Ammonium contents of biotites from Precambrian rocks in Finland: The significance of NH4+ as a possible chemical fossil

0016-7037/85js3.00

Gporhimico A Cawtwhimico Ana Vol. 49. pp. 145-151 $3 p~mon RCSJLti. 1985. Rintui in U.S.A.

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Ammonium contents of biotites from Precambrian rocks in Finland: The significance of NH: as a possible chemical fossil Yumo

ITIHA~

Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558, Japan

and KANENORI SUWA

Department of Earth Sciences, Faculty of Science, Nagoya University, Nagoya 464, Japan (Received February 21, 1984; accepted in revisedform

October 3, 1984)

,&@a~-The ammonium contents of biotites in F’recambrianrocks from Finland havt been determined, to examine the possibility that ammonium in biotites represents a chemical fossil. If biologic activity existed in Precambrian sedimentary environments, biotites formed from the sediments should contain distinctly larger quantities of NH; than biotites from igneous rocks. Biotites from Svecokareiidic metaxxiiments, which were originally products of sedimentation about 2400-1900 Ma ago, have high NH; contents (hundreds of ppm), whik biotites from Sveu~karelidic piutonic rocks of 1800-1900 Ma old and P~~v~o~~iian rapakivigranitesof 1650-1700Ma old have low NH; contents (tens of ppm). The results may indicate that biologic activity was plentiful during sedimentation of the Svecokarelian strata. This agrees with evidence for ancient life found in doiomites and greywacke-slates of the Svecokareiides. Biotites from Presvecokarelidic schists of more than 26002800 Ma old have only tens of ppm of NH;. This value is at the same level as those of biotites from the rapakivi granites and the Svecokareiidic piutonic rocks. Considering the rarity of fossilsin Presvecokarelidic rocks, the low NH; conttnt of biotite from a Presvecokareiidic pelitic schist may indicate less biologic activity during the sedimentation of the original sediments. These data emphasize that ammonium in biotites of Precambrian metasediments may be a useful chemical fossil for identifying the geologic sites of ancient life.

XNTRODUCIION STUDIESOF AMMONIUM inminerals revealed that the

ion is located in the alkafi position of minerals. substituting for potassium because of its similar ionic radius and charge (ERD et al., 1964; VEDDER. 1965; YAMAMOTO and NAammonium

potassium-bearing

KARYAKIN ef al.. 1973; HIGASHI, 1978, 1982;SHGOROVA ef al., 198 I ). The ammonium contents of various rocks have also been determined. KAHIRA, 1966;

(1961), who examined the ammoniacal nitrogen content of 400 rock and mineral samples, showed that paragneisses contain more ammoniacal nitrogen than do orthogneisses. He considers the nitrogen in magmatic rocks to be of primary magmatic origin. MILOVSKIY and VOLYNETS (1966) obtained results similar to W~otzka’s on diffe~nt types of metamorphic rocks. STEVENSON ( 1962) concluded that ammonium in sedimentary rocks is held in secondary silicate minerals and ammonium in igneous rocks is contained largely within primary potassiumbearing minerals. URANO ( 197 1) found that the ammoniacal nitrogen content of two mica granites, which are thought to be derived from sediments, is about 6 times as high as that of biotite granodiorites derived from basic igneous rocks. He regards that the high ammonium content of the two mica granites is due partly to an abundance of potassium feldspar and partly to inheritance of nitrogen from the original sediments. Recently NORRISand SCWAEFFER(1982) WLOTZKA

145

determined the nitrogen content of deep sea basal& and their nitrogen content is about the same with that of orthogneisses determined by WLOTZKA (1961) and MILOVSKIY and VOLYNETS (1966) and of biotite granodiorites by URANO ( I97 1). Norris and Schaeffer estimated from the nitrogen content of a basalt giass ( 10 ppm) that a Lower limit of the nitrogen content of the mantle is 2 ppm. Considering these geochemical data. it seems most reasonable to conclude that there is a recognizable difference between the nitrogen content of magmatic rocks and of metamorphosed peiitic rocks. ITIHARA and HONMA (1979) examined the ammonium content of biotites in Cretaceous granitic and metamorphic rocks, with the view that the ammonium content of ~~iurn-~~ng minerals may be more suitable for di~~mination between meta-igneous and meta-sedimentary rocks than the whole rock ammonium content. The results show that biotites from meta-sediments and migmatites are high in their ammonium content (average, 279 and 475 ppm, respectively) compared with biotites from granitic rocks (average, 67 ppm for rocks from a metamorphic belt, and 22 ppm for rocks from nonmetamorphic terranes). Their results show that the high ammonium content of biotite from metasediments is due to inheritance of nitrogen fmm organic matter in the original sediments. In other words, sediments contained nitrogen as a ~~tuent of organic matter, and the organically bonded nitrogen

146

t’. ltihara

was converted into ammonium ion during diagenesis. The ammonium was then held first within interlayer sites of clay minerals and later within the crystal lattice of biotite which was formed during metamorphism (ITIHARA, 1978; ITIHARA and HONMA, 1979. 1983). If the same events occurred in Precambrian sediments, the ammonium ions would be preserved in biotites of the metamorphosed sediments as a chemical fossil because of the thermal stability of ammonium in mica (VEDDER, 1965: YAMAMOTO and NAKAHIRA, 1966; HIGASHI, 1978). The ammonium ions in biotite could then provide evidence for the existence of life in the Precambrian. HONMA and SCHWARCZ (1979) determined ammonium content of Archean rocks from the Superior Province, and suggested the possibility of prospecting for early life

by the ammonium

content of metamorphic

rocks of

the early Archean.

The ~uroose of this studv is to examine the oossibility that ammonium in biotite represents a chemical fossil. Precambrian rocks from Finland were studied

for this purpose. SAMPLES Samples of biotite were obtained from rocks collected in Finland during the Excursion 001 of the 26th IGC in 1980 (Precambrian bedrock of southern and eastern Finland). The main geologic characteristics of the Precambrian in Finland have been summarized by SIMONEN(1980). The

Gulf

of

and K. Suwa

localities sampled for this study are plotted in hg. 1, which shows the main structural units of the Precambrian m Finland (SIMONEN,1980). The local geology and the occurrences of the rock samples have been described in the guidebook 10 the excursion (HYT~NEN, 1980). On the basis of the information in the guidebook, the biotite-bearing samples are grouped as shown in Table I. Samples of group A are from Postsvecokarelian igneous rocks. rapakivi granites. which were emplaced 1700-1650 Ma ago into the cratonized parts of the Svecokarelidic complexes. Samples of group B are from rocks in the Svecokarelides. which cover most of the Finnish Precambrian. The Svecokarelidic folded zone is composed of three major geological units, the Karelidic schist belt in eastern and northern Finland, the Svecofennidic schist belt in western and southern Finland, and plutonic rocks which were emplaced during the Svecokarelidic orogeny and penetrated the Karelidic and the Svecofennidic schists (Fig. I). in this study, samples of group B are divided into two subgroups. Bl consists of the plutonic rocks and B2 of metamorphic rocks in the Karelidic and rhe Svecofennidic belts. The ages of the plutonic rocks are 1800-1900 Ma. Except for 5-3-B and 9-6, the other samples in group B2 are metamorphic rocks which were originally sediments that accumulated on the Presvecokarelidic basement and in the geosynclinal basins and then were metamorphosed during the Svecokarelidic folding which took place about 1900 Ma ago. The ages of sedimentation of these rocks are considered to be in the range 2400-1900 Ma. Rocks 5-3-B and 9-6 are considered to be migmatitic gneisses related to the emplacement of rapakivi granite and microcline granite. respectively. Samples of group C are from the Presvecokarelidic basement rocks upon which the Svecokarelian sediments were deposited. Rocks 2-la and 2-lb are not younger than 2800

Finland

FIG. I. Main structural units of the Precambrian in Finland (after SIMONEN,1980). and the locations of rock samples collected. 1: Presvecokarelidic basement, 2: Karelidic schist belt in the Svecokarelides, 3: Svecofennidic schist belt in the Svecokanlides, 4: plutonic rocks in the Svecokarelides. 5: Postsvecokarelian rapakivi granites.

NH: as a chemical fossil Table 1.

147

NH*+ contents of biotites and modal contents of biotite, K-feldspar and muscovite in Precambrian rocks in Finland NH4+

Sample

(~ma)

R. Postsvecoka~~ian rapakivi granite l-l (Hb bg bi granite) l-2 (Biotite granite) l-4 Hb-bi porphyritic granite) 5-l IHb-bi granite) 5-3-A (Biotite granite) B. Svecokarelidicrocks 1. Plutonic rocks 6-6 (Hb-bi tonalite) 6-7 (Hb-bi porphyritic granite) 6-8 Biotite granite) 6-9 Ilib-biquartz gabbro) 6-10 (Orbicular granitoid) nucleus of orbicule (Hb-bi trondhjemite) shell of orbicule (Ei diorite) matrix of orbicular rock (Hb bg bi granite) 7-3-B (Biotite granodiorite) 7-7 (Hb-bi granodiorlte) 2. Metamorphic rocks a. Karelian 3-2 (To-mu-bi schist) 3-3 (To-ani-bi-Kf schist) 3-4 (To-st-bi schist) 4-4 (To-sil-and-st-bigneiss) b. Svecofennian 4-5 (Bi schist: mica schist zone) 4-6 (To-Kf-bi gneiss: Kf-sil zone) 4-7 (Kf-bi gneiss: Kf-car zone) 4-8 (Sil-car-ga-Kf-bigneiss: ga-cot-511 zone) 5-3-B (Hb-bi migmatitic gneissf 7-l (To-mu-bi schist) 7-3-A (Ga-mu-Kf-bischist) 7-4 (Mu-Kf-bimigmatitic gneiss) 7-5 (To-mu-bi schist) 9-6 (Pinitired car-mu-bi migmatitic gneiss) C, Presvecokarelidicbasement rocks Z-la (To-bi-hb schist) Z-lb (To-mu-bi schist) 4-1 (Fib-Kf-bi gneiss: core of mantled gneiss dome)

32 14 ;: 67

:: :; 51 :: 202 37 762 329 940 885 522 192 341 57

Modal Bi * ::1 2.8 4.3 5.3

Kf 42.0 38.0 16.4 38.5 38.6

17.0 6.4 2% 4.9* 25.5 13.3 0.0 3.0 3.1 ::: 4.9 30.0 11.9* 4.5 6.4* 3;5 26.1* 56.5 29.4 28.3

0.0 58.2 0.0 0.0

contents

Hu

Others Total 55.9 59.9 80.8 56.9 56.1

loo.0 100.0 100.0 100.0 100.0

83.0 66.7 69.1 86.7

100.0 100.0 100.0 100.0

95.0 95.7 64.6 %

100.0 100.0 100.0 100.0 100.0

00,: 0:O

56.7 11.5 43.5 71.7

100.0 100.0 100.0 100.0

61.4 73.4 81.1 75.8

100.0 100.0 100.0 100.0

81.2 46.7 57.9 74.4 55.7 68.5

100.0 100.0 100.0 100.0 100.0 100.0

87.3 79.8 02.7

100.0 lW.0 100.0

X:"o ao*! 0:O 0.0 0.0 x:: ::: 0.5 0.7 G;O 18.2

24.1 38.6 17.5 15.1

2.5 0.0 ;::

:.z 0'0 0:o

342: 231 289 1558 46

53.4 38.2

5.4 0.0

1::;

17.7 35.1 32.2 21.7'

:.: 010 0.0

x 12:1 9.8

22 40 24

12.7 11.4 13.6

0.0 0.0 3.7

:*os 0:O

Abbreviation for minerals; and: andalusite, bi: biotite, COP: cordierite, ga: garnet, hb: hornblende, Kf: K-feldspar,mu: muscovite, sil: sillimanite,st: staurolite, to: tourmaline. * Biotite is partly chloritized. bg: -bearing

Ma, because the zircon ages of all the granitoidrocks of the besement area fall within an age group of 2600-2800 Ma, and schists in this area are intruded by the granitoids. Basement gneisses from the cores of the mantled gneiss domes, however, are 2400-2500 Ma because of partial recrystallization of the zircon during the later Svecokarelidic metamorphism. EXPERIMENTAL

PROCEDURE

Biotite was separated using an isodynamic separator and heavy liquids. The purity of the samples was more than 90%except for 3-2 which was 80-8596 pure because of the difficulty of separating biotite fmm muscovite. The method for determining the ammonium content of biotite was fun~m~~Ily the same as that used by ITIHARA and HONMA ( 1979), but hydrofluoric acid with sulfuric acid was used to decompose the biotite samples instead of sealed tube digestion with sulfuric acid. Them was no systematic difference greater than analytical error between the ammonium content ~e~~n~ by the two methods Details of the procedure for decomposing the biotite samples using hydrofluoric acid with sulfuric acid have been described by URANO (1971). More than two analyses were done with each sample, and the differences between analyses were less than 10% for samples which had more than 100 ppm ammonium.

RESULTS

The ammonium contents of the biotites are listed in Table 1 together with the rock name and the modal contents of biotite, K-feldspar and muscovite. Figure 2 shows the distribution

of ammonium

con-

in biotites from Precambrian rocks in Finland and from Cretaceous rocks in Japan. tents

Biotitesfrom Poslsvecokarelianigwous rock-s(A) NH; contents of biotites from Postsvecokareiian rocks, rapakivi granites, range From 14 to 91 ppm, This range is contained within the range of m contents of biotites from Japanese Cretaceous gzanitic rocks (D 1, D2 in Fig. 2). Among the samples of’ this group, l-4, which was obtained from a hombkndebiotite granite with ovoids of K-feldspar, and 5-3-A, from a biotite granite which occurs along the contact against the Svecokarelidic mi~ati~c gneiss 5-3-B, show higher NH: contents than the others.

148

1.. ltihara and Ii. Suwa

8

t

8

,

I

I

-.+Fr--T-

Biotite from Precambrian rocks in Finland

?*I

k

_Zci

.‘.

-

82

&C

FIG. 2. NH; contents of biotites from Precambrian rocks in Finland and Cretaceous rocks in Japan. A, B I, B2 and C are the same as those in Table 1. DI: biotites of granitic rocks in non-me~mo~hic terranes, 02: biotites of granitic rocks in the Ryoke belt. D3: biotites of metamorphicrocks in the Ryoke belt. Data for Dl, D2 and D3 are From ITIHARA and I-IONMA (1979). Value for sample 6-10 is plotted as the mean of biotites from nucleus and shell of orbicule and matrix of the orbicular rock.

Biotiles from Svecokurelidic

plulonic

rocks (El)

NH; contents of biotites from Svecokarelidic plutonic rocks range from 10 to 5 I pm except for “7-3B which shows a high content, 202 ppm. This sample is from a biotite granodiorite which contains fragments of micaceous schists (VORMA, 1980). and its NH: content is about the same as that of biotite from a sample of garnet-muscovite-K-feldspar-biotite schist which was contained in the biotite granodiorite (231 ppm of 7-3-A). Except for sample 7-3-B. NH: contents of group BI are divided into two populations: those of biotites in rocks with abundant K-feldspar range from IO to 33 ppm and those of biotites in rocks with little or no K-feldspar range from 35 to 51 ppm. These NH: contents are all within about the same range as those from rapakivi granites (group A), and the range is within that obtained for biotites from Japanese Cretaceous granitic rocks (Dl, D2 in Fig. 2). Biotites from

Svecokarelidic

melamorphic

rocks (B2)

NH: contents of biotites from metamorphic rocks in the Svecokarelides range widely from 21 to 1558 ppm. Samples 3-2 to 4-4 in Table 1 were obtained from rocks in the Kanlidic belt, and sample 4-5 to 9-6 were from rocks in the Svecofcnnidic belt (Fig. I). Karelian me~mo~hic rocks examined are all metapelites containing tourmaline. NHI+ contents of biotites from these metapetites range from 329 to 940 ppm. Among them, those fmm rocks with neither K-feldspar nor muscovite range from 885 to 940 ppm while the NH; content of a rock containing muscovite is 762 ppm and that of a rock containing

K-feldspar and muscovite is 329 ppm. NH; contents of biotites from the Svecofennian metamorphic rocks generally range from 192 to 1558 ppm. although there are a few exceptions (4-8, 5-3-B, 9-6). The NH: content of rocks without K-feldspar range from 348 to I558 ppm and that from rocks with K-feldspar range from I92 to 341 ppm. These results suggest that NH: is also contained in K-feldspar and muscovite to some degree. Taking the modal content of these minerals in Svecokarelidic metamorphic rocks (Tablel) and the dist~bution of NH: in coexisting minerals (HONMA and ITIHARA, 1981) into consideration, however, a large part of NH: in the rocks is considered to be contained in biotite. Four samples, 4-5 to 4-8, were obtained from metapelites that suffered progressive metamorphism (KORSMAN, 1977). These rocks belong to the mica schist zone, the K-feldspar-siliimanite zone, the Kfeldspar-cordierite zone and the gamet-cordierite-siilimanite zone, respectively. Among them, three samples, 4-5 to 4-7, have NH: contents that exceed 190 ppm, and their values are unrelated to the metamorphic grade of the rocks but instead related to the amount of K-feldspar in them. Biotite from sample 4-8, a sillimanite-cordie~te-garnet-K-fe~~par-biotite gneiss, belonging to the garnet-cordierite-sillimanite zone, contains 57 ppm of NH:. Because migmatizing granites occur in this zone in a greater abundance than in K-feldspar-cordierite zone (KORSMAN, l977), the low NH: content of biotite from sample 4-8 may be caused by intrusion of the granites. NH: in mica is not driven off below 400°C (YAMAMOTO and NAKAHIRA, t966; HIGASHI. l978), and there is atso evidence that the maximum loss of NH; occurs at 800-900°C in vacuum (KARYAKIN et al.. 1973). Thus, NH: in mica is stable at low temperature, and the NH: contents of biotites in metamorphic rocks will reflect the nitrogen contents of the original rocks if the rocks did not undergo high temperature metamorphism (i.e.. :> - 6OO“C). If rocks were metamorphosed at high temperature, for example under conditions at which the assemblage sillimanit~~ie~tegarnet-K-feldspar-biotite would develop, NH: might be liberated from biotite. In this case, the NH; content of biotite would not record the existence of nitrogen in the original rocks. WLOTZKA (196 1) also thought that a high NH: content of a rock indicates that the rock is metasedimentary but that a kow NH: content does not necessarily indicate that the rock is not a metasediment. Other samples which show low NHf contents, 53-B and 9-6, are from a hornblende-biotite migmatitic gneiss and a pinitized cordie~te-mu~o~te-biotite migmatitic gneiss, respectively, that occur near rapakivi granite and microchne granite, respectively. Except for these samples and the above mentioned sample 4-8, biotite samples from the Karelian and the Svecofennian metamorphic rocks have high NHf contents.

NH: as a chemical fossil

149

pnrvecokanlidiC basement rocks contain no graphite. Biotites of these rocks do not include carbonaceous NH: contents of biotites from PresvecokaretidiC matter. basenumt rocks range from 22 to 40 ppm. Amow Besides the above mentioned explanation, other the samples of this group, 4-l was obtained from a sources of NK: must be considered for the high hornblende-K-feldspar-biotite trondhjemitic gneisS NH: contents of biotites. One is an abiotic source which occurs as the core of a mobilixed mantled such as volcanic gases. A small quantity of ammonia gnejss dome (PAAvOLA, 1980). Sample 2-la is is often detected in gases from some volcanoes from a magnetiteepidote-tourmaline-biotite-ho~(WHITE and WARING, 1963), and the ammonia is blende schist and 2-lb is from a hematite-calcitebelieved to be magmatic in origin (e.g., Y~HIDA et tourmahne-muscovite-biotite schist. 2-la is considered al., 1964). If volcanic activity occurred, rocks conto be a metamorphosed tuffaceous rock and 2-lb is taminated by the ammonia gas might be found in metamorphosed pelitic sediment. These two samples such an area. It is difficult to believe that the large show low contents of NH& 22 and 40 ppm, respecnumber of the Svecokarelidic metasediments analyzed tively. These values are the same as thoseobserved in this study were all contaminated by ammoniafor biotites from igneous or plutonic rocks. Presvebearing volcanic gases. Another possible origin of cokarelidic pelitic schist 2- 1b is petrographically simhigh NH: contents is post-Precambrian contaminailar to Svecokarelidic pelitic schists 3-2, 7-l and 7-5. tion by fluids enriched in NW: of biologic or@. In These pelitic schists contain tourmaline, muscovite this case, the NH: content of biotite might nflect and biotite, and no K-feldspar, although Presvecobiologic activity but not Precambrian biologic activity. karelidic pelitic schist contains no graphite. The If nitrogen-bearing fluids afFkcted Svecokareltic rocks metamorphism of the mafic metavolcanics of Preat Postsvecokarehan age, not only metasediments but svecokarelidic basement in eastern Finland grades also plutonic rocks would have been contaminated from the conditions of the greenschist facies to the by the lluids. Nitrogen would be supplied to the rocks amphibolite facies (SIMONEN, 1980). throu~out, irrespective of rock type. However, the difference in NH: content between biotites from DISCUSSION metasediments and plutonic rocks is too remarkable to attribute to post-Precambrian nitrogen contamiAs reported in the preceding section, biotites from metasediments in the Svecokarelides show, with a nation. It is unlikely that nitrogen-bearing fluids of post-Precambrian age were the main source of NH: few exceptions, distinctively high NH: contents When the measured contents are compared to those of in biotites in Svecokarelian metasediments. According to SIMONEN(1980), the most convincing biotites from plutonic rocks, the contrast is remarkable. The observed difference in NH*+ contents cor- evidence for ancient life during sedimentation of the responds well to that observed between biotites of Svecokarelian strata are (a) stromatolite structures in the Jatulian dolomites of the Karelids and (b) carCretaceous granitic and metamorphic rocks in Japan (Fig. 2). The similarity between the Precambrian and bonaceous sacs (corycium) in the Svecofennian greyCretaceous rocks probably indicates that the high wacke-slates. In addition, black schists that occur as NH: contents found in biotites from the Svecokareinterbeds between greywackeslates and meuquartzites lidic metasediments are due to the inheritance of have been interpreted as bituminous sapropelic sedorganic matter from the original sediments, as found iments. Therefore a substantial biomass may have with Japanese Cretaceous metamorphic rocks. This existed in both sedimentary basins where the Karelian may mean that biologic activity existed in the sedi- and the Svecofennian were deposited about 2400mentary basins of Finland 2400-1900 Ma ago, and 1900 Ma ago. The evidence from paleontological that much insoluble organic matter such as kerogen data is consistent with our conclusion deduced from accumulated in the sediments. Carbonaceous matter, the high NH: contents of biotites from the Sveeowhich commonly occurs in the Svecokamlidic metakarelidic metasediments. In several samples from the sediments and is included in most of the biotite metasediments, NH: contents exceed 534 ppm which samples from the metasediments, could also be evi- is the maximum obtained from biotites from Japanese dence that the original sediments contained much Cretaceous metamorphic rocks. Because it is believed organic matter. The X-ray dithaction pattern of the that a great increase of plant biomass (mainly algal) carbonaceous matter, separated from a biotite sample occurred 2400-2200 Ma ago, resulting in a change using hydrofluoric acid and sulfuric acid, indicates of the composition of the atmosphere and gnatly that the carbonaceous matter is now crystalline affecting many geological pmcesses on the Earth’s graphite. The graphite amounts to about 0.06% of surface (SALOP, 1983), the high NHf contents may the biotite sample, 3-4, by weight. Nitrogen as NH,+ record the great increase in life at that time. Decomas well as carbon as graphite, which are both present position of organic matter from the organisms would in biotite samples, may possibly have been componot have pmgmssed far under the reducing conditions nents of the organic matter in the original sediments. of the sediments. HONMA and !&HWARCZ (1979) Kapakivi granite!& Svecokarelidic plutonic rocks and determined the NW content of rocks in the Superior Biotites from Presvecokwelidic basement rocks (c)

IS0

Y. Itihara

Province, Canada, and found that volcanic rocks of 2700-2710 Ma are very low in their NH: content, O-6 ppm (whole-rock samples), while a black shale of 2 100 Ma contained more NH;, 130 ppm (a wholerock sample). The content of NH: in the black shale was regarded as evidence of the presence of biota at that age. This view also agrees with the conclusion obtained from the 2400- 1900 Ma old Finnish metasediments. NH: contents of biotites from the Presvecokarelidic schists are only tens of ppm, which is the same level as those in the rapakivi granites and the Svecokarelidic plutonic rocks. These values may be explained in several ways. First, organic nitrogen may have existed originally in the rocks but was removed during metamorphism. This possibility is possible for sample 4-1 which is from the core of a mobilized mantled gneiss dome in which the gneisses were recrystallized during the later Svecokarelidic orogeny (PAAVOLA, I980), but is unlikely for schists 2- 1a and 2-I b, which are a metamorphosed tuffaceous rock and a pelitic rock belonging to the transitional zone between the greenschist facies and the amphibolite facies, respectively (SIMONEN. 1980). Sample 2- 1b is petrographitally similar to some Svecokarelian pelitic schists (32, 7-1. 7-5) whose biotites have high NH: contents, 348-1558 ppm. Nevertheless biotite of schist 2-lb has only 40 ppm NH:. A second explanation is that life existed in the original sediments but the sedimentary environment was not right for nitrogen to be fixed in the sediments. This is a possibility for sample 2-la which is considered to be a metamorphosed tuffaceous sediment, but is unlikely for a metamorphosed pelitic sediment such as sample 2-lb. The third explanation is that no life existed in the original Presvecokarelidic

sediments

and

no post-Presveco-

process added nitrogen to the rocks. This possibility, however, may not be correct when we consider that morphological remains of microorganisms or stromatolite structures have been found in rocks of South Africa and Western Australia 35003000 Ma old (KNOLL and BARGHOORI% 1977; DUNLOP cr al.. 1978: LOWE. 1980: WALTER ef (11.. 1980). karelidic

Furthermore.

even complicated

plants are assumed

to have been present in the sediments of the Witwatersrand (South Africa) of 2700-2300 Ma (HALLBAUER, 1975: HALLBAUER d al.. 1977). These reports

and others (f.~., SALOP, 1983) demonstrate the existence of biologic activity on the earth before 2800 Ma. However. convincing evidence for the existence of ancient organisms has not yet been found in Presvecokareiidic rocks in Finland (SIMONEN, 1980). Considering this lack of evidence in Finland, less or no biologic activity during sedimentation seems to be a reasonable explanation why sizable amounts of organic matter did not accumulate in the Presvecokarelidic pelitic sediments. A lack of biologic activity is probably the best general explanation for the low NH; content of Presvecokarelidic biotite samples.

and K. Suwa

From the above discussion, the large NH., content of biotites in metasediments, which exceeds the NH: contents of biotite from igneous rocks. is considered to be a possible chemical fossil. It may play an important role in studies of the evolution of life in the Precambrian, because (a) NH: in the crystal lattice of biotite is protected from contamination during subsequent geologic processes, e.g., metamorphism, and (b) biotite is a common mineral in Precambrian metapelites. Although other K-bearing minerals such as muscovite or K-feldspar can also hold NH: in their structures, the distribution of NH: between coexisting minerals of granitic and metamorphic rocks shows that biotite can hold the most NH:. Biotite has a more suitable crystal structure and the NH: content of muscovite or K-feldspar is less than half of that in biotite (HONMA and ITIHARA. 198 1). Thus, NH: in biotite may be helpful in studies of ancient life. Acknowledgments-We

are grateful to Dn. A. Simonen and

K. HytGnen of Geological Survey of Finland for their kind guidance in the field, to Dr. M. Itihara of Osaka City University for his help in sampling rocks and his valuable advice during the course of this work, and to Dr. N. Kagemori of Katano high school for the orbicular rock sample. We are also indebted lo Mr. T. Agata of Nagoya University for his kind advice on the ore microscopy. Editorial

handling.

1. M. Ferry

REFERENCES DUNLOP J. S. R., MUIR M. D., MILNE V. A. and GROVES D. 1. (1978) A new microfossil assemblage from the Archaean of Western Australia. Nature 214, 676-678. ERD R. C., WHITE D. E.. FAHEY J. .I. and LEE D. E. (1964) Buddingtonite, an ammonium feldspar with zeolitic water. Amer.

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