1 AT THE MECHANISM OF ALLOCATION OF MINERALS. 3. SOME CONDITIONS OF CRYSTALLIZATION OF MAPHIC ROCKS AND METEORITES MINERALS Makarov V.P. (Geol. Dep. RSGPU, Moscow) email@example.com; Phone: (495) 433- 56-77, Add. 11-78 Research of isotope content of oxygen of minerals of acid magmatic and metamorphic rocks with use of new types of isotope geothermometers (IGT) [1, 2] has allowed to develop a technique of definition of the mechanism of allocation of minerals [3, 4], i.e. revealing of a types of the chemical equation conducting to a mineral formation. The available material allows to decide this task and for the basic and ultra basic rocks. |
From references (Ustinov V.I. et al, 1987; Marakushev A.A. et al, 1988; Murav'eva N.S., 1989;
Taylor H.P. et al, 1962, 1965; Anderson A.T. et al, 1971; Onuma N et al, 1972; Yeser T.K. et al, 1978;
Clayton R.N. et al, 1983; etc.) the data on 99 simples with analyses of isotopes of oxygen (18 = O) in minerals from meteorites and rocks (eclogites, kimberlites, lercolites, tholeiites, alkaline basalts, ultrabasic and the basic rocks, gavajaites, gabbros, adamelites) various globe regions are collected.
Minerals: Na-feldspars (Pl), clinopyroxines (Cpx) and olivines (Ol).
Fig.1. Distribution of isotopes of oxygen between Pl and Cpx. In figures Px = O(Px), Ol = O(Ol), Pl = O(Pl) According to analyses melting inclusions Pl were allocated at 1100-1300оС; Cpx (is more often Dio) - 1100-1300оС; Ol-1100-1300оС (Naumov V.P., 1979; Barsukov V.L. and etc., 1981; Sharygin V.V. et al, 1997; and so forth). Usually temperatures of Cpx crystallization (Тcr) on 20-40оС are higher Pl Тcr. For similar rocks Тcr (Ol) are close to those in Cpx. As a whole it is possible to believe, that minerals crystallize at 1220-1250оС. Geochemical geothermometers for pyroxenes (Px) give underestimated on 200-300оС value of Тcr.
Fig.2. Distribution of isotopes of oxygen between Cpx and Ol It is accepted us, that the isotope structure of oxygen of an investigated mineral of M is in thermodynamic isotope balance (TIB) with joint C (an axiom 1), and pair М-С is a product of disintegration of initial parent substance (protomatter) (an axiom 2) as only formation the TIB in this case is explained. Thus intramolecular isotope balance here takes place. Hence, the task consists (1) in definition of substance of C connection and (2), proceeding from chemical features of M and C, revealing of protomatter substance.
On [1, 2] distribution of isotopes O between minerals is described by the equation O(Px) = SО(Pl) + A, where A = (O(C2) - SO(C1); S = 1000ln(Pl)/1000ln(Px) = [O(Pl) - O(C1)]/ [O(Px) - O(C2)] (the same for Px - Ol). Selecting substances C1 and С2, calculate value S and compare to its actual size; on their concurrence also estimate substance of connections C, isotopic equilibrium to minerals M. It was studied the TIB between minerals on the one hand and H2O, CO2 and CO-with another; influence of gas diffusion with factor of diffusion D  was taken into account and the amendment which is taking into account influence on distribution of oxygen of diffusion 13СО2 was 12 entered. At CO2 diffusion oxygen moves as in forms С16О2 and С18О16О with a parameter of division D = D (12С16О2)/D(12С18О16О) = [M(12C18О16О)/M(12С16O2)] (M (X) - molecule weight of a components X), and in the form 13С16О2. In the latter case division of isotopes O will be determined by a parameter D = D (12С16О2)/D(13С16О2) = [ М(13C16О2)/ М(12С16O2)]. A parameter of division of components under action of individual processes 1 and 2 - product of parameters of individual processes division, i.e. D = D1D2. As at moving components 13СО2 its oxygen 16О migrates in the 18 same side, as oxygen О components С18О16О and dilutes it, the common division of oxygen 18 isotopes will be determined by factor D = [D(12С16О2)/D(12С О О)]/[D(12С16О2)/D(13С16О2)] = D(13С16О2)/D(12С18О16О) = [M (C18О16О)/ M(13СO2)] = 1,045.
For the decision of a task diagrams (fig.1 and 2) in coordinates O (Pl) - O(Px) were under construction (O(Px) - O(Ol)). On the diagrams points well turn in direct lines with the angular coefficient S, interpretive as isotherms in 1250оС. The equations, describing these straight lines, are given in figures. In distribution of pair (Px-Pl) it is observed the samples groups of described by the equations - 1: O(Px) = 1,38О(Pl) - 1.86; 2: O(Px) = 1,33О(Pl) - 2.6; points of samples of group do not get on one of the named straight lines. Strict bond of these samples to the certain types of meteorites are not present, though more often samples of group 1 meet in meteorites such as Ure, Mes, Euc; tests of group 2 are H, L, LL, СЗ, Nk, basalt achondrites, and also to rocks of a terrestrial origin;
samples of group 3 are H, L, L, to terrestrial rocks. As a whole, distributions of these tests are described by one equation (fig. 1).
In result the TIB between minerals it is not established; the hypothesis of balance of minerals with СО2 is most acceptable in view of diffusion СО2 with fractionation depth Nd = 0.4 (at the account of the amendment on influence 13СО2) or Nd = 0.19 (without taking into account this amendment).
Thus, the supplier 18О in minerals is CO2, being coordinated with good solubility СО2 in ultra basic melts at high pressures (Kadik A.A. etc., 1982; Eggler D.H., 1978; Mysen B.O., et al, 1976). At the analysis of influence СО2 on fusion of silicates reactions carbonatization of pyroxenes (Ens) and olivines under pressure (Kadik A.A. etc., 1982) were considered:
Ol + Dio (диопсид) +CO2 Ens + Dol (dolomite);
Ol + Dol + CO2 Ens + Mgt (magnesite);
Opx + Dol + CO2 Mt (magnetite) + Q.
According to infra-red spectroscopy (Mysen B.O., et al, 1976) in pair above peridotite melt, sated СО2, is СО2 and СО3-2; СО3-2 it is established also in the cooled glasses of these melts.
Formation СО3-2 goes under the scheme hypoteticaly:
а). 2(SiO4)-4 (melt) = (Si2O7)-6 (melt) + O-2 (melt);
б). СО2 (steam) + O-2 (melt) = СО3-2 (melt).
In the sum this process is described by the equation:
2 (SiO4)-4 (melt) + СО2 (pairs) = (Si2O7)-6 (melt) + СО3-2 (melt).
The analysis and the received results speak, that the equations do not correspond to real natural systems. Orthopyroxene (Ens) isotopicaly are equilibrium not to carbonates, but only СО2. On isotope data Mt is equilibrium only to rutile, quartz always is equilibrium only to water; Mt with Q in balance never meet . The offered mechanisms do not explain the TIB between gas and minerals though explain the TIB with (Si2O7)-6.
Probably, СО2 forms with Si connections of species Si(CO3)4-4 and at its carry plays a role of transport. Reaction is described by the equation 2Mg+2 + Si(CO3)4-4 = () = Mg2SiO4 + 4CO2. This mechanism explains presence СО2 in high-temperature melting inclusions. In this case role СО2 is similar to a role of water in melts.
Influence of the diffusion being the mechanism of achievement the TIB, is caused by that speed of diffusion of components is less than speed of crystallization of minerals. There of the TIB is established not completely (Grinenko V.A., Ustinov V.I., 2004).
References 1. Макаров В.П. Способ определения глубины диффузионного фракционирования изотопов элементов природных соединений. Авт. свид. 1260906 // Бюлл. изобр. откр. 1986.З6.197.
2. Макаров В.П. Способ определения температуры образования природных соединений. Авт.
свид. 1312504 // Бюлл. изобр. откр. 1987.19.192.
3. Makarov V.P. Some problem of decision of thermodynamic reserve task // Electronic Scientific Information Journal “Herald of the Department of Earth Sciences RAS” № 1(21)2003.
URL: http:/www.scgis.ru/russian/cp1251/h_dgggms/1-2003/ informbul-1/mineral-17e.pdf.
4. Makarov V.P. At mechanism of minerals formation: magmatic and metamorphic rocks // Electronic Scientific Information Journal “Herald of the Department of Earth Sciences RAS” № 1(22)2004.
URL: http://www.scgis.ru/russian/cp1251/h_dgggms/1-2004/informbul-1_2004/izotop-2e.pdf Electronic Scientific Information Journal “Herald of the Department of Earth Sciences RAS” № 1(24)ISSN 1819 – Informational Bulletin of the Annual Seminar of Experimental Mineralogy, Petrology and Geochemistry – URL: http://www.scgis.ru/russian/cp1251/h_dgggms/1-2006/informbul-1_2006/izotop-3e.pdf Published on July, 1, © Herald of the Department of the Earth Sciences RAS, 1997-All rights reserved