| dc.description.abstract | This thesis embodies the results of computational studies of the physico-chemical conditions associated with the condensation of high-temperature phases within the primitive solar nebula. The focus of these studies is on understanding the effect of two compositional parameters within the nebular gas which brought about large separation in the condensation temperatures of metal (represented here by the Fe-Ni-Si alloy) and silicates. The two compositional factors chosen are:
(a) the abundance of oxygen, and
(b) the presence of charged species within the primordial nebula.
The studies have been carried out using the program SOLGASMIX, which utilizes the method of steepest descent for minimizing the free energy of a system consisting of a gas phase, liquid and solid solutions, and stoichiometric phases for a fixed pressure, temperature, and system composition.
Studies to articulate the effect of oxygen fugacity on the condensation of high-temperature phases have been carried out by varying the amount of oxygen in the input data of SOLGASMIX between 12.0 and 24.0 moles (which corresponds to a variation in the C/O ratio between 1.0 to 0.5). The effect of variation in the input amount of oxygen on the fugacities of important gaseous species in the H-O-C-N system (H?, CO, CH?, H?O, NH?, and CO?) reveals an important aspect of the distribution of oxygen in the equilibrium assemblage. It is seen that oxygen preferentially attaches itself to carbon to form CO, and since carbon monoxide consumes more than 95% of carbon, the availability of oxygen for stabilizing other oxygen-bearing species sharply reduces under low f(O?) conditions.
This has an interesting consequence on the condensation temperature of the first-formed silicate, olivine, and the Fe-Ni-Si alloy. The condensation temperature of olivine goes down from 1448 K for computations with 24.0 moles of oxygen to 1152 K when only 12.0 moles of oxygen are included. The Fe-Ni-Si alloy's condensation temperature, however, remains remarkably constant throughout the range of input oxygen, at 1470 K, except under extreme reducing conditions when it rises sharply to 1613 K. Thus, the difference in the stabilization temperatures of these two phases at a pressure of 10 bar varies from 22 K (24 moles oxygen) through 37 K (20.1 moles), 49 K (18 moles), 81 K (15 moles) to 481 K (12 moles). The log f(O?) stability range for olivine varies from -30.8 to -16.9 and for the Fe-Ni-Si alloy the variation is in the range -26.9 to -16.6.
Computations under extreme reducing conditions also indicated a reversal of the condensation sequence between corundum (the first-formed phase under "normal" oxidation conditions) and Fe-Ni-Si, with the alloy condensing before corundum under low f(O?) conditions. Additionally, a new phase, Fe-Ni carbide solid solution, becomes the first phase to condense in low f(O?) environments. The P-T stability fields of the high-temperature condensates under extreme reducing conditions also show the overall depression in the condensation temperatures of the oxide and silicate phases and an increase in the Fe-Ni-Si condensation temperature in the P-T range of 1 to 10?? bar and 1000 K to 2000 K, respectively.
Three sets of computations have been carried out to articulate the abundance, composition, and consequence of the presence of charged species in the condensation of high-temperature phases in the primitive nebula. They include charged species of:
(a) oxygen (O?, O??, O??, O?, O??, O??),
(b) the H-O-C-N system (C?, C??, H?, CH?, NO?), and
(c) the Al-O system (O?, O??, Al?, AlO?, AlO??, AlO?, AlOH?).
The P-T range for the oxygen study was 10 to 10?? bar and 1500 K to 2000 K; for the H-O-C-N system study, 10?? to 10?? bar and 1300 K to 1650 K; and for the Al-O system, 10?? to 10?? bar and 1100 K to 1950 K.
In general, gaseous charged species are seen to be stable in the computed equilibrium assemblage, with the negatively charged species being preponderant. In the first set of computations involving oxygen charged species, for instance, the abundance was in the order: O? > O > O?? > O? > O?? > O??.
Charged species in the H-O-C-N system further indicated that their presence had an effect in modifying the condensation behavior of the first-formed solid phases. Thus, at 10 bar, the condensation temperature of Fe-Ni-Si increased by 63 K, whereas that of melilite, spinel, olivine, and clinopyroxene decreased by 68 K, 46 K, 47 K, and 6 K respectively when compared to the condensation temperatures in computations without charged species. This resulted in an increase in the difference between the Fe-Ni-Si and olivine condensation temperature from 37 K in a neutral gas to 147 K when charged species were considered.
The computations in the Al-O system brought out yet another consequence of the presence of charged species in the condensation process. Since the Al-bearing gaseous charged species are quite stable, they retain most of the input amount of aluminium in the gas phase. The result is that aluminous minerals such as corundum, gehlenite, and spinel condense at much lower temperatures. This happens principally because at lower temperatures as well as higher pressures, the importance of charged species diminishes (as is seen in the reduction of their fugacities), and the condensation pattern of the various phases becomes identical to the situation when no charged species were considered. | |
