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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