Oxygen and Evolution of the Precambrian Iron Banded Formations
New Methodology Principle for Systematics of Igneous Rocks: Silica-Saturation versus Desilication Concept....
Oxygen and Evolution of the Precambrian Iron Banded Formations......
Stress-Shear Metamorphic Differentiation as the Paradigm for Origin of the Kola Banded Iron Formation......
ÒÅÇÈÑÛ íà EUG-XI, 2001
New Methodology Principle for Systematics of Igneous Rocks: Silica-Saturation versus Desilication Concept
Dmitri YEGOROV
Geological Institute, Kola Science Center RAS
Fersman str. 14 Apatity
Murmansk region
184209
RUSSIA
Igneous rocks are composed from the minerals (not directly chemical elements). This is the reason for the idea, that minerals or calculated minal norm statistics are the correct basis for the rocks systematics. In addition to the main conventional chemical systematics of rocks (SiO2 versus (Na2O+K2O)) the proposed method enables a rapid assesment of the rocks chemistry using the concept of “saturation”.
The well known CIPW norm calculation method (Cross et al. 1902) provides a way to such calculate on the basis of the desilication concept. Because of this basis, classical CIPW have some different ways of the calculation for the rock norms with different Al/Na/K ratios, very complicatet algorithm, etc. In opposite, we proposed (Yegorov et al, 1998) program for the CIPW norm calculation consists of steps which combine molecules of normative minerals starting from the smaller ones (e.g. kalsilite before leucite, and leucite before orthoclase) in order to "consume" step-by-step all silica present in an analysis. The sequence of proposed calculations is as follows:
The weight percentages of oxides are converted to molecular amounts;
Accessory minerals (halite, fluorite, pyrite, carbonates, apatite, zircon, chromite) are calculated;
The main chemical oxides are then combined step by step to form progressively more complex rock-forming minerals. The molecular amount of a mineral is equal to the amount of the least abundant component of those required to form that mineral present at this step. Accordingly, if a certain component is already consumed for the construction of previous minerals, than nothing happens during all further steps that involve this component.
The sequence of steps is as follows:
K2O+SiO2=potassium silicate
Na2O+SiO2=sodium silicate
potassium silicate+Al2O3+SiO2=kaliophillite
sodium silicate+Al2O3+SiO2=nepheline
sodium silicate+Fe2O3+3SiO2=acmite
CaO+Al2O3+2SiO2=anorthite
FeO+TiO2=ilmenite
CaO+TiO2=perovskite
FeO+Fe2O3=magnetite
FeO+MgO=periclase+wustite
CaO+(periclase+wustite)+SiO2=monticellite
2CaO+SiO2=larnite
ilmenite+larnite=perovskite+monticellite
2(periclase+wustite)+SiO2=olivine
larnite+SiO2=2 wollastonite
kaliophillite+2SiO2=leucite
monticellite+SiO2=diopside
leucite+2SiO2=orthoclase
perovskite+SiO2=titanite
nepheline+4SiO2=albite
olivine+SiO2=2 hypersthene
The proposed scheme of calulation is almost the inverse to the conventional one which prescribes the successive steps of desilication of mineral molecules. Both the proposed algorithm and the conventional routine give identical CIPW-norm results as to rock-forming minerals. The exeption are perovskite and ilmenite which are not calculated in the same analyses: we propose to calculate norms in a way to avoid the simultaneous presence of ilmenite and calcium silicate which react with each other to form perovskite. The supposed advantage of the new algorithm is its simplicity and correspondence to the natural process of magmatic crystallization.
Cross, W., Iddings, J.P., Pirrson, L.V. and Washington, H.S. (1902) A quantitative chemico-mineralogical classification and nomenclature of igneous rocks. Journal of Geology, 10: 555-690.
Yegorov D.G., Korobeinikov A.N., Dubrovskii M.I. Chempet - calculation for the chemical systematics of igneous rocks based on the CIPW norm // Computers and Geosciences, v. 24, ¹ 1, 1998. Pp. 1-5.
Oxygen and Evolution of the Precambrian Iron Banded Formations
Dmitri YEGOROV
Precambrian Banded Iron Formations (BIF) can be subdivided into two major types: Algoma-type (lenticular deposits closely associated with effusive rocks) and Lake Superior-type (thicker deposits in schists). The Superior-type BIFs all over the world are confined mainly to the Lower Proterozoic rocks; in Europe these are huge deposits in Krivoy Rog and Kursk (2200-2000 Ma; Shcherbak et al., 1990). The Algome-type BIFs occur mostly near the Archean-Proterozoic boundary. In Europe, the largest deposits of this type are located in the Ukrainian Shield (Novopavlovsk, over 3400 Ma, Kosivtsevskaya formation, Near-Azov region, over 3310 Ma, Konsko-Verkhovtsevskaya BIF, 3175-3150 Ma; Shcherbak et al., 1990), and on the Baltic Shield (Kola BIF, Olenegorsk, 2800-2750 Ma, original data; Kostomuksha, 2800-2760 Ma; Bibikova, 1989).
Most researchers interpret the Algoma-type BIFs to be sedimentary or volcanic-sedimentary formations, however (in contrast to the undoubtedly sedimentary BIFs of the Superior-type) this interpretation is more often than not based on insufficient factual evidence. According to our studies of the Kola BIF and the data on the Kostomuksha (Barabanov, 1985) and a number of Archean BIFs of the Ukrainian Shield (Shcherbak et al., 1990), these deposits were formed by metamorphic-metasomatic transformations of primary high-Fe volcanic rocks in Archean granite-greenstone areas.
Isotopic studies of sulphur (Huttori et al., 1983) and carbon (Melezhik and Fallik, 1996) showed that about 2300 Ma ago there was a sharp increase in the oxygen content in the atmosphere, from only a few percent to the modern level. In our opinion, this very event created the necessary prerequisites for huge (as much as 3 km thick) Superior-type BIFs to originate in the course of sedimentation.
And finally, BIFs ceased to originate about 1800 Ma ago. This is usually taken to be the consequence of oxidation of the deep-ocean waters which, as a result, became depleted in iron (Klein, 1997).
Barabanov V.F. Geokhimiya. Leningrad, Nedra, 1985, 424 pp.
Bibikova E.V. Uranium-lead geochronology of early stages of the evolution of ancient shields. Moscow, Nauka, 1989, 180 pp.
Hattori K., Krouse H.R., Campbell F.A. The start of sulphur oxidation in convental environments: about 2.200.000.000 years ago. Science, vol. 221, 1983. P. 549-551.
Klein C. Igneous ferment at Hamersley. Nature, 1997. V. 385. P. 25-26.
Melezhik V.A., Fallick A.E. A widespread positive <delta>-13carb anomaly at around 2.33-2.06 Ga on the Fennoscandian Shield: a paradox? Terra Nova, 1996. V.8. P. 141-157.
Shcherbak N.P., Artemenko G.V., Bartnitsky E.N., Tkachenko M.V., Plotkina T.A. Age stages of BIF formations of the Ukrainian Shield. In: Isotope dating of endogenic ore formations. Abstracts of the All-Union Conference. Kiev, 1990. P. 89-92.
Stress-Shear Metamorphic Differentiation as the Paradigm for Origin of the Kola Banded Iron Formation
Dmitri YEGOROV
Geological, isotopic - geochemical data, mass-balance recalculations show that Banded Iron Formation of the Kola peninsula (Baltic Shield) were derived from the stress-shear metamorphism of primary highly ferruginous basic vulkanites in situ, the gain-loss of the substance being of inferior significance in scales of the ore-bearing Kola series as a whole.
Metamorphic differentiation is the process restricted in its manifestation by diffusion rates of the components. As shown in a number of the works by V.E.Panin, a combination of stress and shear can lead to the transition of the substance to the atom-vacancy state (AVS) with a drastic increase of the diffusion rates (up to ten orders of magnitude). This promotes the occurrence of the metamorphic differentiation processes at scale levels up to hundreds of meters.
A simplest model of the process in question is as follows: the iron oxidizes in the zones traversed by a fluid flux with the formation of magnetite; accordingly, ferrous iron becomes deficient and therefore starts diffusing from the regions surrounding the active zone; the rest of the rock-forming elements, in accord with the mass-balance, are forced out and lost, in doing so aluminium as the most inert element in the metamorphometasomatic process deposits in the direct vicinity of the forming ore bodies and forms alumosilicate gneisses framing ferruginous quartzites lens. The fluid is, herewith, regarded not as a transport agent (the solid-phase process of the substance to the AVS is being assumed) but as an oxidant for ferruginous minerals.
In this case, more intricate synergetic effects associated with a nonlinear interaction of the components in the system are possible. We have performed a numerical modeling of the system describing extraction of iron from ferrosilicate (Fs) and redeposition in the form of magnetite (Mt). We shall designate all the complexes incorporating the block [Fe2+2Fe3+] as X [Fe2+2Fe3+] as Y, Fe2+ as Z, Fe3+ as R.
Model scheme of the reactions (with the kinetic constants of the direct/reverse reactions, respectively):
Fs Z (k1)
Z R (k2 k3)
Z+RY(k4 k5)
Y+RX (k6 k7)
XMt (k8)
X3R (k9)
Mt Z+2R(k10).
Assuming that all the complex formation reactions are equilibrium ones, we shall write the following kinetic equations (kinetics proportion to the degree 2/3 reflect surface-to-volume ratio):
dM/dt= [k8*ZR2 - k10]Mt2/3
dFs/dt= -k1Fs2/3
dZ/dt= k1Fs2/3 – k2Z + k3R -k9*ZR2 - [k8*ZR2 - k10]Mt2/3 + Dfe2Z- u(Z/x)
dR/dt= k2Z - k3R + k9*ZR2 - 2[k8*ZR2 - k10]Mt2/3 + Dfe2R - u(R/x),
where k8* = (k4/k5)(k6/k7)k8; k9* = (k4/k5)(k6/k7)k9; u – speed of the fluid flux.
In the digital calculation of the system the stationary periodic structure of R and Z concentrations forms from the initially homogeneous distribution of all the variables that leads to the wave-like character of distribution of the magnetite being formed.