PajaFormation(Colombia)bylossonignitionand CHNS analysis ...
Transcript of PajaFormation(Colombia)bylossonignitionand CHNS analysis ...
島根大学地球資源環境学研究報告 21,9~16ページ(2002年12月)
Geoscience Rept.Shimane Univ.,21,p.9~16(2002)
囮 Organiccarbonand carbonatecontentsofb且ackshalesfromthelower Cretaceous
PajaFormation(Colombia)bylossonignitionand CHNS analysis:comparisonofmethods
N.O.Campos*,B.P.Roser*and Y.Sampei*
Abstract
Organic matter(and hence organic carbon)and calcium carbonate contents of soils and sediment can be estimated by
simplelossonignition(LOI)deteminations,attemperaturesranging丘om375to550℃and850-1000℃,respectively.
Organic matter(OM)andcarbonate contents of24carbomceous black shale samples were dete㎜inedby LOI and CHNS
methods to compare these two techniques.Two LOI tests were c飢Tied out to dete㎜ine the ignition times required at
550℃and1000℃to ensure complete combustion ofthe OM and carbonate components.Results showed4to6ho皿s of
exposure at550℃were required to ignite the OM,and3ho皿s further ignition at lOOO℃ensured decomposition of the
carbonate.A complete set of sample were then ignited under these conditions,and the results compared with those ffom
CHNS analysis.Although the results correlated well,LOI550values(OM)were consistently greater than C。,,by CHNS.
COrreCtiOn Of the LOI data uSing a generiC OM tO C。,、COnverSiOn faCtOr COmmOnly applied in the literatUre did nOt
improve the results.A third experimental mn with LOI detemlinations at375℃,550℃,and1000℃showed a strong
correlation between LOI375and C。,、(CHNS),but relatively constant loss of weight(2-4wt%)between375℃and550℃
irrespective ofC。,、content.This suggests most ofthe weight loss below375℃was due to combustion of OM,whereas
that between375℃and550℃was struct皿al water lost ffom clays.Nevertheless,ignition at either375℃or550℃can
giVe reaSOnable eStimateS Of C。,、Ontent,prOVided CalibratiOn data fOr indiVidUal Sample SUiteS iS available frOm an
independent technique such as CHNS analysis。The results support proposals that use of generic OM to C。,,onversion
factors is inappropdate.Test ofthe viability ofcarbonate content dete㎜ination was hampered by the non-calcareous
nat皿e ofmost ofthe samples.At low levels,neither LOIlooo nor CHNS seem to give good results.This is probably due to
continued loss of stmctural water and other volatile phases,coupled with weight gains through oxidation of species such
as Fe2+andMn2+.
Key words:Black shales,LOI,CHNS,organic content,carbonate content.
lntroαuction
Loss on ignition method(LOI)is often used as a simple
means of determining approximate organic carbon(C。,g),
organic matter(OM),and carbonate contents of soils,
sediments and sedimentary rocks,particularly when they are
Ca1CareOUS and Clay-pOOr (Ball, 1964; Dean, 1974;
Sutherland,1998).The method includes a first heating at
375to500-550℃to ignite the OM,followed by a second
ignition at950-1000℃to release carbon dioxide ffom
carbonate minerals.Contents・are derived丘om gravimetric
data.However,many caveats s皿round the method.A
number of authors(Sutherland,1998;Heiriαα1.,2001,
among others)point out that losses of other compounds
may also occ皿 (e.9.volatile salts,stmctural water,
inorganic carbon, pyrite), depending on the ignition
temperat皿e.Such losses can thus influence the accuracy of
the C。,g and carbonate detenninations.Heiri8∫α1.(2001)
further noted that LOI is also influenced by heating time
and sample size.
Chromatographic CHNS elemental analyses can also be
USed tO aCCUrately meaSUre Ct。t,C。,g and CarbOnate COntentS,
and to detemine the composition of the organic matter
(Minoruαα1.,1999;Pimmel&Claypoo1,20011Sampei&
Matsumoto,2001).In this method samples are combusted
in a reactor at1000℃in a temporarily enriched oxygen
atmosphere.Temperature reaches1800℃d皿ing flash
combustion.The reaction products are passed through a
glass column packed with a tungsten trioxide and copper
reducer to form CO2,N2,and SO2.These products are then
analyzed chromatographically,by calibration against an
organic standard.
This paper examines the OM,C。,g and carbonate contents
of a suite of black shales from the P勾a Formation of
Colombia,as detemined by both the LOI and CHNS
methods.The primary p皿pose is to compare the results
obtained by the two techniques,and detemine ifthe LOI
technique is apPropriate for rocks of this type.
Geologic Setting
*DepartmentofGeoscience,ShimaneUniversity,Matsue690-8504
The Eastem Cordillera of Colombia is a NNE-SSW
trending fold belt composed of Precambrian to Paleozoic
basement covered by a thick sequence of Mesozoic to
9
10
Organic carbon and carbonate contents ofblack shales from the lower Cretaceous
P句aFomation(Colombia)bylossonignitionandCHNSanalysis:comparisonofmethods
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50
■StudyArea
Fig.1.Geology of the northem segment of the Eastem Cordillera
and location of the study area(modified from Cooperαα1.,
1995).
Tertiary sedimentary rocks.The Eastem Cordillera formed
during Miocene-Pliocene times by inversion of two Late
J皿assic-Early Cretaceous basins,the Tablazo-Magdalena
basin to the west and the Cocuy basin to the east.These
former basins are separated by the Santander High,which is
characterized by the occuITence of faults running Parallel to
the general trend of the cordillera itself (Colletta 8∫ α1.,
1990,Dengo&Covey,1993,Cooperαα1.,1995).
The study area is located in the Velez-Arcabuco
anticlinorium in the westem flank of the East Cordillera
(Fig.1).Micritic limestones of the Valanginian-10wer
Hauterivian Rosablanca Formation are conformablyoverlain by siliciclastic rocks ofthe Hauterivian-Aptian P司a
Fonnation (Royero & Clav麺。, 2000; and references
therein).The present study focuses on surface samples of
the Paja Fo㎜ation.This sequence consists predominantly
of organic-rich black shales,and minor intercalated silty,
very fine-grained sandstones,massive organic-poor beige
shales, and occasional massive organic-poor green
mudstones.The black shales are generally massive,but
occasionally show very thin lamination and foliation
parallel tO the OVerall Orienta.tiOn Of the Strata.TheSe
laminations usually consist of fine-grained quartz or other
tenrigenous detritus.
ground using a ROCKLABS(Model RC)ring millequipped with a100g capacity tungsten carbide head.The
method used followed that described in Roserαα1.(1998).
Chips showing any sign of oxidation,alteration or
weathering were discarded.Maximum mill times were
30seconds.Subsamples ofthe powders were stored in glass
vials and held at110。C for at least24hours prior to LOI
determinations.
Powders for the CHNS analysis were prepared by
manually crushing10-20g subsamples of the original hand
specimens to<1mm,followed by grinding in an automatic
agate pestle and moltar for10minutes.The powderedsamples were stored in glass vials and oven-dried at70℃
for24hours prior to analysis.
Methoαology
Cr配sh∫ng
Samples for LOI deteminations were chipped,washed in
distilled water,and dried at110℃for24h.They were then
Lossonなni∫ionD8∫εrn¢inα∫ions‘LO1フ
The parameters used to determine organic matter and
carbonate contents by the loss on ignition method vary
widely between studies.For determination of OM,ignition
temperatures used vary between375and550℃,with
ignition times ranging between one and seventeen hours
(Ba11,1964;Dean,1974;Sutherland,1998;Heiriαα1.,
2001; and references therein). For carbonate contents,
temperat皿es used range between850and1000℃,and
times between30minutes and two hours.The method
suggested by Heiriαα1.(2001)for assessing organic matter
and carbonate contents re(luires four hours of exposure at
550℃to ignite the organic matter,and two ho皿s further
ignition at950℃to complete combustion of the carbonate
component.We used the550℃temperat皿e for assessing
organic matter following Heiriαα1.(2001),but carTied out
thecarbonateignitionat1000℃ratherthan950℃.
Wefirstmadetwo tests to determinereliable combustion
times for each component.In the first test,five samples
were selected at random and processed individually.After
ovemight oven drying at1100C the samples were ignited at
550℃to determine OM content.Weights of the crucibles
were taken every hour until the ninth hour,with the
crucibles retumed to the fumace after each weighing.Each
ho皿一long ignition was timed from when the fumace
regained550℃.The samples werethen ignitedfor afurther
12hours at550℃,and the weights again meas皿ed.The
fumace temperat皿e was then raised to1000℃.After
constant temperature was reached,the samples were ignited
for another two hours,cooled in a desiccator,and
reweighed.The crucibles were then retumed to the furnace
for another three ho皿s.Weights were remeas皿ed every
hour,asinthe550℃experiments.
In the second test,five duplicates of sample PES-31
varying in net weight from2.7to3.4g were ignited using
the same conditions as above,to test the influence of sample
Slze.
Following the above experiments,a complete set of
24samples was ignited using optimal ignition times as
determined from the trial samples.Percentage loss on
N.0.Campos,B.P.Roser and Y.Sampei 11
ignitionat550and1000℃aregivenbythefomlulae:
(CSH・一CS55・)一(C-Ci)
%LO155。= *100 Sllo
(CS55・一CS1。。。)一(C-Ci)
%LOll〔)。。= *100 SIlo
where CSニweight of crucible+sample;Cニcrucible
weight before ignition;Ci=crucible weight after ignition;
Sニsample weight(after Heiri8∫α1.,2001).LOI550thus
corresponds approximately to the amount of OM in the
sample,whereas the corresponding value at1000℃ is
related to the carbonate content.
Following the above,the CHNS analysis,and preliminary
interpretation,we ca皿ied out a further experiment(test
three)on a selected subset of samples.In this test,the
samples were first ignited at375℃for16hours(after Ba11,
1964;Dean,1974),followed by six hours at550℃,and
three hours at1000℃.Weight losses were dete㎜ined at
each step.
C研VSαnαlysis
Total CarbOn(Ct。t),OrganiC CarbOn(C。,g),H,N,and S
contents ofthe suite of24samples were determined using a
CHNS automatic elemental analyzer(FISSON,EA llO8).
Total carbon contents were first
powdered samples apProximately
Accurately weighed samples were
capsules,which were crimp-sealed
dete㎝ined, using
10mg in weight.
placed in tin foil
prior to analysis.
Organic carbon contents were detemined in a separate run.
For these analyses lO mg ofrock powder were placed in Ag
foil containers,and the samples treated by addition of l N
HCl to dissolve the carbonate fraction.If samples reacted
strongly due to high carbonate content,HCl was added
repeatedly until reaction ceased.The Ag capsules were then
dried on a hot plate for45 minutes at110℃,and
subsequently sealed.They were then placed within tin
capsules,which were sealed in tum.
FOr bOth the Ct。t and C。,g analySeS,prOdUCtS Of the
combustion were calibrated against BBOT standard[2.5-Bis
一(5-tert.butyl-benzoxazol-2-YI-thiopen)].The Eager 200
software was used to run the equipment,and to store and
manipulate the data.TOtal CarbOn(Ct。t)and OrganiC CarbOn
(C。,、)COntentS Were Obtained direCtly frOm the eqUipment,
whereas carbonate carbon(Ci。。,g)was calculated ffom the
difference between C、。t and C。,g.The error inherent in this
type of analysis is about±3%relative.
Results
Lossonlgni∫∫on
Results for the first test show that weights of the two
samples with lowest LOI(PES-9,PES-14)stabilized after
only three hours ho皿of combustion at550℃(Fig.2).No
significant additional weight loss was observed after
12hours further exposure at that temperat皿e.In contrast,
for samples PES-1,PES-25and PES-35weights did not
stabilize until six hours had elapsed(Fig.2).In the second
test,weight losses converge after six hours ofcombustion at
550℃,and are identical after nine hours(Fig.3).No
significant weight loss was caused by a further12h
expos皿e at550℃.In both tests,constant weight was
achieved in vi血ally all samples after six ho皿s at550℃.
However,weight losses vary considerably with less than six
hours combustion.This variability may be attributed to
differences in OM content(test l)and sample weight(test
25
0 5 nU 5
コ ヨ
(ぷ筆)ω8=8一ΦタΦ>蚕コ∈δ
0
550。C
溝
一十PES-31+PES25十PES-1十PES-14+PES-9
051015202530 1gn而on time(hours)
Fig.2.Cumulative LOI(wt%)vs time for five black shale samples
ignitedat550℃(h・urslt・21),and1000℃(h・urs22t・27).
25
0 5 0
2 1 1
(ぷ琶ω8==9睾Φ>穂3∈δ
5
550℃
纏
hitial weights
+2.73g+2.80g+2.939+3.28g+3.43g PES-31
0 5 10 15 20 25 30
1gnition time(hours)
Fig.3.Cumulative LOI vs time for five duplicates of a single shale
sample(PES-31),with differing initial weights.Ignition at550℃
(hours l to21),and lOOO。C(hours22to27)as in Fig.2.
12
Organic carbon and carbonate contents ofblack shales ffom the lower Cretaceous
P司aFomation(Colombia)bylossonignitionandCHNSanalysis:comparisonofmethods
Table1.Loss on ignition and CHNS analyses ofblack shales from the P司a Fo㎜ation,Colombia。
SaNr
LossOnlgnition(wt%)
Tota1 550gC 1000gC Ctot
CHNS Elemental Analyser(wt%)
CorgCinorgH N S
PES-1
PES-2
PES-3
PES-4PES-5
PES-8
PES-9
PES-10PES-11
PES-12PES-14PES-15PES-16
PES-18PES-20PES-21
PES-23
PES-24PES-25
PES-27PES-28
PES-29PES-30PES-31
816739717058806353109434
079879058580087104299267
902913564585544486943060
112 41 1 1 2
405022607485546598347997
249404624945349741056889
101913464484443376843956
14ーハ∠ 41」1 」- 』-
312617120673369965862557
830475434535737352133377
701000000000000000100003
3
560189943964854941877404
795583100457362780431629
156417121040000002300404
』i 」一 ハ∠ 」1 詞i
050196369674583963153694
495563198327241769120909
156437110040000001300304
イー
イー イー イー -
500093684390371088734710
300010001120111010311610
ααααaααααααααααααααααααα
016897693217638404969311
458536455766756406875008
000000000000000010000110
646376191805402028306794
237224222121020131630415
000000000000000000000000
910054000000000000400000
114040000000000000300000
ααααtααααααααααααααααααα
2).As could be expected,the larger the sample and the
higher the organic content,the more time needed for
combustion.
In both tests,increasing the temperature to lOOO℃
caused additional weight loss(Figs.2&3).Most of the
additional loss occurred within two ho皿s at1000℃,
implying that carbonate ignition was largely complete.
Weight loss afterthree ho皿s f皿ther ignition was minima1,
and a pla.tea.U WaS reaChed.
The above tests suggest that six hours ignition at550℃
was sufficient to bum off the OM,and three hours at
1000℃ completed decomposition of the carbomte
component.These times were adopted for the LOI
dete㎜inations of the fhll suite of24samples;results show
thatLOI550was>10wt%in seven samples,with a maximum
value of21.95%(Table1).Among those with LOI550<10%,
the majority yielded values in the range4-6%.In contrast,
LOIlooo values were generally much lower,with only
samples PES-1,一5and-31having significant values(7.83,
30.71,and3.77wt%,respectively).These three samples
also have the greatest CaO contents (5.89, 35.83 and
1.61wt%;Campos,unpubl.data),suggesting the presence
of some calcium carbonate.All other samples have low
LOIlooo(0.25-1.01wt%)and CaO(0.03-1.49wt%;Campos,
unpubl. data), suggesting their carbonate contents are
minima1.
Results of the third test(sequential ignition at375,550
and1000℃)showedthatinhalfthe samples the bulkofthe
cumulative weight loss at550℃actually occ皿red after
ignition at375℃(Fig.4).In three samples the rate of loss
was almost constant through to550℃,and in one sample
(PES-20),the greatest loss in weight occurred during
ignition at550℃.The only significant weight loss at
1000℃was recordedinthe singlehighly calcareous sample
(PES-5),as expected.
40
ハU nU O
3
∩∠ -
(ぷ峯)ω8=昼睾Φ>量コ∈δ
0
375。C 550℃
ゆ一一PES-3一一汕鼈黷oES-8
十PES-20十PES-25十PES-29一一Z一一PES-1
一一「・一一PES-5
一一m二}一一PES-31
a’
1000℃
⑬
1’ .’口
“.・禔轣h一一
一._’一〇一’”’一が 一””
0 5 10 15 20
1gnition time(hours)
25
Fig.4.Cumulative LOI vs time for eight black shale samples of
equal weight(3.00±0.02g)ignited at375℃(hours l to16),
550℃(hours l6-22)and lOOO℃(hours22to25).Solid symbols
-carbonate-poor s&mples;open circles-carbonate-rich.
N.O.Campos,B.P.Roser and Y.Sampei 13
25
0 5 0 5
ハノ で り
(」
ウω≧O⊆OのZ工Q)ぷ峯◎、。Q
0● ●
PES-5
0
●
PES-1
●
PES-31
0●
1:1
y=一3,72+0、89x Rニ0.98
0510152025 0rganic matter(LOI550wt%)
Fig.5.Comparison of OM content based on LOI550with C。,g
content determined by CHNS method.Open symbols(carbonate-
rich samples)are excluded from the regression.
25
0 5 0 5
コ て
ぷ一タ(のZ工Q芯、。Q℃Φ」コωOΦΣ
0
y=一3.18+1.54x R=0.98
●
○
○
○ ●
1:1
0 5 10 15 20 25
Estimated Corg(LO1550/1.724)wt%
Fig.6.Comparison of estimated C。rg from LOI550with measured
C。rg(CHNS amlysis),using the generic OM to C。rg conversion
factor(1.724)commonly applied in the literature(Sutherland,
1998).
CH/VSα加n4αnc8s
CHNS abundances in the suite of24samples show
considerablevariation.Organiccarbon(C。,、)contentsrange
from O.03to16.5wt%,and the greatest C。,、values are all
associated with high LOI550values.Although Ci,。,、varies
from O.00to8.19wt%(Table1),in only one sample(PES-
5;8.19wt%)is it significant;all others are<0.67wt%.
Hydrogen(0.44-LO3wt%)and nitrogen(0.02-0.76wt%)
were detected in all samples(Table1).Nitrogen and C。,、
contents are moderately correlated (R=0.77); this
correlation improves when three sample with both high C。,,
and LOIlooo(PES-1,一5,and31)are omitted from the
regression (Rニ0.89).However,no such relation exists
between hydrogen and C。,、(R=:0.26).Sulphur was deteCted
in only six samples,and with the exception of PES-5
(1.45wt%),even in these samples abundances are low(0.04
-0.40wt%).This suggests that sulphide minerals are not
significant components ofthis suite.
Discussion
As expected from the LOI tests,ignition time and sample
size seem to be the main factors influencing the results,
especially the determinations at550℃ (Figs.2&3).
Samples must be ignited at550℃for sufficient time to
ensure total ignition of the OM component,and hence
preclude any effects on the carbonate determinations at
lOOO℃(Heiriαα1.,2001).This is particularly true for
samples with high organic content.Heiriαα1.(2001)
suggested thatfourhours exposure at550℃was sufficient
to complete ignition of most OM,and that two hours of
ignitionat950℃weresufficienttocompletedecomposition
of carbonate.From our results,we found that six ho皿s
exposure at550℃are required to ens皿e complete weight
loss due to combustion of the OM in samples weighing
<3.3g,and three hours f皿ther exposure at1000℃
guaranteed total loss of the carbonate fヤaction.
COmpariSOn Of LOI550and C。,、(CHNS analySiS)ShOwS
that a strong linear correlation exists between OM content
(LOI550)and measured C。,g when three SampleS(PES-1,一5
and-31)are excluded from the regression(Fig.5).The
compositions of these three samples differ ffom the others,
as noted above.Although the observed correlation is strong,
the intercept is non-zero.This situation is not unusual,as
evident f士om the summary of regression models between
C。,g(dry CombuStiOn)and LOI given by Sutherland(1998);
slopes may also vary significantly between suites.However,
given the strength of the co∬elation,LOI550values in this
Suite Seem tO be reaSOnable prediCtOrS Of C。,g uSing the
equation derived.
COnVentiOnally,C。,,iS eStimated ffOm10SS On ignitiOn
data by dividing LOI550by a conversion factor of1.724,
based on the assumptions that LOI550is related to loss of
OM,and thatC。,g compriSeS58%OfOM(SUtherland,1998;
and references therein).Applying this factor to the P勾a
samples produces overestimates at low values(<5%C。,g by
CHNS)and overestimates at higher values(Fig.6).
Sutherland(1998)highlighted the danger of assuming a
fixed conversion ratio of L724 by summarizing an
extensive body of literature data,and noted that factors
actually ranged from l to as high as l6,depending on the
nature of the OM In his study of Hawaiian fluvial
sediments,he found the factors ranges from4.2to33.9.
Conversion factors in given suites are approximated by the
reciprocal ofthe slope of regressions between OM(LOI550)
and meas皿ed C。,g(Sutherland,1998),yielding a conversion
14
Organic carbon and carbonate contents ofblack shales from the lowerCretaceous
P司aFormation(Colombia)by loss on ignition and CHNS analysis:comparison ofmethods
25
20
5 0
婦『 』-,
器一夕のト。り一〇」
5
0
a.
●
●
111
y=1、57+1」25x Rニ0、99
5 10 15
Co凹wt%(CHNS)
20
25
20
5 0
1 1
訳一夕Reり,8』O」
5
0
b
●
0 25
●
25
1:1
25
0
20
5 0
1 」-
(ま峯)㌍.粁\霞。っ6」
5
て
Corgwt%(CHNS)
20 25
C.
●
1:1
y二1.41+1.01x R二〇、99
20
5 0
イー -
O創工H工雷OΦ,。昭一〇」
5
d
●
●
● ●
y=0.40+0、94x Rニ0.99
●
&
O PES31
1:1
0 5 10 15 20 25 0 5 10 15 20 25
Corgwt%(CHNS) Co四+N+40%H+S
Fig.7.Plots of(&)measured C。,g(CHNS)vs weight loss after16h ignition at375℃(LOI375)in the samples from Fig.4,excluding the
highly calcareous sample PES-5;(b)Weight loss between375。C and550℃in the same samples compared to measured C。,g;(c)
estimated C。,g(LOI375/L12)vs measured C。,g(CHNS);(d)Modelled OM content from the LOI550data in the entire sample suite,
assuming60%Of measured hydrOgen iS lost as water,COmpared to measured C。rg+N+H,esidual+S(CHNS).PES-5and PES-31were
excluded from the regression.
0 0
fa.CtOr Of l.12 in thiS CaSe (Fig。 5). ThiS SUppOrtS
Sutherland’s contention that indiscriminant application of
conversion factors can lead to serious e1Tors in estimation of
C。,g from LOI data.
The non-zero intercept of the relation between OM
(LOI550)and measured C。,g(Fig.5)is most likely due to loss
of water.Although temperatures of500to550。C are used
by many workers to estimateOMcontents(e.g.Dean,1974;
Heiriαα1.,2001),10ss ofwater from clay minerals at lower
temperatures may be significant.Ba11(1964)stated that
most water loss f士om oven-dried clays occ皿s between450
and600℃,and cited work by Keeling(1962)that found
prolonged ignition(16h)at375℃removed90%ofcarbonaceous matter without affecting structural water in
clays.Clay minerals are a significant component of the
shales analyzed here.XRD analysis has shown the clay
mineralogy consists of primarily of illite,chlorite,and
smectite,with lesser montmorillonite and pyrophyllite in
some samples(Campos,unpubl.data).The samples are also
highly aluminous,with up to27.3wt%A12030n a hydrous
basis(Campos,unpub1.data),suggesting that the clays are a
m句or component.
The results of the third test(successive ignitions at375,
550and1000℃)also suggest loss of struct皿al water from
clay minerals during ignition between375and550℃.A
StrOng COrrelatiOn eXiStS betWeen LOI375and meaSUred C、)rg,
and the slope is near unity(Fig.7a).In contrast,weight loss
between375and550℃(LOI,,。.,7,)is more or less constant
(2-4 wt%)irreSpeCtive Of C。,g Content(Fig.7b).This
suggests that the bulk of the loss during Prolonged ignition
at375℃is due to combustion of OM,and a more direct
eStimate Of C、)rg Can be Obtained by ignitiOn at the IOWer
temperature.The correlation between LOI375and C。rg can be
imprOVed by applying the LOI5500M tO C。,g COnVerSiOn
factor(1.12)derived from Fig.5.A plot of the converted
data shows the slope is virtually unity and the intercept is
near zero(Fig.7c).
The laCk Of CO∬elatiOn betWeen hydrOgen and C、,rg
contents noted above also supports loss of water during
ignition.Crude modelling gives some estimate of the
amount of water that may have been lost.If an arbitrary
60%ofthe hydrogen in the CHNS analyses is recast as H20
N.O.Campos,BP.Roser and Y.Sampei 15
8
7 6 5 4 3 2 1
ぶぎ7(ooo』O」)のOQ邸QコΦ一応∈眉ω山
0
勘 ●. ●
●
●
1:1
012345678 Estimated CaCO3(CHNS)wt%
Fig.8.Estimated CaCO3contents in carbonate-poor samples by
LOllooo and CHNS methods.
and deducted from LOI550,and the residual H summed with
C。,、,N,and S tO better refleCt the COmpositiOn Ofthe organiC
matter,LOI550and CHNS results become comparable(Fig.
7d).In the entire sample suite,only PES-5and PES-31
diverge from a l:1relationship to any extent.The latter
sample has the highest H content(L27wt%)as well as high
C。,、(1353wt%),and a differing prOpOrtiOn of the hydrOgen
may have been lost.
The comparison ofresults at375and550℃suggests that
ignition at either temperature can provide an estimate of
OM COntentandhenCe C。,g,prOvided ignitiOn iS OfSUffiCient
duration to oxidize all organic matter.Structural water
appears to be a significant component of the LOI550weight
losses,but the strong correlation with CHNS C。,g still allows
reasonable estimates to be made.Although the results at
375℃giveamoredirectcomparison,thelongerignition
time required limits sample throughput.In either case,
determinatiOn Of C。,g in repreSentatiVe SampleS by an
independent method such as CHNS analysis is essentia1,so
that calibration for individual suites can be made.This is
necessary given the variation in conversion facto「s
highlighted by Sutherland (1998). Generic conversion
factors are clearly inapPropriate.
Comparison of the LOIloooand CHNS methods of
determining carbonate contents was limited here by the
range in carbonate contents of the suite analysed.Only three
samples(PES-1,一5,一31)have significant LOIlooo values;
they were also the only samples which effervesced during
acid addition in the CHNS method.Results for PES-5,a
highly calcareous shale,are good.Assuming all weight loss
at1000℃isCO,,andallC、。.,、iscontainedincalcite,
calculated CaCO3contents are69.84wt%(LOIlooo)and
68.29wt%(CHNS).The results for PES-1and PES-31are
very poor.PES-l has an indicated CaCO3content of
17.81wt%by the LOI!ooo method,but only2.94wt%from
reCa1CUlatiOn Of Ci。。,g frOm the CHNS analySiS.Neither
seems correct.Assuming all CaO in this sample(5.89wt%,
hydrous basis)is present as calcite,then maximum CaCO3
content should be10.5wt.%.PES-31retums an estimate of
8.56wt%from LOIlooo,but Ci.。,g was not detected in the
CHNS analysis.The LOIlooo estimate is unrealistic,because
the CaO content ofthis sample is only1.61wt%,(Campos,
unpub1.data),equivalent to2.87wt%CaCO3.The cause of
the poor results for these two samples is unknown.Both
samples had lower a(Uusted LOI550than expected(Fig.5).
They may contain a carbonaceous phase(e.g.graphite)that
was not lost until the1000℃ignition,or contain other
hydrous phases that s皿vived the lower temperature
ignition.However,their whole-rock elemental compositions
and XRD mineralogy (Campos,unpub1.data) are
unexceptional.
Indicated CaCO3contents of the remaining samples by
both methods are<6%,and most are less than2%(Fig.8).
PES-14and-29have much greater CaCO3contents by
CHNS than by the LOI method.Their low CaO contents
(0.69and O.38wt%respectively;Campos,unpub1.data)
suggest that the CHNS data are incorrect,possibly due to
dissolution of non-carbonate minerals by the acid addition
USed tO determine C。,、,and COnSeqUently Ci。。,、.AlthOUgh
some of the remaining samples yield comparable values by
the two methods,a number have greater estimates from
LOIlooo.These include five samples in which Ci,。,g was not
detected,but which have LOLooo calcium carbonate values
ranging from O.6to2.25wt%.
The poor estimates of carbonate contents by both
methods at low contents may be partly due to loss of other
volatiles between550℃and1000℃,as suggested by Heiri
αα1.(2001).This could include more tightly-bound
stmct皿al water from hydrous silicate minerals other than
clays.Other volatile species may include SO2fromoxidation
of sulphide minerals,and F and Cl.Moreover,at these
temperatures,weight gain through oxidation of Fe2+to Fe3+,
and ofMn2+to higher oxidation states may also occur.The
combination of these effects could obscure the results and
contribute to the variability observed.
Conclusions
Simple loss on ignition procedure is a popular method for
estimating OM and carbonate contents in sediments and
soils。Tests of the method used by Heiriαα1.(2001)and
others suggest that the OM in P句a Fomlation black shales
waslostafterthreeto sixhours heating at550℃,andthree
further hours of ignition at1000℃ were sufficient to
completely decompose the carbonate fraction.OMand
eStimated C。,g COnCentratiOnS determined by the LOI methOd
were greaterthan those determined using a CHNS elemental
analyzer,even when modified conversion factors were used
assuggestedbySutherland(1998).Igniti・ntestsat375℃
16
Organic carbon and carbonate contents ofblack shales from the lower Cretaceous
Paja Formation(Colombia)by loss on ignition and CHNS analysis:comparison ofmethods
give a better COrrelatiOn betWeen OM and C。,,,and SUggeSt
that much of the weight loss between375and550℃is
structural water from clays,which are an important
component of the samples analysed.However,ignition at
either temperature iS viable,prOVided C。,g data frOm an
independent method such as CHNS analysis is available for
CalibratiOn,and SpeCifiC OM tO C。,g COnVerSiOn faCtOrS Can
be derived for the sample suite concemed.The viability of
estimating calcium carbonate contents from LOIlooo data
could not be tested in our sample suite due to its generally
carbonate-poor nature.Neither the ignition method nor
CHNS analysis produced reliable results at the low levels of
CaCO3present in most samples.Continued loss of volatile
species and water from other hydrous phases may have
contnbuted,and oxidation reactions causing weight gain
may have also obscured the results.
Acknowleαgements
This work forms part of an M.Sc.study by N.O.C.at
Shimane University,financially supported by a scholarship
from the Inter-American Development Bank(IDB).The
laboratory workwas supportedby aMonbukagakusho grant
-in-aid to B.P.Roser.
References
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Organic Carbon in Non-Calcareous Soils.Joumal ofSoil Science,15,84-
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A.,J.,and Taborda,A.1995.Basin development and tectonic history of
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Blum,P.,and Herbert,T.D.(Eds.),Proc8α11ngs犀‘h80cθαn Dr∫ll’ng
Progr伽,Sc’8n峨’cR8sμ1‘s,162,209-216.
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Roser,B.,Sawada,Y.,and Kabeto,K.1998.Crushing performance and
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(Received:210ct.2002,Accepted:310ct.2002)
(要 旨)
N.O.Campos・B.P.Roser・三瓶良和,2002,コロンビア下部白亜系パジャ層黒色頁岩の有機炭素濃
度と炭酸塩濃度の測定=イグニッションロス法分析結果のCHNS元素分析法による検討,島根大学地
球資源環境学教室、21、9-16.
堆積岩や土壌の有機炭素濃度と炭酸塩炭素濃度は,一般に,簡便なイグニッションロス法(LOI法)
を用いて,それぞれ375-550℃と850-1000℃下で加熱して求めることができる.本研究では,24個
の有機物に富んだ黒色頁岩試料を用いて,有機炭素濃度と炭酸塩炭素濃度を2つの方法(LOI法と
CHNS元素分析法)で分析し,簡便なLOI法で得られる値の正確な意味について議論した.検討に
先立ち,有機炭素濃度と炭酸塩炭素濃度を求めるLOI法の最適加熱時間を決めるため,550℃と
1000℃下で時間を変えて予備加熱実験を行った.その結果,有機炭素の燃焼には550℃下で4-6時
間(LOI55。),炭酸塩の加熱には1000℃下で3時間(LOI1㎜)がそれぞれ必要であることが分かった.こ
れらの条件下で黒色頁岩試料のLOI分析が行われCHNS元素分析結果と比較したところ,両者には
強い相関関係が見られるものの,値には差が生じた.有機炭素濃度は,LOI,,。の方がCHNS元素分
析値よりも一貫して高くなった.この理由については,LOI,,。では約375℃で有機物が燃焼し,375
-550℃間でほぼ一定量(2-4wt%)の粘土鉱物の構造水が放出されることが原因と推察された.従っ
てこれらを回帰計算で補正することにより,LOI,,。でも有機炭素濃度を正確に求めることができる.
一方,炭酸塩炭素に関して,特に低濃度の場合にLOll㎜とCHNS元素分析値はバラついて一致しな
かった.その理由には,高温下でも粘土鉱物構造水の放出が継続していることや他の揮発性元素の
逸散,また,Fe2+やMn2+が酸化して重量が変化することなどが示唆された.