Sulfidation and Oxidation States of Calc-Alkaline Granitoids

EsseneEAR-0230098本工作将评价钙碱性花岗岩的岩浆硫化和氧化状态。这些测量将在犹他州宾厄姆-帕克城带、智利中部斑岩铜带和墨西哥西部马德雷山脉中部的普罗维登西亚-康塞普西翁-德尔奥罗地区的花岗岩套上进行。不同钙碱性花岗岩类的氧逸度在5个数量级以上，硫逸度在3个数量级以上。对黑云母中Fe~(3+)/Fe~(2+)的穆斯堡尔谱和H_2O的测定将限制黑云母的氧化还原反应，并为评价黑云母花岗岩的氧化还原状态提供另一种方法。此外，还将对同一套钙碱性花岗岩中的锆石进行Ce4+/Ce3+的系统测定。预计这些结果将表明，花岗岩类在整个结晶历史中具有独特的氧化还原特征。岩浆的氧化状态很可能是源区遗传的，而不是后期结晶、围岩混染、热液蚀变或脱气历史造成的。这一研究结果有望对钙碱性花岗岩类的源区和演化提供线索。这项研究的更广泛的意义涉及(1)与这些花岗岩类有关的含硫火山喷发对气候的影响，以及(2)斑岩铜矿体在结晶过程中的形成。高度氧化的花岗岩类中的硫主要以硫酸盐形式存在，在主要火山喷发时释放形成硫酸。这些释放对地球气候产生了可测量的影响，在大喷发期间。菲律宾的皮努图博和墨西哥的El Chichon。岩石记录中的斑岩铜矿床与氧化花岗岩类侵位密切相关。了解这种关系对铜矿的定位很重要，大多数铜矿都产于这些矿床中。

EsseneEAR- 0230098This work will evaluate magmatic sulfidation and oxidation states in calc-alkaline granitic rocks. These measurements will be carried out on granitoid suites from the Bingham-Park City belt of Utah, the central Chile porphyry copper belt, and the Providencia-Concepcion del Oro area in the central Sierra Madre Occidental of Mexico. Calculation of oxygen fugacity ranges over five orders of magnitude and sulfur fugacity over three orders of magnitude in different calc-alkaline granitoids. Mossbauer determination of Fe3+/Fe2+ and measurement of H2O in biotites will constrain redox reactions for biotite and provide another way to evaluate the redox state of these granitoids. In addition, systematic determinations of Ce4+/Ce3+ will be undertaken on zircons from the same suites of calc-alkaline granitoids. It is anticipated that these results will show that the granitoids possessed their unique redox signature throughout their crystallization history. The oxidation state of the magmas is most likely inherited from source regions rather than being imposed by late stage crystallization, wall-rock contamination, hydrothermal alteration or degassing history. Results of this study are expected to shed light on the source regions and evolution of calc-alkaline granitoids. Broader implications of this study relate to (1) impacts on climate by sulfur-bearing volcanic eruptions that are related to these granitoids, and (2) formation of porphyry copper ore bodies during their crystallization. The highly oxidized granitoids have sulfur mainly as sulfate, which is released in major volcanic eruptions to form sulfuric acid. These releases had a measurable impact on earth's climate during major eruptions of Mt. Pinutubo in the Philippines and El Chichon in Mexico. Porphyry copper ore deposits in the rock record are strongly correlated with emplacement of oxidized granitoids. Understanding this relationship is important for location of copper ores, most of which occur in these ore deposits.