RARE-EARTH ELEMENT BEHAVIOR IN PHOSPHATES AND ORGANIC-RICH HOST : AN EXAMPLE FROM THE UPPER CARBONIFEROUS OF MIDCONTINENT NORTH AMERICA

ANNA M. CRUSE,* TIMOTHY W. LYONS Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA AND DAVID L. KIDDER Department of Geological Sciences, Ohio University, Athens, Ohio 45701-2979, USA

ABSTRACT: Four distinct -normalized rare-earth element (REE) patterns are preserved in the black and gray phosphatic shale facies of the Upper Carboniferous of Midcontinent North America. Two of these patterns [flat and middle REE (MREE) enriched] are observed in both nodular phos- phate and the shale host, but MREE-enriched patterns are more common in the nodules. MREE-depleted patterns were observed only in the shales. Subtle negative Ce-anomalies are rare and occur only in samples of nodular phosphate. MREE-enriched patterns in phosphate are interpreted to have formed during very early diagenesis. Coprecipitation of phosphate and REE may have depleted pore waters in MREE to the extent that host shales, which lithified later, are characterized by MREE depletions. Phosphate diagene- sis can thus be a critical factor controlling REE patterns preserved in shales. Flat REE signatures characterize gray shale samples that have low con- centrations of phosphate and organic carbon. Such samples were thus not affected by processes of remobilization and scavenging of REE. Black shale samples that have phosphate concentrations intermediate of those in samples characterized by MREE-enrichments and -depletions also display flat patterns. Such patterns likely reflect a composite signal of MREE-enriched phosphate and MREE-depleted shale. Conversely, flat patterns in phosphate nodules could reflect later diagenetic migration of REE from MREE-depleted pore waters into later cement phases in the nodules. Subtle Ce-depleted patterns in phosphate nodules likely reflect the minor exchange of Ce with seawater during episodes of sediment reworking.

INTRODUCTION could be preferential conduits for later flow of subsurface fluids. We report here on REE analyses of nonskeletal phosphate (nod- The rare-earth elements (REE), La to Lu, are widely utilized ules and laminae) and the shale hosts from largely unreworked as tracers of a range of geological processes because of their sections in the Upper Carboniferous of Midcontinent North strikingly similar electronic configurations that give rise to sub- America in an effort to constrain the sources of and mechanisms tle, predictable differences in chemical behavior along the series for REE concentrations observed in phosphates and the general (Elderfield, 1988). Recent studies have revealed that REE pat- pathways of REE partitioning in organic-rich shales. terns in sedimentary systems are influenced by both deposi- tional environment (e.g., Murray et al., 1990, 1992) and diagenetic processes (e.g., Milodowski and Zalasiewicz, 1991; GEOLOGIC SETTING Murray et al., 1992). Distributions of REE also vary consider- ably within modern ocean waters as a function of depth Phosphatic black shales from Upper Carboniferous (Murphy and Dymond, 1984; Sholkovitz et al., 1994); latitudi- (Pennsylvanian) cyclothems in Iowa, Kansas, and Oklahoma are nal gradients (Murray and Leinen, 1993); uptake of REE by emphasized in the present study (Figs. 1, 2A). Cyclothems in organic and/or oxyhydroxide grain coatings (Sholkovitz et al., this region of North America consist predominantly of alternat- 1994); and variations in water chemistry, in particular redox ing limestone and shale units. Based on extensive study of conditions, between pore water and overlying ocean water (e.g., deposits within the Midcontinent, Heckel (e.g., 1977, 1990) Elderfield et al., 1990). summarized the idealized pattern of cyclicity (Fig. 2B). Within Phosphates are often analyzed for REE signatures because this scheme, the basal unit consists of a nearshore sandy shale phosphate mineralogy commonly stabilizes during early diage- that may contain coal, sandy layers, and soil horizons. This nesis with minimal subsequent alteration, and phosphate miner- shale is overlain by a thin transgressive limestone, which is in als are naturally enriched in bulk REE concentrations due to turn overlain by the black and gray shales analyzed in the pre- substitution for Ca in the apatite lattice (e.g., Wright et al., sent study. Based on biofacies models of water column oxy- 1987). However, analysis of the relationships between phos- genation, the bioturbated gray shales were deposited under an phate and their host sediment is complicated by the oxic to dysoxic water column, while the black shales were common occurrence of phosphate in reworked lag deposits, deposited under anoxic waters (Rhoades and Morse, 1971; which allows for the possibility that chemical exchange Savrda and Bottjer, 1991). The black and gray shale horizons occurred with overlying seawater at some unknown time after are overlain by a thick regressive limestone, and the sequence is deposition. Winnowing also separates the phosphate typically capped by a second nearshore shaley unit that marks from the sediment in which it originally formed. Finally, lag the beginning of the next cycle. The black shale facies has been deposits that are more permeable than the surrounding host rock interpreted as the deepest, most offshore part of the cycle (e.g., Heckel, 1977, 1990), although other workers have favored a shallower depositional environment (Zangerl and Richardson, *Present address: Woods Hole Oceanographic Institution, Department of 1963; Merrill, 1975; Coveney et al., 1991). Marine Chemistry and Geochemistry, MS#4, Woods Hole, MA 02543, Phosphate occurs in the black shale facies as distinct layers email:[email protected]

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66 Copyright 2000 SEPM (Society for Sedimentary ), ISBN 1-56576-064-6, p. 445-453. 446 ANNA M. CRUSE, TIMOTHY W. LYONS AND DAVID L. KIDDER

of nodules or bedding-parallel microlaminae that are typically 0.5–1 mm thick. The layers either occur in zones proximal to the gray-black transitions or in multiple zones throughout the black, organic-rich horizons. In the latter case, nodules are generally largest near the upper and lower boundaries of the black shale (Kidder, 1985).

METHODS

Documentation of REE variation was conducted on two dif- ferent scales. Phosphate nodules were examined via a broad reconnaissance of shale outcrops across a region extending from northeastern Oklahoma to northeastern Kansas (Fig. 1). These analyses focused on regional and stratigraphic distributions assessed within the petrographic framework established by Kidder (1985). Analyses of the shales were conducted at a more detailed level. Numerous stratigraphic levels within gray and black shales were analyzed at high-resolution, centimeter-scale intervals across paleoredox gradients to examine local, fine- scale variation within organic-rich shales and between black and gray shale facies. No effort was made to separate phosphatic phases [a nodule found in one sample (289.3-4BR) from core C- TW-1 or laminae in all others] from the shale prior to analysis. For the shale study, samples from two drill cores (IRC, Iowa; C- TW-1, Oklahoma; Fig. 1) were analyzed to avoid alteration FIG. 1.—Map showing position of northern midcontinent shelf and the limit related to recent surface weathering effects. of Upper Carboniferous (Atokan-Desmoinesian and Missourian) surface expo- Analytical methods for the nodular phosphate consisted of sures (stippled pattern). Filled circles mark positions of drill cores analyzed in extracting powders using a carbide-tipped engraver. Centers of the present study; filled squares represent localities from which analyzed phos- phate nodules were collected.

FIG. 2.—(A) Generalized stratigraphy of cyclic units analyzed in this study after Heckel (1990) and Kidder and Eddy-Dilek (1994). The Hushpuckney, Muncie Creek, and Eudora Shales are the units from which analyses are reported, although some of the other units are referred to in the text. (B) Generalized “Kansas” cyclothem modified after Heckel (1994). See text for discussion. RARE-EARTH ELEMENTS IN PENNSYLVANIAN PHOSPHATES AND SHALES 447 phosphate nodules were preferentially sampled to minimize from an arbitrary baseline that connects the three outcrop loca- weathering effects. These samples were analyzed for REE con- tions that were furthest from the shelf edge). Correlation was tents using inductively coupled plasma mass spectrometry (ICP- generally reasonable (r2 = 0.71–0.77), with MREE-enrichment MS). Details of analytical methods can be found in Kidder and increasing with distance from the shelf when samples from all Eddy-Dilek (1994). eight shales were treated as one data set as well as when sam- Bulk shale samples were crushed in a ceramic mill and ana- ples were grouped according to host shale. One exception was lyzed by instrumental neutron activation analysis (INAA) with the value of r2 = 0.35 for the HREE depletion in phosphate appropriate corrections applied for the effects of U-fission from the Muncie Creek Shale. This unit, which displayed the (Glascock et al., 1986). Recognizing that artifacts can arise most variability in REE patterns, was affected by local deposi- through the use of INAA on black shales, a selected suite of tional topography and is thus not indicative of regional diage- samples from core IRC (Fig. 1) was also analyzed for REE netic trends. The overall relationship between MREE using ICP-MS to cross-calibrate the two methods. Generally, the enrichment and geographic position thus appears to be robust two methods gave comparable results except for a spurious (Fig. 4). enrichment in Sm that was observed in the INAA results, despite Minor negative Ce-anomalies occur in phosphate nodules the corrections for U-fission (Glascock et al., 1986). Although collected at three localities in the Stark, Tackett, and Muncie the specific cause of this artifact is presently unknown, possibil- Creek shales. The phosphate from one of these localities is ities include previously unidentified interferences from U-fis- clearly a reworked lag deposit from which the host siliciclastics sion that are not accounted for in the correction algorithms or have largely been winnowed (Kidder, 1985; Kidder and Eddy- 187W (R. Hannigan, pers. commun., 1997). Dilek, 1994). The phosphate nodules in the Stark and Tackett Concentrations of total and inorganic carbon for the shale shales, however, do not appear to be reworked. samples were determined coulometrically through bulk sample combustion and acidification, respectively. Organic carbon Shale Patterns (Corg) concentrations were calculated by difference. Inorganic phosphate (Pinorg) in shales was extracted by reacting 0.2 g shale Patterns depleted in MREE occur in samples from C-TW-1 with 25 ml of 1 N HCl for 14–16 hours (Aspila et al., 1976; with Pinorg concentrations generally less than 0.2 wt. % as P2O5 Ruttenberg, 1992; Ingall et al., 1993). Pinorg concentrations in (Table 2; Cruse, 1997). The relationship was observed in both the filtered solutions were then measured spectrophotometri- black, laminated and gray, bioturbated host shale from core C- cally (Murphy and Riley, 1962). Representative results for all TW-1 collected in northeastern Oklahoma (Figs. 3, 4). C-TW-1 analyses are reported in Tables 1 and 2. More complete data can was collected in the general region characterized by MREE- be found in Kidder and Eddy-Dilek (1994) and Cruse (1997). enriched phosphate nodules in the study of Kidder and Eddy- Dilek (1994). Black shale samples from C-TW-1 that are RARE EARTH ELEMENT PATTERNS AND DISTRIBUTIONS enriched in inorganic (authigenic) phosphate (1.2-12.1 wt. % as P2O5; Table 2; Cruse, 1997) display an MREE enrichment. Four distinct shale-normalized REE patterns in the phos- Pinorg concentrations in IRC typically range from 0.1 to 0.8 phates and their host shales have been documented (Kidder and wt. % P2O5 in the black shales (an exception is sample 1036.7- Eddy-Dilek, 1994; Cruse, 1997). These are: (1) flat, (2) MREE- 4BR with a concentration of 4.4 wt. %) and are less than 0.1 wt. enriched, (3) MREE-depleted, and (4) patterns characterized by % P2O5 in the gray shales (Table 2; Cruse, 1997). These con- a negative Ce-anomaly (Fig 3). Flat shale-normalized patterns centrations are less than those characteristic of samples in C- are observed in phosphate nodules, black laminated facies, and, TW-1 that display an MREE-enriched pattern and generally most commonly, in gray bioturbated shales. MREE-enriched greater than those that display a MREE-depletion. However, all patterns are also observed in both phosphate nodules and in samples from core IRC display flat, shale-normalized patterns black shales with elevated concentrations of phosphate. Patterns (Fig. 3). with a negative Ce-anomaly are confined to phosphate nodules, while MREE-depleted patterns are observed in both black and gray host shale samples. DISCUSSION MREE-enriched and -depleted patterns have not been Phosphate Nodule Patterns observed in modern settings, but they have been documented in other ancient apatite samples [nodules and microfossils] In nodular phosphate, there appears to be a correlation (McArthur and Walsh, 1984; Wright et al., 1987). Analyses of between geographic position and type of REE pattern observed foraminifera tests by Palmer (1985) and numeric modeling by (Fig. 4). Flat REE patterns are most common in northeastern Grandjean-Lécuyer et al. (1993) have shown that adsorption of Kansas, whereas MREE-enriched patterns are most common in REE by iron oxyhydroxides can result in MREE-enriched pat- southeastern Kansas and northeastern Oklahoma. Kidder and terns. However, petrological and geochemical analyses demon- Eddy-Dilek (1994) plotted the slopes of the depletions of LREE strate that iron in the nodules of the present study dominantly (Sm/La) and HREE (Sm/Yb) for nodular phosphates from eight occurs as pyrite (Kidder, 1985; Ece, 1990). Also, considering different host shale units against basin position (i.e., distance the presence of water-column anoxia during black shale deposi- 448 ANNA M. CRUSE, TIMOTHY W. LYONS AND DAVID L. KIDDER

TABLE 1.—RARE-EARTH ELEMENT ANALYSES FOR PENNSYLVANIAN PHOSPHATE NODULES AND BLACK SHALES FROM IOWA, KANSAS AND OKLAHOMA

Locality La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Avg. Shale1 41 83 10.1 38 7.5 1.61 6.35 1.23 5.49 1.34 3.75 0.63 3.51 0.61

Shale Analyses

Hushpuckney Shale Iowa 1036.7-4GR 15 29 3.3 13 2.4 0.50 2.13 0.55 1.91 0.41 1.36 0.21 1.69 0.28 1036.7-4BR 49 93 11.5 45 8.3 1.82 8.93 1.25 7.07 1.42 3.79 0.49 2.92 0.46 1036.7-5BR 17 29 3.8 15 2.7 0.62 2.70 0.39 2.26 0.44 1.35 0.22 1.49 0.26 1036.7-8BR 3 11 0.7 3 0.5 0.12 0.43 0.08 0.40 0.10 0.30 0.05 0.47 0.09 1039.2-4BT* 13 23 — 7 9.1 0.44 — 0.40 1.69 — — — 1.56 1.11 1036.2-3BT 5 16 1.5 6 1.1 0.26 1.08 0.17 1.01 0.20 0.61 0.10 0.56 0.09 1039.2-1BT 2 14 0.6 2 0.4 0.09 0.33 0.05 0.30 0.06 0.17 0.03 0.22 0.03 1039.2-3GT* 5 8 — 4 0.9 0.10 — 0.10 0.60 — — — 0.40 0.10 1039.2-4GT 3 6 0.7 3 0.6 0.15 0.60 0.11 0.51 0.11 0.29 0.05 0.27 0.06

Oklahoma 289.3-14GR* 42 81 — 45 5.8 1.10 — 1.45 4.13 — — — 2.87 0.39 289.3-3GR* 33 50 — 17 2.5 0.50 — 0.50 2.20 — — — 1.90 0.30 289.3-4BR* 196 373 — 536 61.6 13.77 — 7.23 50.86 — — — 14.44 2.58 289.3-7BR* 26 39 — 11 2.3 0.40 — 0.20 1.60 — — — 1.90 0.40 294.0-8BT* 81 182 — 177 23.6 4.60 — 3.30 16.00 — — — 5.70 0.70 294.0-1BT* 31 53 — 18 3.7 0.69 — 0.71 3.10 — — — 2.47 0.42 294.0-7GT* 34 62 — 26 4.9 0.99 — 1.06 3.17 — — — 2.67 0.45

Phosphate Analysis

Muncie Creek Shale IA3 K-32 257 558 91.5 364 76.9 15.90 59.60 10.90 60.60 11.10 29.10 3.30 16.50 2.50 IC3 KTL 144 251 56.4 249 60.7 14.00 50.30 8.80 46.30 7.80 20.00 2.10 10.40 1.40 IC3' KTL 50 180 32.1 161 46.5 9.30 31.60 5.10 24.40 3.70 8.40 0.80 3.80 0.50 IE4 CAN 84 243 48.0 235 58.9 10.60 37.80 6.50 29.30 4.30 8.90 0.70 2.90 0.40 IF2 NEO 109 250 44.5 217 51.7 10.00 38.70 7.30 39.40 7.00 17.60 1.90 8.90 1.10 IF2' NEO 120 298 49.7 225 52.7 10.80 40.40 7.40 38.10 6.40 16.60 1.70 8.50 1.00

Eudora Shale EA1 CPD 122 385 62.1 314 95.7 19.20 66.00 10.90 48.10 7.30 16.50 1.70 8.00 1.00 EB2 K-10 58 167 25.9 102 21.2 4.70 17.00 3.00 15.80 2.80 7.10 0.80 3.70 0.50 EC1 NQEB 96 269 53.7 237 56.3 12.80 45.20 7.70 38.70 6.30 14.50 1.50 6.30 0.70 ED1 PTC 243 544 79.0 299 59.2 13.70 50.90 9.30 49.20 8.90 24.20 3.00 15.50 1.90 EE1 TYQ 85 405 67.6 320 82.4 18.10 63.70 10.90 55.20 8.60 19.80 1.90 8.40 1.10

1Shale values from Piper (1974). *INAA results. —: not analyzed. BR, BT: black shale sample. GR, GT: gray shale sample. RARE-EARTH ELEMENTS IN PENNSYLVANIAN PHOSPHATES AND SHALES 449

TABLE 2.—ELEMENTAL ANALYSES (WT.%) FOR PENNSYLVANIAN PHOSPHATE NODULES AND BLACK SHALES IN IOWA, KANSAS AND OKLAHOMA

Locality SiO2 Al2O3 FeO CaO MgO Na2OK2OP2O5 TiO2 MnO F2OCorg Cinorg

Shale Analyses1

Hushpuckney Shale Iowa 1036.7-4GR — 14.63 6.23 3.18 3.20 1.29 3.09 0.12 0.83 0.05 — 0.58 0.60 1036.7-4BR — 12.04 4.54 6.91 3.20 0.94 2.67 4.36 0.48 0.02 — 9.62 0.69 1036.7-5BR — 9.70 4.71 6.79 3.23 0.70 2.31 0.51 0.46 0.02 14.41 1.46 1036.7-8BR — 10.62 5.24 4.66 3.10 0.75 2.40 0.14 0.50 0.03 — 15.96 1.03 1039.2-4BT — 7.77 3.88 3.00 2.64 0.59 1.94 0.28 0.34 0.03 — 33.08 0.62 1039.2-3BT — 8.11 4.69 6.09 2.91 0.57 2.06 0.73 0.38 0.02 24.34 1.24 1039.2-1BT — 11.49 4.59 5.08 3.79 0.82 2.71 0.31 0.57 0.05 — 15.25 0.99 1039.2-3GT — 0.94 0.93 50.84 1.60 0.11 0.17 0.04 0.09* 0.03 — 0.41 10.85 1039.2-4GT — 1.62 0.81 45.42 2.29 0.18 0.26 0.04 0.09* 0.05 — 0.34 10.29

Oklahoma 289.3-14GR — 16.96 4.86 0.65 3.16 0.91 3.47 0.11 0.75 0.08 — 0.81 0.31 289.3-3GR — 15.57 5.51 0.81 2.57 0.91 3.44 0.05 0.55 0.03 — 1.16 0.07 289.3-4BR — 7.56 10.97 16.77 1.53 0.56 1.54 12.16 0.17 0.06 — 9.35 0.68 289.3-7BR — 12.91 8.37 1.27 2.83 0.79 3.08 0.04 0.52 0.05 — 12.25 0.36 294.0-8BT — 12.13 6.18 7.14 2.67 0.66 2.67 1.99 0.44 0.08 — 6.08 1.39 294.0-1BT — 13.62 4.18 8.03 2.80 0.83 2.96 0.08 0.57 0.10 — 2.30 1.90 294.0-7GT — 13.69 4.70 10.41 2.40 0.78 3.27 0.27 0.45 0.10 — 1.16 2.11

Phosphate Analyses2

Muncie Creek Shale IC3 KTL 0.03 0.02 0.01 54.54 0.00 0.23 — 37.93 0.01 0.02 7.21 — —

Eudora Shale EA1 CPD 5.44 1.37 1.91 52.91 0.15 0.50 — 37.69 — 0.03 0.00 — — ED1 PTC 11.34 6.25 0.90 44.99 0.43 0.29 — 30.72 0.03 0.03 5.03 — — EE1 TYQ 6.21 3.30 0.21 48.54 0.00 0.49 — 35.37 0.02 0.01 5.86 — —

1Shale samples analyzed via INAA. Total element reported as oxide. 2Phosphate nodules analyzed via electron microprobe. Oxides recalculated to 100% on a volatile-free and reduced iron basis. *Values represent detection limits. —: not analyzed. BR, BT: black shale sample. GR, GT: gray shale sample.

tion (Heckel 1977, 1990; Cruse, 1997), the presence of appre- Pinorg concentrations reflects this mechanism. This idea is ciable iron oxide phases in the black shales also seems unlikely. strongly supported by the MREE-depleted host shales from Recent experimental results of Byrne et al. (1996) demonstrated Oklahoma (C-TW-1), an area in which phosphate nodules are that coprecipitation of REE-phosphates with apatite will result commonly enriched in MREE. A similar correlation between in enrichments in MREE in the solid phase, provided there is phosphate-enrichment and MREE-enriched and -depleted host minimal competition from carbonate ligands in the solution. We shales was also observed in organic-rich shales of the uppermost suggest that the relationship of MREE-enrichment in phosphate Carboniferous (Virgilian) from southeastern Kansas (Baker, nodules and phosphate-rich black shales and MREE-depletions 1995). in overlying and underlying gray and black shales with low This remobilization likely proceeded via one of two path- 450 ANNA M. CRUSE, TIMOTHY W. LYONS AND DAVID L. KIDDER

FIG. 3.—Representative examples of REE patterns preserved in phosphate nodules, host black shale, and associated gray shale. Sample numbers refer to locali- ties listed in Tables 1 and 2. (A) MREE-enriched pattern. (B) Flat pattern. (C) Subtle Ce-depleted pattern. (D) MREE-enriched pattern (294.0-8BT) and MREE- depleted pattern (289.3-3GR) observed in core C-TW-1. (E) Flat patterns observed in IRC. ways that differ only in the relative timing of diffusion of REE cause desorption of the REE from sediments. Since MREE were through pore waters and adsorption of REE by the sediments. preferentially precipitated with the apatite, their concentrations Coprecipitation of REE-phosphates during apatite precipitation would have been proportionally lower in porewaters. This would could have resulted in the establishment of concentration gradi- result in enhanced remobilization of MREE from the nonphos- ents in the pore waters. As REE diffused to the sites of copre- phatic sediments. However, in Buzzard’s Bay, Massachusetts, cipitation, REE concentrations may have been sufficiently diagenetic remobilization of REE into pore waters is insufficient depleted in the pore-waters of the nonphosphatic sediments to to affect sediment concentrations (Elderfield and Sholkovitz, RARE-EARTH ELEMENTS IN PENNSYLVANIAN PHOSPHATES AND SHALES 451

FIG. 4.—Map showing regional variation in shale-normalized REE patterns from phosphate nodules and for two localities of the shale host (IRC and C-TW-1). Plots corresponding to “h” on map are for drill-core samples of the Hushpuckney Shale. Phosphate nodules from the Muncie Creek Shale are represented by “m”; those from the Eudora Shale are indicated by “e”. Heavy dashed lines show the position of Bourbon Arch.

1987). A more plausible mechanism is one in which coprecipi- using electron microprobe analyses. The Al2O3 content of the tation of REE with apatite caused preferential depletion of four samples, used as an indicator of the detrital content, ranges MREE in pore waters prior to adsorption of the REE by sedi- from 0.02 to 6.25 wt. % (Table 2). The concentration of REE ments. Our understanding of REE behavior during water-rock associated with the detrital component can be calculated from interactions is nascent and further work is needed to fully con- the following formula: strain the exact mechanism of this process. In addition to the inorganic, selective extraction of MREE REEdetrital = Al2O3sample*(REEshale/Al2O3shale), from solution during coprecipitation of phosphate (Byrne et al., where REEshale and Al2O3shale refer to the concentrations of a 1996), there may be organic influences on the MREE-enrich- specific REE and Al2O3 in a shale standard, and Al2O3sample is ment in phosphate. Stanley and Byrne (1990) showed that under the concentration measured in a given sample. Using a value of certain conditions, algae will preferentially take up MREE. It is 16.90 wt. % Al2O3 for average shale (Gromet et al., 1984), a plausible that bacteria may be capable of the same process. Thus, maximum of only 10% of the measured concentrations of each we speculate that bacteria present during degradation of phos- REE can be attributed to incorporated detrital material. phate-rich fecal matter may also contribute to REE diagenesis. Considering that REE concentrations in the nodules are Kidder and Eddy-Dilek (1994) interpreted the apparent rela- enriched two to three times over average shale concentrations tionships between REE patterns and geographic distribution as (Table 1), the calculated detrital component is not great enough reflecting detrital influence. They suggested that REE from to affect the observed patterns. detrital material within the nodules could mask the original The origin of the flat patterns in phosphates could thus relate MREE-enriched pattern. Phosphate nodules from shales in to timing of different diagenetic processes. Although the phos- northeastern Kansas, which tend to display the flat pattern, are phate nodules lithified during early diagenesis as evidenced by closer to potential detrital sources in the Appalachians and the shale laminae that commonly compact around them, there are two Canadian Shield than equivalent facies in northeastern distinct nodule types. Kidder et al. (1996) illustrated that nodules Oklahoma. Thus, REE in authigenic phosphate nodules from from Kansas and northeasternmost Oklahoma (including those shales closest to the basin margin have greater potential to be analyzed for REE in the present study) tend to have a more open affected by incorporated detrital material. However, preliminary structure that is minimally compacted, and they are characterized geochemical analyses do not support this hypothesis. The major by micronuclei of radiolaria and peloids on which phosphate and element concentrations of four nodules have been measured 452 ANNA M. CRUSE, TIMOTHY W. LYONS AND DAVID L. KIDDER later cements precipitated. These “Kansas-type” nodules may nesis. This MREE migration is the result of preferential uptake have lithified in the manner suggested by Mozley (1989), in of MREE from pore waters due to coprecipitation with apatite. which early lithification consisted of cementation of micronuclei Host sediments thus acquire an MREE-depleted signature via an distributed throughout the concretion. Diagenetic fluids generated adsorptive process that occurs later. Biogenic factors may also during compaction and dewatering of host shales may have infil- contribute to the MREE-enriched pattern observed in phosphate trated these nodule types and imparted the flat REE pattern to late nodules. Thus, the presence of authigenic phosphate nodules cement phases. Unfortunately, we are aware of no data for the and laminae appears to be a key factor controlling the REE pat- REE composition of fluids derived from dewatered shales. terns observed in ancient sediments. Organic- and phosphorus- Microanalysis of the REE composition of late cement phases rich black shale samples from southeastern Iowa display flat (apatite, , and silica) in the phosphate nodules could test shale normalized REE patterns, suggesting that the preservation this hypothesis. Conversely, “Oklahoma-type” nodules may have of a phosphate-related signal depends upon the relative abun- fully lithified early as indicated by textural relationships within dance of authigenic phosphate. Conversely, flat REE patterns in the nodules. This early lithification could have prevented over- these shale samples could also reflect the reworked nature of printing of the original MREE-enriched pattern during late diage- authigenic apatite whereby the signal of the remobilization asso- nesis. As the name implies, this nodule type is most common in ciated with apatite precipitation was preserved in different sedi- Oklahoma (Kidder et al., 1996), where MREE-enriched patterns ments. are generally observed in phosphate nodules (Figs. 3, 4). Flat shale-normalized patterns in phosphate nodules are a Overprinting of flat patterns in phosphate nodules during late-stage diagenetic phenomenon. We hypothesize that phos- later diagenesis of shales is consistent with the trace element phate nodules with open, uncompacted structures may acquire movement from shale to limestone suggested by McHargue and this overprint from detrital clays in the host shale as they Price (1982). They noted that limestone immediately above and undergo diagenesis. Phosphate nodules that formed in areas below black/gray shale horizons in the Anna Shale is more where high sedimentation rates induced early compaction may heavily dolomitized than the limestone farther above and below have become impermeable before the appropriate shale diagen- the shale, and suggested that this effect was due to release of esis took place. Mg2+ during shale diagenesis. These workers further noted that Further work is required to fully constrain the behavior of the dolomite in these limestones was a late diagenetic phase and REE during diagenesis. These results suggest that shale-nor- interpreted it to have formed during generation of fluids from malized REE patterns in authigenic phosphate and host shales the dewatering of shales during diagenetic compaction. can vary temporally during diagenesis. Thus, the patterns If phosphate diagenesis exerts a primary control on the REE observed today may reflect the timing of authigenic nodule pre- patterns observed in host shales, MREE-enriched and -depleted cipitation and subsequent lithification in addition to paleoenvi- patterns should also be observed in IRC. Samples from this drill ronmental changes (e.g., pore water redox conditions). While core contain phosphate concentrations greater than those in this has negative implications for the use of REE as simple indi- samples from C-TW-1 that display an MREE-depleted pattern cators of paleoenvironmental conditions, these results suggest but less than those from C-TW-1 displaying an MREE-enriched that the REE could play a critical role in modeling phosphate pattern (Table 2; Cruse, 1997). Thus, the observed flat patterns diagenesis and the systematics of nodule formation. in samples from IRC may reflect a mixed signal of MREE- enrichment in the phosphate and MREE-depletion in the shale. ACKNOWLEDGMENTS This implies that preservation of MREE-depleted and -enriched patterns indicative of phosphate diagenesis depends on a mass M. Glascock, S. Herrera, and R. Hannigan provided invalu- balance between REE concentrations in authigenic phosphate able assistance with INAA and ICP-MS analyses. We thank D. and surrounding shale. Conversely, the apatite present in lami- Piper and R. T. Watkins for thoughtful reviews that helped nae in IRC could have been reworked from another location improve the manuscript. This work was supported in part by a rather than having precipitated in situ. In this case, MREE remo- grant from the University of Missouri Research Board. bilization caused by apatite precipitation occurred in a different location and would not be observed in the sediments that ulti- mately hosted the phosphate. However, since the apatite grains REFERENCES in the laminae did not appear to have been reworked when ASPILA, K. I., AGEMAIN, H., AND CHAU, A. S. 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