spivack et al., 1987) and thus has thepotential to lower the pore fluidd11B from the original valuenear seawater (39.5‰). The effect of this desorption woulddepend on the temperature, fluid/rock ratio, and degree ofcompaction, and would compete with other diagenetic reac-tions. Nonetheless, above 120°C there should be no adsorbed-Bon the sediment, and experiments (Williams et al., 2001) haveshown that at.70% illitization (similar to conditions at 4-kmburial) recrystallization and long-range ordering of I/S allowssignificant substitution of B into illite. This process will depletethe pore fluid supply of10B and would be expected to leave thefluids 11B-enriched.
The interesting coincidence, however, is that oil is generatedat the same temperature (depth) that illite neoformation begins(Eberl, 1993). It was shown (Williams et al., 2001) that kerogenfrom the source rock for oil in these reservoirs (Sassen, 1990)has a B-isotopic composition of24‰ to 210‰ (Williams etal., 2001). It is not known how much boron was in the organicmatter before it was thermally mature, but at the present tem-perature ('125°C) the B-content is 140 ppm. This is a potentialsource of 10B for reservoir waters and would significantlydecrease thed11B of the waters from the surficial pore fluidcomposition. If the B isotopic ratio of the reservoir fluids can belinked to the source rock, it could be a useful tracer of hydro-carbon migration.
Fig. 3. Summary of data discussed in the evaluation of B-isotopetrends and fluid/rock interactions in the Gulf of Mexico sedimentarybasin.
1787Boron geochemistry during diagenesis: Application
6.2. Norphlet Fm
Deeper in the Gulf Coast Basin ('22,000 ft; 6.7 km) theJurassic Norphlet sandstone is host to significant gaseous hy-drocarbon reserves. The unit has a number of stylolites withassociated authigenic quartz and muscovite (1 Md) thought tohave formed from pressure solution (Thomas et al., 1993).There are also pore-filling muscovites and muscovite pods inthe formation. Thomas et al. (1993) determined the timing ofstylolitization by age dating the large 1 Md muscovite crystals,indicating formation at 516 9 Ma (Fig. 3.16). The pore-fillingmuscovite is 776 22 Ma, and the pods are 866 16 Ma. Fromthis it is deduced that the stylolites formed when the sandstonewas buried to'18,000 ft (5.5 km). Thermal maturity indicatorsshow that the muscovite grew at the end of wet gas generationand beginning of dry gas generation (Fig. 4) (Thomas et al.,1993). It was proposed that the stylolites formed during meth-ane leakage associated with fracture of the overlying Smack-over Fm that seals the Norphlet reservoirs.
Oxygen isotope analyses of the stylolite muscovite (Table 1)show an averaged18O of '12‰. It is not known what thed18Oof the pore fluids was at the time; however, by using the burialhistory curve from Thomas et al. (1993; Fig. 4), one canestimate that the maximum temperature of crystallization of themuscovite was'165°C. Under equilibrium conditions then(neglecting pressure effects), thed18O of the water must havebeen' 14‰. This is within the range ofd18O values mea-sured in present-day sandstone reservoirs at that depth (Fig. 2)(Land and Macpherson, 1993).
The utility of SIMS analyses of B-isotopes in the muscovitesis notable in this case, where crystallization of muscoviteoccurred at different times, each representing different condi-tions of formation. Analyses of thed11B were performed on the
stylolite in thin section (Fig. 5). The fixed-B analyses averaged22.9 6 0.3‰ with a B-content of 297 ppm (Table 1). TheB-fractionation (Dmineral-water) at 165°C is 221‰ (Fig. 1),indicating precipitation of the muscovite in equilibrium with119‰ water. This is consistent with B-isotope measurementsof the formation waters at 4 to 5 km (Fig. 2B) and supportsformation of the muscovite at a shallower depth than presentday.
Analyses of the pore-filling muscovite show ad11B value of22 6 2‰, overlapping values determined on the 1 Md mus-covite, which also shows overlap in age dates. The pore-fillingmuscovite is isotopically similar to the pore-filling I/S in sand-stone reservoirs currently at 4 km (as discussed above). Themuscovite pods displaying the oldest ages (866 16 Ma)(Thomas et al., 1993) have ad11B value of 210 6 2‰,however, these isotopically light measurements were made onthe insoluble residue of the stylolite that contains intermixedkerogen that and may be responsible for the lowd11B value.Other measurements of kerogen and coals in thin section showa range of values from22 to 220‰ (Williams, 2000). Thesedata support the idea that B released from kerogen duringthermal maturation should promote10B-enrichment of the flu-ids that migrate with hydrocarbons. However, if10B is prefer-entially incorporated in illitic clay minerals that precipitatealong the migration path, then the fluid might fractionate (be-coming heavier) during migration.
Other possible mineral sources of B include tourmalines andborates. Detrital tourmalines in the Norphlet sandstone wereanalyzed and found not to be a likely source of11B-enrichmentbecause of their lowd11B (26.7 6 0.2‰) and because disso-lution is unlikely at these reservoir conditions (Henry andDutrow, 1996). Jurassic bittern salts, however, contain boracite
Fig. 4. Burial history curve for the Norphlet Fm showing the interpreted timing of stylolite formation producing 1 Mdmuscovite and earlier pod and pore-filling muscovites (modified from Thomas et al., 1993). The thermal maturity curveestimates the stages of hydrocarbon generation during mineralization.
1788 L. B. Williams et al.
with 131‰ d11B (Macpherson and Land, 1989). Althoughthey cannot be ruled out as a source of11B, their dissolutionwould be expected to have a more regional effect on the watersof the deep Gulf Coast basin. Instead, a negative trend ind11Bof waters with depth (Fig. 2) is observed, not a positive trend(Macpherson and Land, 1989).
Most importantly, the Norphlet Fm muscovites show that theirB-isotope composition reflects the fluids present at the time ofcrystallization and retains that signature at least through tempera-tures as high as the current burial depth ('200°C). The experi-mental work (Williams et al., Part I) showed that B is incorporatedin illitic clays at 350°C. It is clear that authigenic clay minerals(illite and muscovite) precipitating at diagenetic temperatures canbe useful monitors of changes in the fluid chemistry of hydrocar-bon reservoirs. The equilibrium fractionation curve between aque-ous-B and I/S may be refined through further experiments andfield data, but the application showed here indicates that it givesreasonable results in agreement with field observations.
7. BORON ISOTOPES IN THE PIERRE SHALEMETA-BENTONITE
7.1. Background
The Walsen dike intrudes a thick organic-rich section of thePierre shale where a single bentonite layer (altered volcanic
ash) was studied. The,2-mm size fraction of the bentonitelayer is dominated by smectite and illite, with minor kaoliniteand chlorite. Earlier studies by Lynch and Reynolds (1985)showed that significant mineralogic changes in the shale occurnear the dike including Na-metasomatism (albitization), anincrease in quartz, and a decrease in detrital mica and K-feldspar. The increase in quartz near the dike may be due toexcess SiO2 produced during breakdown of detrital mica andfeldspar. Lynch (1985) concluded that neoformation of authi-genic illite resulted from the short-lived thermal pulse associ-ated with dike intrusion and that it occurred in a system closedwith respect to major elements. Nonetheless, the system wasnot necessarily closed with respect to trace elements or porefluid, and we will explore this hypothesis by using the boronanalyses of the dominant clay mineral I/S.
8. RESULTS
8.1. Mineralogy
The bulk mineralogy of the Pierre shale is presented as afunction of distance from the dike/shale contact in Table 2. Theclay mineral content of the shale is'24%, with the exceptionof one sample'4 m from the contact with 14% clay. Othermineral modes show the following variations: quartz 36 to52%, K-feldspar 8 to 22%, plagioclase 9 to 20%, carbonate#7%, and muscovite,7%. The dominant clay mineral reaction isthe illitization of smectite with increasing temperature. The per-centage of authigenic illite in the I/S (% I(I/S)) increases withproximity to the dike (Fig. 6; Table 2), with the sample.25 mfrom the dike contact showing 20% I(I/S) and the sample,2 mfrom the dike showing 100% illitization of the I/S.
8.2. Boron and Oxygen
The least illitized samples in the bentonite layer (.8 m fromthe contact) show a range in B-contents from'200 to 300 ppm,and d11B values from17.8 to 18.7‰. The d11B and B-contents decrease as temperatures increased with proximity tothe dike (Fig. 7; Table 3). The isotope ratios are not greatlyaffected, however, until illitization has reached'70% and R1ordering (ISIS) of the mineral structure has begun ('350°C).The recrystallization of smectite to illite continues toward thedike as the temperatures reached a maximum of'500°C nearthe contact. The sample taken,2 m from the dike shows asignificant increase in B-content ('500 ppm) with a corre-sponding drop in thed11B to 212‰.
O-isotopes do not show a consistent or significant trendacross the contact aureole (Table 3). Between 3 and 25 replicateanalyses were made on the clay minerals extracted from wholerock samples. Thed18O values of the I/S vary from 20 to 26‰with 6 1‰ errors (1s).
8.3. Organic Matter
The total organic carbon (TOC) content across the contactaureole of the Pierre shale is between 0.5 and 1%. Table 2 liststhe range of values for organic content and vitrinite reflectancevalues associated with that interval (Bostick and Pawlewicz,1984). The range of temperatures interpreted from vitrinitereflectance are shown in Figure 6.
Fig. 5. Photomicrographs of the 1 Md-stylolite muscovite from theJurassic Norphlet Fm, Gulf of Mexico sedimentary basin. SIMS ana-lytical craters are'20 mm in diameter.
1789Boron geochemistry during diagenesis: Application
9. DISCUSSION
9.1. Boron Trends
No significant change in thed11B is observed until the I/S isdominantly illitized ('70% at'15 m from the dike). Closer tothe dike, the amount of authigenic illite increases to 100%,
whereas thed11B decreases by'20‰. This observation is inclose agreement with results from hydrothermal experimentson I/S (Williams et al., Part I) that also show no significantchange in B-isotope ratios until'70% illitization. This sug-gests that major recrystallization of the mineral must occur
Table 2. Mineralogical data for (A) bulk powders, (B)#2-mm size fraction, and (C) organic matter.
Fig. 6. Plot showing the mineralogical changes in I/S as a function ofmaximum estimated temperatures across a metamorphic contact aure-ole in the Pierre shale. Vitrinite reflectance values (in parentheses)(Bostick and Pawlewicz, 1984) are the basis for the temperature scale.Long-range ordering of the I/S (R1–R3) is found within'8 m of thedike where temperatures exceeded 350°C.
Fig. 7. Trends in B-content andd11B across the contact aureole in thePierre shale. Changes in thed11B do not begin until ordering of the I/Soccurs at'70% illitization.
1790 L. B. Williams et al.
before B isotopes begin to equilibrate with the fluid. For ran-domly ordered I/S, the original boron isotopic composition ofthe source (ash) is apparently retained.
In the Pierre shale, the high temperature of contact metamor-phism (.200°C) caused recrystallization of the smectitic sed-iment to illite. Adsorbed-B is not significant at temperaturesabove 120°C (You et al., 1995); therefore, the changes ind11Bwe observe represent only the changes related to B-substitutionin the tetrahedral layers of the authigenic illite.
Under normal conditions of burial diagenesis, the B-contentof authigenic I/S increases with illitization (Perry, 1972). Thisrequires a source of B that could be other unstable detritalminerals such as micas, or perhaps organic matter (Williams etal., 2001). In the case of the Pierre shale, there is an insufficientquantity of detrital mica and organic material to provide asignificant source of B (Table 2). The metamorphosed Pierreshale samples show a slight decline in B-content of the claywith increasing temperature. This trend could be related todepositional variability, but it is more likely a result of theliberation of water during the reaction of smectite to illite.Boles and Franks (1979) wrote the general reaction for theillitization of smectite showing:
4.5 K1 1 8 Al31 1 smectite3 illite 1 3 Si41
1 ~cations1 water!
The water released from the interlayer of smectite ('5–8wt.%) (Pytte, 1982) will cause dilution of the pore fluid in theimmediate vicinity of the authigenic clay.
A change in the B-concentration of the pore fluid due todilution from the smectite dehydration requires re-equilibrationof the B in the newly formed illite crystals. This would causea lower B-content in the authigenic phase (Fig. 7), reflecting thepore water dilution. During dike emplacement, a greater vol-ume of water would be expected immediately in samples thatexperience higher temperatures, due to more extensive dehy-dration. The amount of fluid that has exchanged with thebentonite was calculated from isotopic mass balance (Taylor,1977) for closed and open isotopic systems (Table 4). Thecalculations indicate that near the dike (excluding the metaso-matic zone) there was nearly a threefold increase in fluidcontent of bentonite during recrystallization that could causedilution of the B-content.
The very high B-content (500 ppm) of the 100% recrystal-lized illite near the dike contact suggests that metasomaticfluids introduced with the dike had a much higher B-contentthan the marine pore fluids trapped with the sediments. The factthat the illite incorporates this much B in a sample that reached
'500°C is evidence that authigenic illite is not a source of Bduring diagenesis or even moderate grades of contact metamor-phism. Furthermore, there is no significant change ind11B ofthe sample despite the large increase in B-content. This indi-cates that B-fractionation is temperature dependent, not con-centration dependent. This fact was also observed in experi-ments on B-partitioning in basaltic melts (Hervig and Moore,2000). Illite is a host for B at temperatures much higher thanpreviously acknowledged. The illitization of smectite does notrelease B in sedimentary basins as has been suggested (e.g.,Moldovanyi et al., 1994; Land and Macpherson, 1992).
9.2. Isotope Modeling
In a closed system, we would expect B-isotope ratios in I/Sto track the fractionation between I/S and aqueous species ofboron in the existing pore fluids (presumably seawater). Theexpected fractionation has been determined through experi-ments on I/S (Fig. 1) as a function of temperature (Williams etal., Part I). Assumptions required to apply this fractionationmodel include the following: (1) boron in authigenic illite istetrahedral and (2) boron in pore water is trigonally coordi-nated. This is reasonable for a high-temperature, low-pH envi-ronment (Palmer and Swihart, 1996) like a metamorphosedblack shale.
A batch volatilization model (Nabelek et al., 1984) can beused to predict the isotopic composition in a closed systemwhere the liberated water is in equilibrium with the rock.Application of this model (Table 5; Fig. 8) shows a finalpredictedd11B of I/S of only 22‰ (at 500°C), which is 10‰heavier than measuredd11B values of I/S. Because this does notfit our observations, it indicates that the bentonite was notclosed with respect to volatiles. Therefore, a Rayleigh volatil-ization model was applied that accounts for illitization accom-panied by loss of water from the rock (Valley, 1986).
The O-isotopes in this contact aureole vary so little (withlarge errors) that it is difficult to constrain a mechanism ofisotope exchange. Both the batch and Rayleigh models fit thed18O measurements within error (Table 5). This shows that thelarge B-isotope variations compared withO-isotopes make it
Table 3. Isotope ratio analyses for B and O in clay mineral samplesfrom the Pierre Shale meta-bentonite.
D min-wat is taken from the experimental fractionation equation(Williams et al., Part 1).
d11B water5 d11B (I/S) 2 D.F/Rclosed5 ?(d final rock 2 d initial rock)/(d initial fluid 2 (d final
rock 1 D))?.F/Ropen 5 ln (F/R closed1 1).
1791Boron geochemistry during diagenesis: Application
more useful in interpreting geochemical changes accompany-ing recrystallization.
Volatilization modeling for boron requires that the initiald11B of the rock is known, and an approximation of the reactionprogress must be made to determine the fraction of reactantremaining. An initiald11B of 18‰ was assumed on the basisof the average of the least altered samples analyzed (R0 I/S). Inaccordance with the observation that thed11B of I/S does notchange until long-range (R1) ordering of the crystal structureensues (Williams et al., Part I), we have defined the B-fixationreaction as beginning at this point (f5 1) and ending at 100%illite (f 5 0). The fraction of reactive material remaining (f) isproportional to the degree of illitization determined by XRD onsamples with$R1 ordering (Table 5). Thea used in eachcalculation was based on the maximum temperature indicatedby vitrinite reflectance, and the corresponding fractionation(Dmineral-water) predicted for that temperature from the experi-mental results (Table 5; Fig. 1).
The Rayleigh model for isotopic fractionation results in largedecreases in thed11B of I/S, greater than the magnitude of ourobservations (Fig. 8). The calculations of the predictedd11B ofI/S based on our simplified modeling of the f parameter (basedon the apparent rate of illitization) are shown in Table 5.However, a better fit to the data can be made by using a
Table 5. Isotope ratio predictions based on (A) batch and (B) Rayleigh volatilization models.
(A) Batch Volatilization Model df 5 di 2 (1-f)*1000 ln a
T°C Temp. K 1000/T(K) Dmin-wat a %I(*/S) % rxn.prog f* d11Bpred d11Bmeas d11Bwater
* From Savin and Lee, 1988.The value for f (fraction of reactant remaining) is defined to begin with Rl ordering of I/S (f5 1). The % illitization was used as an estimate of
the reaction progress (f*). The valued11Bpredis based on f*, however, the best fit to the data (d11Bmeas) is attained by using values of fcalc that indicatea slower isotope exchange than estimated by % illitization. The difference between f* and fcalc indicates a 10 to 20% difference in reaction rate.
Fig. 8. Volatilization models for predicting the changes ind11Bexpected in a contact metamorphic aureole. Two end-member models,batch and Rayleigh, show limits for the predictedd11B values (graycurves). Circles indicate thed11B measured on I/S, with errors indicated bythe symbol size. The best fit to the data is shown by the dark curve thatrequires modification of the f parameter by 10 to 20% (see Table 5).
1792 L. B. Williams et al.
Rayleigh volatilization model based on an f parameter adjust-ment to a 10 to 20% slower rate (Table 5, fcalc). In other words,it seems that the B-isotope exchange occurred at a slower ratethan is indicated by the final degree of illitization and maxi-mum temperatures achieved.
The small adjustment to the reaction rate necessary to fit aRayleigh model can be explained by reaction kinetics. Ourassumption that the dominant exchange reaction began with R1ordering of the I/S is validated by experimental results (Wil-liams et al., Part I); however, the kinetics of this recrystalliza-tion depend not only on the reactants, but also temperature,time, and fluid/rock ratio. We have assumed that the recrystal-lization of each sample occurred at the maximum temperaturethat affected that sample, but it is possible that isotopic ex-change occurred at a lower temperature, yet was retained in theauthigenic illite as temperatures increased to the degree indi-cated by vitrinite reflectance. Recrystallization at lower tem-peratures would be slower than at the higher temperature, inaccordance with the reaction rates indicated by fitting the data.
The relatively close agreement of the Rayleigh volatilizationmodel with the measuredd11B of I/S in this contact metamor-phosed shale shows the utility of B-isotopes for evaluatingfluid/rock interactions and shows that B-isotopes can be moreinformative thanO-isotopes due to the large isotopic variationsobserved. The results show that illite is an important mineralhost for B and that it retains the equilibriumd11B even insamples that have sustained temperatures of'500°C. Mostimportantly, this establishes that authigenic illite will retain theequilibrium isotope ratio acquired during recrystallization atlower diagenetic temperatures.
10. CONCLUSIONS
The application of the B-isotope fractionation equation (Wil-liams et al., Part I) to illitic sediments from diagenetic andcontact metamorphic environments shows that the experimen-tally derived fractionation equation is a good approximation tothe isotope measurements made in natural geologic settings.These studies support the simple linear fractionation equation(Bmineral-water5 210.12 (1000/T(K))1 2.44) for tetrahedralB-substitution in minerals, showing that B-isotopes can be auseful geothermometer from low temperatures to high temper-atures. These case studies indicate that B is not released fromI/S clay minerals during diagenesis, but rather is incorporatedwith an isotope ratio reflecting thed11B of the fluids. BecauseB-substitution requires breaking of tetrahedral Si–O bonds,there is a coincident change inO-isotopes, which we have usedto evaluate the isotopic composition of paleofluids.
Application of the new B-isotope fractionation curve toauthigenic clay minerals from sandstone reservoirs Gulf ofMexico sedimentary basin showed in each case (Eocene andJurassic reservoirs) that the fluids in equilibrium with the clayminerals must have been more10B-enriched than the presentday pore fluids. Combining theO-isotope and B-isotope datawe deduced that the temperatures of authigenic clay mineralprecipitation coincide with present-day depths of burial in theWilcox reservoirs.
Boron isotope variations in the contact metamorphosed ben-tonite of the Pierre shale show that illite is a reservoir for10B,even at metamorphic temperatures as high as 500°C. Boron is
incorporated into illite in isotopic proportions reflecting thecoordination change between water (trigonal) and illite (tetra-hedral). Measuredd11B values of I/S match values predicted bya Rayleigh distillation model for isotopic fractionation, indi-cating that the bentonite was not closed with respect to porefluids during the contact metamorphism. There is good agree-ment between the field results and the experimentally derivedB-isotope fractionation curve indicating that it is a fair assess-ment of the temperature dependence of B-isotope fractionationsbetween silicates and water. These fractionations are largecompared with oxygen isotope fractionation at the same tem-perature (Savin and Lee, 1988) and, therefore, may be moreuseful in identifying processes involved in fluid/rock interac-tions.
Acknowledgments—This research was funded by a grant from the U.S.Department of Energy (DE FG04 97ER14414). Samples were collectedwith Dr. Ray Ferrell from Louisiana State University (LSU). Wegratefully acknowledge the technical assistance of Wanda LeBlanc(LSU) and Al Higgs (ASU) who keeps our SIMS in excellent operatingcondition. Dave Pevear kindly provided the Norphlet Fm sample.
Associate editor:L. M. Walter
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