Low-temperature aqueous mobility of the rare

Journal of the Geological Society, London, Vol. 152, 1995, pp. 895-898, 4 figs, 1 table. Printed in Northern Ireland
stability of apatite during diagenesis is likely to have a
strong influence on the behaviour of the REE. Furthermore,
S m - N d isotopes are used in sedimentary provenance and
hydrocarbon reservoir correlation studies. However, the use
of Nd model ages requires the assumption that a system has
remained closed to R E E fractionation, at least on a sample
scale, since deposition (e.g. Mearns et al. 1989). If the R E E
are mobile and fractionated during late diagenesis, then
there is the potential for the 'resetting' of Nd-model ages
(Awwiller & Mack 1991), which carries strong implications
for the use of S m - N d isotope systematics in reservoir
correlation.
In this paper, we describe in detail the occurrence and
R E E geochemistry of diagenetic francolite (a carbonate
fluor-apatite containing CO2 and F > 1 mol%; McConnell
1973) overgrowths on detrital apatite cores from the Jurassic
Statfjord Formation (North Sea). We show that the R E E
are greatly enriched in the overgrowths relative to cores and
that R E E transport is likely to have occurred over a
considerable distance. This enables the distinction of
diagenetic and detrital apatite grains. In addition, we
demonstrate that transport and fractionation of the R E E
during diagenesis can potentially have a profound influence
on Nd model ages.
Low-temperature aqueous mobility of the
rare-earth elements during sandstone
diagenesis
J O N E . B O U C H 1, M A L C O L M
J. H O L E l,
NIGEL H. TREWIN 1 & ANDREW
C.
MORTON 2
1Department of Geology and Petroleum Geology,
University of Aberdeen, Aberdeen, AB9 2UE, UK
2British Geological Survey, Nicker Hill, Keyworth,
Nottingham, NG12 5GG, UK
Diagenetic francolite (carbonate fluor-apatite) occurs as overgrowths on detrital apatite grains in sandstones from the Lower
Jurassic Statfjjord Formation of the North Sea. The francolite
overgrowths are considerably more enriched in the rare-earth
elements (up to 1 wt % C%Os), Sr and F than the detritai cores.
These elements were carried in aqueous solution, probably in the
form of complex ligands involving organically sourced carbon and
halogens. It is possible that reported aberrant neodymium isotope
model ages within the Statfjjord Formation are the result of
mobilization and relative fractionation of Sm and Nd during
diagenesis, rather than a result of changes in provenance.
Keywords: North
144Nd/143Nd"
Sea,
diagenesis,
francolite,
rare
Data and analytical techniques. The Statfjord Formation (Brent
Field, northern North Sea) is the lower member of the Upper
Triassic to Lower Jurassic Banks Group. It is a continental fluvial
sequence comprising channel sands and overbank flood deposits on
which palaeosols with variable maturities characteristic of an arid
climate have devoloped (Dalrymple et al. in press). The diagenetic
apatite species described here is from a medium-grained channel
sand within part of the sequence domminated by stacked channel
sands. Similar material occurs throughout the Statfjord Formation.
Polished grain mounts prepared from heavy mineral separates
were initially investigated using SEM backscattered imaging
(BSE-SEM); (see Lloyd 1987 for review). On the basis of this,
several points on 2-3 grains from each sample were chosen for
wavelength dispersive electron microprobe analysis (EPMA).
Importantly, analysis of individual mineral grains avoids the
problem of averaging of chemical characteristics, which occurs when
bulk samples are studied (e.g. Dill 1994). Analyses were performed
on a Cambridge Instruments Microscan 5 electron microprobe using
a beam accelerating voltage of 15 kV, a current of 20 nA (Ca and P)
or 50hA (other elements), and a focused spot of approximately
2-5p~m diameter. Total counting times (peak plus background)
were between one and two minutes, depending on the abundance of
the element being analysed. Raw count data were matrix corrected
using Link Analytical ZAF-4/FLS software. All analyses except
carbon were conducted using a carbon sample coating. For the
analysis of carbon, the initial carbon coating was removed and the
sample recoated with aluminium.
Classification of apatite and francolite requires the determination of F, Cl, OH and C concentrations. The light elements (F, O,
C) have characteristic X-rays of very long wavelength making it
difficult to separate these lines for analysis using conventional
diffracting crystals. Analyses for F and C were therefore conducted
using a synthetic multilayered dispersion element (MLDE) with a
2d spacing of 62.2,A. Ports & Tindle (1989) describe the use of such
a MLDE for the quantitative analysis of F and show that the third
order reflection of the phosphorus Ka line, which is a potential
problem in the analysis of fluorine in apatite, is absent. Carbon
contamination by oil condensed on the sample surface from oil
reservoirs within the microprobe vacuum diffusion pumps is a
earths,
It is becoming accepted that the assumption that the
rare-earth elements ( R E E ) are immobile in aqueous systems
is not always valid. Banfield & Eggleton (1989) reported
R E E mobilization and fractionation resulting from the
dissolution of apatite on a very small scale during
weathering of a granite. Hole et al. (1992) demonstrated that
the R E E , Zr, Nb and Y were mobilized during late
diagenesis within permeable
sandstones
of mixed
fluvial/aeolian origin, transport within aqueous fluids at
<200 °C probably being enhanced by formation of halogen
complexes. Rasmussen & Glover (1994) described early
diagenetic florencite (Ce, La, A1 phosphate) and xenotime
(YPO4) preserved by later bitumen envelopes within
sandstones. Additionally, Ohr et al. (1994) came to the
general conclusion that the R E E can be fractionated by
diagenetic and prograde metamorphic processes in argillaceous rocks.
Variations in the modal abundances of detrital 'heavy'
minerals within clastic sedimentary rocks are commonly
used as a tool in aiding the correlation of hydrocarbon
reservoirs (Morton & Hurst 1995). Consequently, it is
important to differentiate between diagenetic heavy mineral
phases and original detrital grains, a process that is not
always straightforward. Apatite (Ca,,(PO4)6(OH, F, CI)2) is
a common component of detrital heavy mineral suites and,
together with other heavy minerals such as monazite, zircon,
garnet and titanite, is potentially a site for a large proportion
of the total R E E budget of a clastic sedimentary rock.
Apatite can be dissolved by low-pH meteoric fluids during
early diagenesis (Morton 1984, 1986), and consequently the
895
896
J.E.
BOUCH ET AL.
potential problem in quantitative carbon analysis by EMPA (Potts
1987). The presence of a build up of carbon at the point of analysis
was checked for by measuring the intensity of the carbon Ka line
for ten minutes (much longer than the time required for analysis)
without moving the beam position. Over this time period no change
in peak count intensity was observed suggesting that any carbon
build up was insignificant relative to the carbon concentrations of
the mineral grains being studied. However, the possibility that the
earlier carbon coating was not completely removed, coupled with
poorly known matrix correction coefficients for carbon, and the
possibility of C build up at the sample surface, means that the
carbon analyses should be considered as semi-quantitative only.
Representative analyses are given in Table 1, together with the
appropriate EPMA lower limits of detection and standard errors.
Mineral chemistry. Figure 1 shows a BSE-SEM image for a
single grain (12636.9 a p # 1 ) from the Statfjord Formation.
The grain consists of a small (detrital) core (black) which
has been overgrown by the euhedral (bright) diagenetic
overgrowth. Vague sector twins and growth zones which are
clearly observable under transmitted cross polarized light
microscopy can be seen as very subtle brightness variations
within the overgrowth. N u m b e r e d points on the BSE-SEM
image represent E P M A analysis points (Table 1).
As implied by the BSE-SEM image there is a major
difference in chemistry and also mineralogy between the
detrital core and the diagenetic overgrowths. Figure 2 is a
plot of phosphorus concentration against total ( O H + F +
C1) for 18 analyses from three grains in the same sample
superimposed on the compositional fields for apatite and
francolite plotted from analyses given by D e e r et al. (1962).
The overgrowths have fluorine concentrations in excess of
the stoichiometric maximum (>2 formula units) for
fluor-apatite and show a trend of decreasing phosphorus
content as this excess fluorine concentration increases. The
observed compositional fields suggest that the detrital cores
are near stoichiometric fluor(chlor)apatites and the overgrowths are francolite.
The francolite overgrowths contain appreciable concentrations of R E E (up to 1.1 wt% Ce203; Table 1).
C h o n d r i t e - n o r m a l i z e d R E E profiles for the overgrowth
,..,
0+
4
a:
©
3
+
2
"---<2
1
0
5.2
I
I
I
5.4
5.6
5.8
12636.9 ap#l). Numbered points represent positions of analyses
whose results are presented in Table 1.
J
6.2
6.0
P
Fig. l. Plot of P v. total (OH + F + C1) for 18 analyses of apatite
from the Statfjord Formation. All data are presented as formula
units normalized to 10 Ca + Sr + REE + Y. The compositional fields
for apatite and francolite are plotted from analyses given in Deer et
al. (1962). Symbols; filled circles Statfjord Formation detrital cores;
open circles diagenetic overgrowth; filled diamonds Deer et al.
(1962) apatite: open diamonds francolite.
Table 1. Representative analyses of diagenetic and detrital apatite
12636.9
ap#1
CaO
SrO
Y203
La20 3
Ce20 3
Nd~O3
Sm203
Gd203
P205
CO2
F
C1
Total
F=- O
1
2
52.52
0.24
1.55
0.27
1.13
0.82
0.23
0.36
37.47
2.57
4.48
bd
51.21
0.27
1.51
0.28
1.08
0.75
0.18
0.33
36.75
2.50
4.79
bd
3
4
5
6
5 1 . 0 9 5 2 . 6 7 51.74 54.94
0.30
0.25
0.26
0.07
2.13
1.64
2.26
bd
0.24
0.12
0.22
bd
1.02
0.71
0.86
0.06
0.74
0.84
0.72
0.20
0.20
0.15
0.30
bd
0.33
0.35
0.34
bd
3 6 . 3 3 3 6 . 6 4 36.21 41.19
2.62
2.36
2.23
1.69
4.66
4.80
4.77
2.41
bd
bd
bd
bd
std
err
0.32
0.06
0.20
0.09
0.08
0.29
0.14
0.12
0.52
0.41
0.10
101.64 9 9 . 6 3 9 9 . 6 5 100.52 99.89 100.55
- 1.89 -2.02
- 1.96 -2.02 -2.01 -1.02
Formula units (10 Ca etc.)
Ca
9.66
9.67
Sr
0.02
0.03
Y
0.14
0.14
La
0.02
0.02
Ce
0.07
0.07
Nd
0.05
0.05
Sm
0.01
0.01
Gd
0.02
0.02
P
5.45
5.48
C
0.60
0.60
F
2.43
2.67
CI
bd
bd
O
23.76 2 3 . 7 2
Fig. 1. BSE-SEM image of an apatite/francolite grain (grain
I
9.61
9.69
9.62
9.98
0.03
0.03
0.03
0.01
0.20
0.15
0.21
bd
0.02
0.01
0.01
bd
0.07
0.04
0.06
0.00
0.05
0.05
0.04
0.01
0.01
0.01
0.02
bd
0.02
0.02
0.02
bd
5.40
4.33
5.32
5.91
0.63
0.55
0.53
0.39
2.59
2.61
2.62
1.29
bd
bd
bd
bd
2 3 . 6 4 2 3 . 2 7 23.22 24.92
Figure 1 shows the positions of analyses, 1-5 are from the diagenetic
overgrowth, analysis 6 is from the detrital core. Probe
operating conditions are given in the main text. Lower limits of
detection (3 sigma oxide wt%) for the trace elements are:
SrO 0.03; Y203 0.09; La203 0.07; Ce203 0.06; Nd203 0.14;
Sm203 0.08" Gd203 0.10; CO 2 0.25; F 0.12. Oxygen is
calculated by stoichiometry, bd, below detection.
REE MOBILITY D U R I N G S A N D S T O N E D I A G E N E S I S
105
I= 104
~ 103
i =
I
I
=
r~
102
I
I
I
I
I
I
I
I
IIO1"I/1.
La Ce
Nd
Sm
value
(ppm)
0.34 0.91
0.64
0.195 0.26
I
Gd
I
I
Y
(=Ho)
2.0
Fig. 3. Chondrite-normalized REE & Y plot for authigenic
francolite overgrowth analyses given in Table 1. Error bars for REE
& Y analyses from spot#1 are shown shifted down by a factor of
ten. Normalizing values taken from compilation in Henderson
(1984).
(Fig. 3) are generally flat (average Cen/Yn = 1.37), with
absolute R E E abundances around 10000× chondrite. The
detrital cores contain low R E E concentrations and only Ce
is consistently detectable. E P M A is unable to distinguish the
subtle variations in chemistry responsible for the slight
sector and growth zonation.
Discussion. Diagenetic francolite overgrowths on detrital
apatites can readily be identified by BSE-SEM, as they are
chemically very different from their detrital cores. The
detrital apatite cores in this sample are very small indicating
an early phase of extensive corrosion. Similar grains from
another channel sand sample (not shown here) have
chemical compositions that are indistinguishable from the
grains described here, but have only slightly corroded
detrital cores and considerably smaller authigenic overgrowths. The corrosion of the detrital grains was probably
caused by low pH meteoric waters relating to soil-forming
processes during the time the sediments were stored on the
fluvial floodplain. The later precipitation of the francolite
overgrowths with their high REE, Sr, and F concentrations
is likely to have occurred as conditions changed as the
sediments were buried. These overgrowths are also clear
evidence that the REE were present in diagenetic fluids and
therefore mobile at the time of francolite crystallization.
The early dissolution of detrital apatite grains is liable to
have released some REEs to the fluids. However, the REE
enrichment observed in the later diagenetic overgrowths (up
to 20x relative to cores) would require the dissolution of
considerably more apatite than was re-precipitated as
francolite. This suggests that the detrital apatite is unlikely
to be the sole source of REE. Alternatively the REE could
have been derived through dissolution of other detrital
REE-bearing minerals. R E E mobility is likely to be
enhanced through formation of complexes with ligands such
as fluorides, phosphates and carbonates (Wood 1990).
Francolite contains (by definition) concentrations of P, F
and C and therefore it is conceivable that any combination
of these species could have been responsible for enhanced
R E E mobility. Francolite is more usually associated with
897
phosphorite deposits and black shales (see Jarvis et al. 1994
for a recent review), both of which have organic
associations. It is possible therefore that the REEs present
in the diagenetic francolite were derived from organic
components and that organic species may have been
responsible for complexing. However Jarvis et al. (1994)
note that francolite associated with organic mudrocks tend
to have lower CO2 and F concentrations than those from
carbonate associated phophorites. The sector growth
observed in these francolites indicates disequilibrium
between the mineral and fluid during growth, making it
difficult to relate trace elements within the mineral to fluid
compositions. Dissolution of apatite and other detrital
phosphate phases may have controlled the amount of
phosphorus available in the fluids, thus limiting the amount
of francolite which could re-precipitate.
Mearns et al. (1989) successfully used Sm-Nd isotopes to
distinguish different provenance areas for the Upper Triassic
Lunde and Lower Jurassic Statfjord Formations in the
Snorre Oil Field (northern North Sea), and to subdivide the
Statfjord Formation on the basis of discontinuities in the
patterns of Nd-isotope model ages. However, Mearns et al.
(1989) also document that some samples have anomalously
high or low Sm/Nd ratios, and that some closely spaced
sample pairs have widely different model ages. Mearns et al.
(1989) accounted for these variations by changes in
provenance, although they recognized the possibility that
fractionation of Sm relative to Nd could have occurred
between the coarse- and fine-grained lithologies.
The diagenetic francolite described here has approximately 7000 ppm Nd and 1700 ppm Sm giving an Sm/Nd ratio
of c. 0.24. Such high R E E concentrations coupled with
relatively high Sm/Nd ratios, means that the addition or
removal of very low abundances of authigenic francolite in a
sample will produce large variations in bulk Sm/Nd ratios.
Figure 4 shows that the addition of trace concentrations of
3000
sst
5
Nd
29
Sm/Nd 0.175
Sm
2800
franc
1700
7000
0.243
~j 2600
e~0
2400
©
~
2200
'
2000
0
saaVe~ageneTDMUR 2100 Ma
I
I
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
0.6
0.7
% of diagenetic REE in sample
Fig. 4. Plot of % of Sm and Nd introduced into a bulk sample of
sandstone as a result of precipitation of diagenetic francolite, versus
model age (Tt)MUR) of the mixed sample. The mixing calculation is
based on the concentration of Sm and Nd in the bulk rock sample,
taken as an average of Statfjord Formation sandstones (Mearns et
al. 1989), and that in the francolite overgrowths (see inset). The
measured 143Nd/la4Nd ratio (0.51150) was chosen to reflect the ratio
observed for samples in the Statfjord Formation (Mearns et al.
1989). 143Nd/144NdDMUR = 0.513113, J47Sm/144NdDMUR = 0.222.
898
J.E.
BOUCH
authigenic francolite of the composition described here can
produce large variations in the calculated model age of a
bulk sample. If the remobilization was on only a sample
scale then any perturbation of the isotope system would not
be reflected in bulk sample analysis. However, if
mobilization and fractionation occurred on a scale greater
than that of sampling, as the significant concentrations of
REE observed in these francolites would seem to require,
then the Nd model age of bulk sample will reflect a
diagenetic component in addition to the provenance-related
isotopic signature. We therefore suggest that the precipitation of even minor amounts of diagenetic francolite in
porous and permeable sedimentary rocks could account for
the aberrant model ages observed in some provenance
studies (e.g. Mearns et al. 1989). In addition, because it is
clear that diagenetic effects can have profound effects on
apatite disolution and reprecipitation, the use of REE
fingerprinting of bulk apatites for correlation purposes (e.g.
Dill 1994) should be treated with caution.
The REE-rich diagenetic francolites described here
provide a potential means of investigating the migration of
fluids through a hydrocarbon reservoir during diagenesis,
and their influence on bulk sample REE profiles. If apatite
crystallization was relatively early during diagenesis then
variations in REE profiles between different authigenic
francolite samples within the Statfjord Formation may be
useful as a tool for reservoir correlations. Alternatively they
may hold information enabling reservoir connectivity during
diagenesis to be assessed.
J.B. gratefully acknowledges financial support from Shell UK.
A.C.M. contribution to this paper is published with the approval of
the director of the British Geological Survey (NERC). M,
Dalrymple and other members of the Statfjord Research Group are
thanked for informative and lively discussions, and E. Mearns is
thanked for discussion regarding Nd modal age provenance studies.
J. Still is thanked for his assistance with EMPA analysis.
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Received 20 April 1995; revised typescript accepted 23 June 1995.