FORMATION AND DIAGENETIC HISTORY. Yehoshua KOLODNY and. Boaz LUZ. Department of Geology, The Institute of Earth Sciences. The Hebrew ...
ISOTOPE
SIGNATURES
FORMATION
IN
AND
PHOSPHATE
DIAGENETIC
Yehoshua
DEPOSITS:
HISTORY
KOLODNY
and
Boaz
LUZ
Department of Geology, The Institute of Earth Sciences The Hebrew University of Jerusalem Jerusalem,
Most
sedimentary
rocks
Israel.
contain
less
than
1%
of
P.
The
formation of rocks that are highly enriched in P205, and in which apatite
is the main mineralogical
conditions, in
requires
during both deposition and diagenesis.
phosphate
essential
constituent,
rocks
nutrient
is
not
only
of both plants
academic.
Our
special interest
Phosphorus
and animals;
hence,
is
an
repeated
growth of crops on limited land, demands the artificial addition of P to the soil. Application of phosphate fertilizers became an essential
part
scientific
of modern
and economic
agricultural importance
an object of extensive geological,
techniques.
of phosphate
The
combined
rocks made them
mineralogical and geochemical
research for over a century. Research has intensified after World War II, and peaked in the seventies and eighties. these
studies
are
books
(Cook,
1976;
Kolodny, is
a
summarized Cook
1981; Sheldon,
summary
application
of
the
in
and
several
McElhinny,
1981; Baturin, contribution
of isotopic methods
The results of
thorough 1979;
reviews
Bentor,
and
1980;
1982). The present chapter
which
to the
has
been
made
by
study of phosphate
the rich
70
rocks, that
as
well
may
theme
be
as
an
applied
of this
attempt in the
chapter
will
components
of n o n - p h o s p h a t i c
fish
drawn
The
mineral
Both,
francolite
have
dispute.
The
combines
with
lattice.
The
( C O 3 - F ) -3
in
the
a key
imbalance,
additional
x=y+a+2c,
Marine
rocks
in
and
confined
to
Cambrian
m.
in
Cook
phosphogenic
apatite
vertebrates,
Tethyan
and
the
research
be
that
of
province
as
The
and
in
PO4 -3
and
OH-
general
the
distorted ion
and Belov,
F-,
(CO 3" F)y
1940; charge
for P04 -3,
formula
that
,1969;
is
the
all
to the
main
present.
have
shown
the
from
be
Phosphoria the
random,
and
which
and
the
and
form
are
the
Georgina
Formation Upper
Mediterranean,
the
phosphorites
epochs, of
are
distribution
to
that
prominent
U.S.,
ages
The
(Kazakhstan)
Western
mineralogical
seem
phosphogenic
1984).
Phosphorites
geological
(1979)
Permian
y=O.4x on the
Nathan,
not
most
(SO4)x F2
zero to 1.5,
does
of
a
structural
such
of
crystallites
substitutions
phosphorites.
Karatau
in the
(CFA)
extensive
time
the
the
variety
is
practically y. B.P.)
more
composition
both
and Lehr
as
apatite,
tetrahedral
(CO3)x-y-z
specific,
formations
a
apatite
McElhinny
(Australia),
to
from near
to
space
of
is
francolite
proposed.
(PO4)6-x
provinces,
deposits
associated
been
which
of
(2,200
phosphorites
Eocene
other
points
biogenic
chemical
of
substitutions,
referred
sediments
Proterozoic
seems
(McClellan
in
of
range
resolve
and x varies
are
size
for francolite
and b=O.4a.
constituent
to
of
Borneman-Starynkevitch To
have
suggested
of
replacing
1952;
Calo-a-b Naa Mgb
Basin
central
biogenic
rocks
the
subject
small
1980).
Ca 2+ for Na +,
are
The
phosphatic
rocks:
rich
and
extensive
tetrahedron
et
however.
methods
but at several
carbonate-fluoride
substitution
al.,
of
isotopic
purpose.
of
teeth
phosphate
the
very
et al.,
found
study
and
- the
extremely
(Altschuler
average
that
sedimentary
structure
been
Bacquet
where
the
bones
francolite
specifically of apatite.
and
to
those
brachiopods.
main
has been
future
to
(ichthyoliths),
conodonts,
outline
will be phosphorites,
attention
of
be
to
and
Cretaceous the
Plio-
71
Miocene has
deposits
been
noticed
phosphorites most
of the
(Strakhov,
occur
frequently
It is quite
accompanied that
by the b i o l i m i t i n g by
the
at e x p l a i n i n g
the
strong
problem
relationship
of
organic matter
into
stress
the
rich
areas up,
formed
It was
CO2,
Ever
between
upwelling.
deep
(McKelvey
The main advocate
et
al.,
between
occurrences
shelves
and
upwelling ties,
certainly formation.
of
obvious
requirement neither The
a
many
occur
for
this
among
to
extracted
to
phos-
deep
water
may
form
surface water
genesis
in
warms
in its
sole
The
sedimentary
"chemical Survey
correlation day marine to
sixties
equating
and
is very
seven-
likely
deposits,
condition
realization
marine
Geological
extended
the
have been on
on present
upwelling
a
first
falling
the
such
theory
phosphorite nor
is
to the
1964).
In
-
apatitel
been
whereas
sufficient
reasons
phosphorites
that for
and
surface,
nodules has
high
organic
the
speculations
formation.
to the -
the
phosphorites
Sheldon,
upwelling,
The
sediment
while
group of the U.S.
of p h o s p h o r i t e
of
The P-containing,
rises
of Kazakov's
1967;
phosphorite
it b e c a m e
necessary
most
areas
and
1953,
that
and
use
in d i s s o l v e d
water
water
high which
and P.
the
was
zone.
gradient
the
made
of
of p h o s p h o r i t e
studies
" form has been the p h o s p h a t e
to
and
of
upwelling
phosphate
precipitates
theories
with
of
oxidized
surface
reaching
and therefore
tied
then
-
of P for u n d e r s t a n d i n g
all
suggestion
upon
areas
may be r e d u c e d
(1937)
cycle
concentration
since Kazakov,
intimately
Kazakov
are
p.21),
theories, have
matter
in the photic
cold,
and
cycle
Practically
rich",
of upwelling;
loses
water.
Kazakov's
1974,
decomposition
of the marine
particles
"phosphate
-
association
productivity
organic
that
rich deposits.
recent
formation
oceanic
sediment
A sharp
thus
organic
of
by organisms
sea-floor.
when
out
formation.
from sea-water
masses.
the
interstitial
organism-produced
is
in
importance
phosphorite
phate
between
- raining
matter
(Broecker,
Most
it
can be e x p l a i n e d
of p h o s p h o r i t e s ,
phosphorite
a break
productivity
the
genesis
lithologic
elements
hand,
1980)
and o r g a n i c
of P and Si these
other
Kolodny,
association,
in the ocean.
the
of e x p l a i n i n g
location
latter
between
productivity
On the
1970;
by cherts
the
nature
connection
aim
U.S.
in a c h a r a c t e r i s t i c
likely
biological
southeastern
it
for
were
several:
sequences
with
a is
their (a) clear
72
imprints
of
rather
shallow
water
deposition,
room for upwelling from great depth; formation
was
(Baturin,
1969,
below),
identified
there
nodules
is
coasts
1973;
that
interstitially rather
(Froelich
et
of
Burnett
indications
of sediment,
water
studies,
the
et al.,
many
forming
centimeters bottom
Veeh
were
off
not
leaving
Namibia
and
and Veeh, the
within
the
1988);
and
Peru
1977;
apatite
uppermost
(c)
in
see
of these
than by p r e c i p i t a t i o n
al.,
much
(b) when Recent phosphorite
few
from the
laboratory
Mg has been shown to inhibit crystallization of apatite
from sea-water. precipitation
Thus Kazakov's from
"classical" theory of phosphorite
sea-water
has
been
gradually
replaced
by
a
strongly altered version of it - his compatriot's G.N. Baturin's theory
of
phosphorite
formation
by
ocean
dynamics
(Baturin,
1971a, b, 1982).
Baturin's
theory
envisages
the
formation
of
a phosphorite
rock in two principal stages: (I) the formation sediment,
of apatite pellets,
which
by growth from interstitial water,
high sea stand,
by remobilization
form within the
during a relatively
of organic matter phosphorus.
The apatite pellets are at this stage sparsely distributed within the sediment; (2) the formation
of a phosphorite
a mechanical concentration stage. stand,
as
It is tied to a lower sea level
and is a stage of winnowing
to a major long
finally
as
rock component. I0
Ma
resulted
could in
the
rock, which is principally
in which apatite
Compton et al. have
separated
enormous
(1990) the
two
phosphorite
is enriched
suggested that stages
deposits
which of
the
southeastern U.S. Even
in
between
Baturin's
version
phosphorites
of
phosphogenesis
and upwelling
relationship has been established,
remains
the
strong.
connection Once
such a
it may be used both as a means
for explaining the origin of phosphorites,
or in its inverse form
- the distribution of phosphorites may be used for the tracing of past areas of upwelling. J.
Parrish
and
Parrish et al., in
Paleozoic
her
The latter approach has been applied by
co-workers
1982) oceans.
in
recent
years
for the reconstruction The
relationship
(Parrish,
1982;
of upwelling
zones
between
phosphatic
sediments and upwelling also encompasses the other bioproductive
73
constituents rocks.
It
related 1982).
of
is
to
not
occurrence
for
surprising
southeast
United type
ocean
dynamics
1984;
Compton
Current
States.
could
et al.,
observations.
In
organic
as
deposits
the
most
well
in large
(Waples, where
no is
phosphorites
of
proposed
explained
by
been
found
can
young
rich
have
in areas
upwelling
be
models,
these
invoking
paleo-
scale u p w e l l i n g
(Riggs,
1990).
of p h o s p h o g e n e s i s
two
rocks
are
are
result
The t w o - s t a g e of
Such
phosphorites
that
ideas
occurrence
context.
and
phosphorites
source
west-coast-type
in this
"east-coast"
- chert
that
of p h o s p h o r i t e
simple,
problematic the
hence
sequences
petroleum-producing
The
evidence
sedimentary
model
are
based
of Baturin
essentially
different
upon
petrological
is s u p p o r t e d types
of
by the
phosphatic
rocks. I. P h o s p h a t i c common 1987;
in b r o w n 1990).
contain bear
rocks
amounts
distinct
evidence
contrast
rocks
winnowing
shales
pellets
or carbonates
are laminated,
of having
been
lenses
(Garrison
organic
carbonate
and
rich,
and/or
of such
rocks
et
al.,
and u s u a l l y silica.
They
formed
in a
diagenetically
The P205 content
are
does u s u a l l y
a few percent.
In
usually
nodules,
of b i o g e n i c
environment.
not e x c e e d 2.
or black
These
large
reducing
micronodules,
to
which
these
bear
clear
and enrichment.
in o x i d i z i n g
rocks,
economic
evidence
Phosphorites
conditions.
They
of
were
phosphorites ciastic
most
show m e c h a n i c a l
are
reworking,
probably
sorting
formed
and traces
of boring. Most
apatite
in sedimentary
rocks
sometimes
termed
phosphorites, described, grains
are
francolitic internal "ovulites",
concentric
structure
grains
are
fossil
rock
that
phosphorite"
but it
is The
just
applied.
fragments. sometimes
cement
to
are
usually
Fine
are
common common
reach as
matrix
a in
grained
were
ovular, referred
the type
an
such
with
no
to
as
internal
term of
oolite
apatitic
in p r a c t i c a l l y
such p r e d o m i n a n c e "bone
also
of p h o s p h a t i c
When
in the pellet,
These
or
are
Another
they
referred
which
"pellets"
is o b s e r v e d
is u s u a l l y
phosphorites,
or
The m a j o r i t y
Those
structure,
"ovules",
"ooid"
pellets.
as grains.
phospho!utities
but are by far less common.
discernable
or
occurs
bed rocks
"
or is
all
in the "bone usually
74
calcitic,
sometimes
- francolitic. skeletal (e.g.
mass
fragments
has
1888;
been
mortalities
studies
phosphate
by
as
interpreted,
fish
and
and
Thus,
sulfur-oxidizing California,
have
in phosphate
Formation
of
rich
in
Cretaceous
East
Australian
phosphorites
continental
Champetier,
1983;
Soudry,
and
(e.g.
1985)
Privet
The
understanding
an
apparently
other
hand,
oceanic
composition repeatedly formation changing approach.
of
a
these
oceanic
identified
in sediments
They suspect Miocene
phosphorites
off
similar Monterey
(Israel)
identified and
(Soudry
of studies
demonstrated
in and
by Lucas bacterial
FOR THE ORIGIN OF PHOSPHATES
under
of dual nature.
and
In this
set
of
rocks,
environment.
apatitic
aspect
task
chemical
is
conditions but
rather
Table
in
fossils
the
apatite
which
phosphatic
On one hand,
we aim
stages of phosphogenesis,
challenging
sedimentary
recurring of
(1983)
Negev,
conditions
and
phosphorites
of
recent
biological
the
of the different
difficult
conditions.
in
as
the
A sequence
of
of Ca carbonate by apatite.
form is a challenge
a better understanding
In
were
margin
1987).
ISOTOPES AS RECORDERS
sediments
of
aspects
structures
experimentally
in replacement
STABLE
Bacterial
Later
cases of apatitic
Beggiatoa of
and
pellets
formations.
Reimers
sediments
1957) o
chemical
identified
and
cause
was related to
apatitic
more and more
been
of the genus
California.
both
mediation
the
the
and from the Gulf of California.
structures
investigators
as reflecting
inorganic
Williams
bacteria
some
formation
stressed
principally
cements
precipitates.
1969)
viewed
years this tide has turned again: pellets
by
and apatitic
(Brongersma-Sanders,
Kazakov
formation
rocks
phosphorites
case phosphorite
of
influenced
between
Gulbrandsen,
In the extreme
phosphorite
and in the high grade phosphorites
The association
Penrose,
effect.
siliceous,
I is
not
a
which it
itself. are
On the
records
(and
reflection result
reflects
a preview
of
isotopic)
in the
of
of
a
the ever
such
an
75
Table
I:
Isotopes
Element
in apatite:
what
Measured Parameter
Genesis of phosphorites
Sr
87Sr~6Sr
Sr - Stratigraphy
Nd
ENd
do t h e y
tell
us?
About the world
Ca-sites
Land-sea distribution
U(IV)AJ(VI) 234U/~SU 230Th/234U 231pa/235U
Age, Rate of growth, Redox, Submarine weathering
818Op, 818Oc
Temp. of formation,
PO4 sites O
Paleoclimates
T of diagenesis
All
stable
time
C
613C
Diagenetic mode
S
534S
Diagenetic mode
and
measuring
as trace
other
devices
constituents
isotopic itself,
sedimentary
may
attempt
S refer
However,
to c o n s t i t u e n t s
structural
trace Thus,
elements as
the
charge
its
above,
divalent
rare
accompanied
being
earths by
a
feature
by
to
tracers,
summarize
element our
by
carbonate balanced
may by
a
or t e t r a v a l e n t
U
charge
element,
conclusions
of
number coupled
balancing
such as Na + for Ca 2+ or CO3 2+ for F-" Below,
dffferent
that
elements
a large
lattice
difference
trivalent
Ca 2+
to a c c o m m o d a t e in
explained
Similarly, for
several
such as U, Sr and Nd are c o n s t i t u e n t s
is the ability
the
of CFA
components,
latter
or
occur
whereas
The unique
substitute
shall
rocks.
deposits
to the lattice.
ion.
the
of p h o s p h a t i c
by or a d m i t t e d
phosphate,
treat
as t r a c e r s
These
apatite
substitution
used
in the lattice.
substitutions.
fluoride
C and
are
elements
different
replace
that
study
of its major
tracers
some major
are c a p t u r e d
in the
of O,
a part
isotopic
isotopes
in the apatitic
composition being
replace
of
radioactive
into
and an
we shall then
we
integrated
76
picture of what isotopes can tell us about phosphorite
formation
and about the world in which they formed.
Oxvaen IsotoDes
Oxygen occurs in apatite as four oxyanions. The major site is in the phosphate and hydroxyl.
separated by
Oxygen mineral,
reaction C02
the
ones
sites: 1950;
phosphate
can
in carbonate,
simple to
carbonate
of the mineral
(McCrea,
be
in apatite
and
analyzed
sulfate
isolate the O
with phosphoric
Kolodny
acid,
Kaplan,
by
can be and
1970b).
dissolving
the
precipitating the phosphate as ammonium phosphomolybdate
and Mg-phosphate to ensure purity, phosphate
(Tudge,
Karhu
Epstein,
1986).
does
exchange
and
phosphate solutions. and
are
it is rather
two of these
of
in
the other
Fortunately
from the major evolution
ion,
the
1960;
not
Hence,
and finally precipitating Bi-
Longinelli,
1965;
Throughout
Kolodny et al.,
this
isotopica!ly
procedure,
O
1983; in the
with O in the aqueous
the final BiPO4 precipitate can be fluorinated
resultant
O
analyzed,
assuming
that
it
adequately
represents the isotopic composition of the original apatite. recently
developed
Ag3PO 4 is used,
procedure
is less time consuming,
more precise
results.
of phosphate
0 as
the
a geochemical
Cretaceous, isotopic the
first
they
phosphate
and
(1991)
as
old as (1951)
its
indicator
of ancient
carbonate
in
is almost
estimates
dependence
of this problem
(1965)
The
on
the
sea water.
sediments.
The
of
the
knowledge
Their
first
measurements
of phosphate
were (1960).
in carbonate
Upper of the
suggestion
was the analysis
using the procedure of Tudge
trace amounts
yet
When Urey and his co-workers
fractionation
made
by
of
O
of water
Longinelli
Longinelli
shells,
for
of coexisting
between the two solid phases would then be independent composition.
The
in which
and yields identical,
paleotemperature
noted
composition
elimination
et al.
The idea of using the isotopic composition
stable isotope geochemistry. presented
by Crowson
analyzed
measured 8180 of
the environmental water in which they deposited their shells, and determined the growth temperature from 8180 of the carbonate. initial
efforts
of
this
experimental difficulties,
pioneering
research
were
marred
The by
and it was not until eight years later
77
(Longinelli and Nuti,
1973), that the following phosphate - water
temperature equation was proposed:(1) t°C
where ~
(1983)
111.4 - 4.3
(1)
(~ - ~w)
refers to the 5180 of O in the phosphate and ~w to that
of the water,
fish.
=
both on the SMOW scale
confirmed
Eq.l
by
analyzing
(Fig. bones
la). and
Kolodny et al.
teeth
of
living
(Fig !b).
A major advantage of the phosphate O paleothermometer is that isotope exchange between phosphate O and water is extremely slow in inorganic
systems,
catalyzed reactions inorganic
exchange
laboratory several
but
is very
(Kolodny et al., is best
method
rapid
for
in biochemical
1983).
enzyme-
The sluggishness
demonstrated by the validity
apatite
isotopic
analysis:
steps of dissolution and precipitation,
it
of
of the
involves
some of them at
elevated temperatures.
o
28
28
24
24
20
2O
16
16
0 12
12 o • 8
8
4
4
•
0 t8
,
4
,
~
20
,
i
22 p
,
24
-= 26
•
m
b
0 18
2O
22
w
24
p
26
w
Figure 1: Temperature vs. (Sp-6w) of mollusc shells. After Longinelli (1965) and L o n g i n e l l i and Nuti (1973a) . (b) Average temperature vs. ( ~ - ~ ) of fish bones and teeth. Data from Longinelli and Nuti (1973a) and Kolodny et ai. (1983). Data points in which the temperature range is larger than 5°C, were rejected. After Luz and Kolodny (1989) Throughout phosphate
those
in apatite
laboratory fluorinates
steps
the
isotopic
is preserved
precipitated in
the
BiPO4,
procedure
composition
and remains which
given
is by
the
of
unchanged
in the
precipitate
Tudge
(1960).
the
one This
78
sluggishness apatite
suggests
and water
at
that
isotopic
exchange
low temperatures
even on a geologic time scale.
between
should be
extremely
On the other hand,
biochemical mechanisms have shown
when
enzymatically
catalyzed.
biomineralization
Although is
not
the
exact
clear,
1977)
exchange
of O between
water
and phosphate
apatitic shells. Kolodny et al. of this
exchange
bones
is
determined
teeth,
and
growing
in a pond
the isotopic composition of the by
the
temperature-dependent
and
water.
apatites
the properties
recorder:
in bones,
with trout
only
between
aqueous
probably affect
bone-phosphate
fractionation have
of
reactions
and 6w. The trout were fed marine fish
meal with much heavier O. Still, trout
is rapid
(1983) demonstrated the high rate
in an experiment
with constant temperature
of
that
mechanism
similar
catalyzed by such enzymes as alkali-phosphatase,
slow
researchers
(e.g. Boyer at al.,
isotopic exchange between water and dissolved phosphate
phosphate
solid
required
for an
Thus,
ideal
biogenic
geochemical
they respond sensitively during their formation to the environment,
and they
preserve
very
well
their
record
after death of the organism.
30 -0
t
O O
o
cO
20
~+:" . ' ~ b :
.
"
-20
W-J
o
-40
0
0 •-
D
10
[]
[] 0 0
0
0
.......[]+ FreshMarine '
'
' 100
+
'
'2o-o'
'
'3+o'o
'
'
+ 400
+
Age (Ma) Figure 2: The secular change of ~180 in phosphate of marine and fresh-water Luz (1992) The
fish;
calibration
of
Devonian to Recent.
the
phosphate
After Kolodny and
thermometer
enabled
the
application of this method to the fossil record. This application included
the
analysis
of ~p in conodonts
of the
North
American
79
Paleozoic
(Luz
Cretaceous
and Tertiary
countries
et
al.,
1984), of Israel
(Kolodny and Raab,
information
about
of and
1988).
temperatures
fossil
from
the
from other Mediterranean
The
that
fishes
latter
analysis
prevailed
during
yields
the
Late
Mesozoic-Early Cenozoic in the tropical parts of the Tethys. thermal
record
carbonate
O
is well
isotope
correlated
thermometry
Pacific
(Douglas and Woodruf,
record
for
Northern
temperatures
with
of
1981)
Europe
the
record
foraminifers
The
obtained
from
the
by
deep
and with the contemporaneous
(Loewenstam,
1964).
The
deduced
for the Cretaceous range between 20°C and 34°C, some
I0 ° higher than Cretaceous surface water temperatures
in Northern
Europe. A survey of the isotopic record of phosphate in fish from their earliest appearance
in the geological record to the Recent
has recently been completed a plot
of 8p in fresh
million values This
years. which
is
not
persistence
and marine
this
time
only
a
of
fresh
demonstration
the
in apatite
temperature,
1992). Figure 2 is
fishes
in the
water
of
cycle
essential
the
of
O. No clear time trend
fish.
long
it but
post
400
had 6p
marine
very
as we know
lack
last
fishes
lower than 8p of contemporaneous
of the hydrological
reconfirmation exchange
Throughout
are
(Kolodny and Luz,
water
term also a
depositional
in 8p, and hence
in
is evident from figure 2.
Much effort has also been directed towards isotopic analysis of phosphate of non-skeletal origin. Shemesh et al. that
ichthyoliths
Formation rocks.
have
in
the
phosphorites
of
In a study of phosphorites
Shemesh and Kolodny
phosphorites
close
1988). Furthermore, of
event
correlated
interpreted similar
shows with
an onset
conclusion,
phosphogenic (1987,
Israel
as
those
of
(1988) fish
an
the
major
increase
of an upwelling
i.e.
province,
that
the
has
onset
been
Mishash
from the
same
showed that 8p of
(Kolodny
a stratigraphic analysis of ~
Formation is
to
Campanian
pellets
showed
from the Tethyan-Mediterranean
Mesozoic province, were
the
same 5180 as apatitic
(1983)
Raab,
in the Mishash
phosphorite-forming
in
8p, w h i c h
regime
of upwelling
proposed
and
by
in the in the
Grandjean
1988) on the basis of REE pattern analysis.
can
be
area.
A
Tethys et
al.
80 Longinelli and Nuti phosphorite Similarly, Recent
whole
(1968)
rocks
with
Shemesh et al.
phosphorites
noted a trend of decreasing ~
(1983)
varies
increasing
geological
in
age.
have shown that whereas 8180 in
between
20
and
25
per
mil,
it
decreases to 15-20 per mil in Mesozoic rocks, and drops further 24-
m
L B
20 J Ii
16
e~
§
•
O
,,,|
u
Him m
m
12
'
n
0 I~
" ~0
•
•
' $00 " 700 " 900 "II00
1300
l
•
1500
!
1700 '1900
AGE(Ma)
The s e c u l a r change of 8180 phosphorites. After Shemesh et al. (1988) Figure
to about been
in
3:
i0 per mil in the Archean
observed
for
8180
of
carbonates,
cherts
1962; Knauth and Epstein,
O'Neil,
alternative
1984).
the
Four
isotopic
exchange
with
time
fresh
trends:
water,
of
(Fig. 3). A similar trend has
(Degens and Epstein,
for
phosphate
glauconites
1976; Keppens and
explanations (i)
and
have
been
post-depositional
(2) changing
8180 of
offered isotopic
sea-water
with
time,
(3) changing temperature of deposition of phosphorite with
time,
and
(4)
explanation (1962)
and
was
high
temperature
favored
for
for phosphorites
alteration.
carbonates
by
by Longinelli
Whereas Degens
the
and
and Nuti
first
Epstein
(1968),
the
main proponent of an ocean with changing isotopic composition has been E.C. Perry and his co-workers Shemesh
et al.
strengthened
(1983)
(e.g. Chase and Perry,
rejected the
in their belief
first hypothesis;
in the
resistance
1972).
they were
of phosphate
to
late isotopic exchange by evidence such as the narrow range of in the Mishash
apatites
compared
coexisting carbonates and cherts
to the broad
range
(see also Vengosh et al., 1987).
Hence they favored higher temperatures of deposition phosphorites.
Such
higher
of 8180 in
temperatures
reflection of a global phenomenon,
i.e.
could
be
for ancient either
of warmer oceans
a
in the
81
geological
past,
formation been
in
debate
and
Epstein
warm
as
in
the
for
has however (1976)
et
the
of
phosphate
early
Precambrian
value the
of the
past
Karhu,
ocean
has
of
(see
If
chert then
and the
phosphate two types
and 8c the 8180 of C032-) In such
chert They
were
of K n a u t h
have
conclusion
varied
more
also
Perry,
that
as
that
of
and
cherts
the
that:
very " The
80°C and decrease
was
than
the
Karhu
Phanerozoic
and
as
of
water
by
concluded
as h i g h
been
by a n a l y z i n g
taken
and
has
1977);
resolution
approach
the
Ma
(Savin,
or 8180 of ocean
that:
"The
8180
mil
over
a few per
1990,
carbonate
the
and
Epstein
and
any
attempt of
(1986)
pointed
entire
temperature
marine
fractionation
The a broad
to
out
calcite
precipitated
of the slope
water
estimate
range, is
coefficient
marine
i0
per
mil
whereas
I)
et al.,
as
to the
1953)
(2)
in the carbonate observation
paleotemperatures
is
Karhu
and
impossible
at room t e m p e r a t u r e
same water,
for such a
to b e h a v e
a disappointing
composition.
because
coexisting
(Eq.
(8c - 8w) 2
such p a r a l l e l i s m
about
from the
was
isotopic
that
two
candidates
(Eq. 2, after Epstein
paleothermometers
independently
as
(Sp the 8180 of PO43-,
paleothermometer
(8c - 8w) + 0.14
similarity
and p h o s p h a t e
treated
8c of C F A - C O 2 is a s s u m e d
phosphate
t = 16.5 - 4.3
be
may be even better
paleothermometer
The striking
can
of O in apatite
a treatment,
in C a C O 3. C o m p a r e
for
i00
1990).
phases,
pair.
in
that
not
years"
past
might
The
in them.
temperatures
corollary
3x109
oceans
Precambrian
silica
phosphorite
could be a c h i e v e d
is the
analyzed
of
of
in the
1986).
rocks
This
8180
site
by the p r o p o s i t i o n
it was t e m p e r a t u r e
who
A
caused
al.,
in ancient
amounts
time".
the
ocean
Precambrian
small
with
in
of 8180 in c a r b o n a t e s
been
that
phases.
(1986)
both
shift
A warmer
(Veizer
low ~
cogenetic
Epstein
a
on the basis
of w h e t h e r
caused
of
ocean.
60-70°C
dilemma
O
the
suggested
much
or
higher
over
the
the 8180 of
than
they m u s t
Epstein
apatite
achieve
zero
paleothermometer
over
at high temperature.
establishment temperature
of a p h o s p h a t e - w a t e r range
requires
experimental
equilibration
of
82
phosphate results
water
pairs
(Shemesh
et al.,
10001n~
where
10001n~
slope
here
analogous
at
different
1988),
= 2.12
of 2.12
should
yielded
and
T
subtracting
(3)
paleotemperature
again
Shemesh
et
(4) ,
because
the
than are
(1983)
the p h o s p h a t e very
well
depositional sites
O
one.
exchange
be
rature
and ~w can be estimated. and
2.12/2.78) isotopic
of
from
each
figure
lower
above had
100°C. to
be
therefore sites
as an overlay
surface
The
water
well
above the
between
interaction
zero.
isotopic of late
values. diagram
occur
then
a
8c a n d 8p
that
equal)
Shemesh
et
diagenetic
both
0.76
in the
most
well
samples, concluded
both
apatite
metamorphic?)
It is also
at a r e l a t i v e l y
the
data points
(1988)
(to early
be
upper
within
these
in
=
Assuming
6180 can
temperatures
0
If
tempe-
of
well
al.
and fluids.
for
(with unity
points
with
of
post
degree.
and
8180w
if
both
slope
However,
composition
took place
that
so
3 isotherms
in e q u i l i b r i u m
the apatite
exchangeable
shown
indicate
of
more
formation
and
a
8 c - 8p p a i r ,
4),
Most
is
Initially,
on the data points. of
temperatures
of the
6w of
which
the
pairs,
618Oc - 8180p pair.
earth
is a r e f l e c t i o n
such
temperature
use
(with
from the
(5)
necessarily
8180w
The
independent
it must
In figure
constant
4 yield
(5)
it was
equilibrium
left part
that
interactions that
of
are p l o t t e d
range of r e c o r d e d in the
are
equilibrium,
estimated part
lines
of 2.78
(Sx - By) .
(Fig.
°K.
(4)
much
assumed, not
8p of
to be
however
(though
4
to
correlated
8c a n d
slope)
figure
appeared
is to
in a c o m p a r a b l e
about
in
1969):
obtain
reluctant
Later
linearly
et al.,
(106/T 2) + 0.09
equals
were
carbonate
temperature
in 8180p - 6 1 8 0 c space,
10001n~(x-y )
equation:
(3)
w i t h that
we
Preliminary
following
(106/T 2) - 2.89
from
equation
al.
the
(O'Neil
10001n~(c- p) = 0.66
where
is
be c o m p a r e d
for calcite
10001n~(c- w) = 2.78
By
the
(106/T 2) - 2.98
(~p - 8w),
equation
temperatures.
suggested
low w a t e r / s o l i d
83
ratio in a semi closed system of pore waters. We can conclude by stating
that
phosphorites,
although
temperature
may
be
recorded
by
it is not always the depositional temperature. Most
phosphorites as well as phosphate bearing rocks may thus not tell us much
about
temperatures of ancient
seas
(Karhu and Epstein,
1986). Rather they tell us about conditions of burial. conclusion
The latter
is supported by the location of phosphorites
Miocene Monterey Formation on figure 4 (Kastner et al.,
50
•
,
,
t
o/
from the 1990).
/1
'/' //
¢- ,.>sH /
25
./
0
/
20
t.,O
/
15
/
ci
0
.2"./O~zo
//
// ~o o"
/. / ~ -
o
,o/- d
