Portrait from the past

Studies concerning the past temperature of the earth?s surface help in discovering minerals

A group of Brazilian researchers are involved in the study of the temperature variation of the earth?s crust over millions of years. Besides the knowledge on the thermal history of the continent, they have developed economic applications relevant to the identification and the characterization of minerals. “We are using a dating technique that is helping in the discovery of deposits of crude oil, natural gas, diamonds, iron ore and bauxite, among others, because in the manner in which it helps us to understand the process of erosion and the depositing of sediments that occurred millions of years ago, it gives us clues concerning the existing minerals below the Earth”, says the geologist Peter Christian Hackspacher, from the Petrology and Metalline Department of São Paulo State University (Unesp), of Rio Claro.

“Right from the beginning of our studies, we have contributed with the linkage between the occurrence of bauxite and the temperature of the rocks in the past in the deposit near the town of Poços de Caldas, Minas Gerais, explored by the companies Alcoa and Companhia Brasileira de Alumínio (CBA) among others”, explains geologist Hackspacher, the coordinator of the thematic project funded by FAPESP on this theme. The age of the formation of an ore bed such as this or a crude oil deposit revolve at around about thirty million years and depend on various geological factors. In the case of the bauxite deposit, the increase in temperature in the crust in a determined period, such as that at Poços de Caldas, influenced the raising up of the rocks of an area, allowing for a process of lixiviation (selective removal of ions, with loss or gain of electrons of silica and the consequent enriching of aluminum (this chemical element appears in nature normally together with silicon) which linked to the mineral.

In the case of crude oil the situation is different. A region warmed because of various geological factors, among others tectonic processes (the study of force fields that act upon the earth’s crust bringing about surface movements), would be favorable to the formation of crude oil. This is the so-called phenomenon of maturation, when the organic material found the sedimentary deposits in sedimentary basins – an example is the Santos basin or the Campos basin – attains the range of  60oC and 120°C.

In this situation, the organic material transforms itself into hydrocarbons (crude oil and/or natural gas). The method could also be used for the evaluation of aquifers and their respective zones of reloading, the regions where the water table is replenished. In another project, the group, which also included researchers from the State University of Campinas (Unicamp), worked in partnership with Petrobras and on projects financed through the Petroleum and Natural Gas Sectorial Fund (CT-Petro), of the Ministry of Science and Technology, in conjunction with the Federal University of Rio Grande do Sul (UFRGS). “Our objective is to improve our knowledge concerning petroleum fields in different regions of the country, that stretch from Rio Grande do Sul to Espírito Santo, making a thermochronology map (the study of rock temperatures throughout time) of Brazilian geological relief”, says Hachspacher.

“The quality of data generated by the thematic project, which through its multi-disciplinary character had many national and international repercussions in this area of study, caught the attention of private companies and Petrobras. The state company has agreed to enter into partnership in the CT-Petro projects that as yet do not have consolidated results.” In relation to the thematic project, one of the consequences was the founding of NuclearGeo, in November of 2003, a company housed at the Technology Based  Incubator of Unesp in the town of Rio Claro (Incunesp). NuclearGeo’s objective, the company having been founded by ex-students and professors of Unesp, UFRGS and Unicamp, is to make use of the techniques and knowledge developed during the execution of the thematic project for the exploration of minerals and the identification of aquifers and their properties.

Surface temperature
In order to achieve the results that already extrapolate into the academic environment, Hackspacher’s group concentrated their studies around the identification of surface and sub-soil temperatures in determined eras of the planet’s history. This allowed them to model the evolution of ridges, plateaus, and other landscapes, as well as to determine the processes of raising and erosion of the earth’s surface in distant geological eras. An example of this scenario is the Mantiqueira Ridge,  a mountainous massif located in the southeast region of the state, which previously had been much more imposing.

Some 120 million years ago, when the separation of Pangaea had been in course, the massive continent that our planet had formed, placing on opposite sides of the Atlantic the South American and African continents, the Mantiqueira had passed through a gradual process of being raised (lifting movement) and reached an altitude of around 4,000 meters higher than it is today. Some time afterwards, the massif  was progressively eroded until it attained the maximum height of 2,800 meters. The resulting sediments of this abrasion were deposited on the Brazilian continental platform, in the area that makes up the current basins of Campos, in the state of Rio de Janeiro, and the Santos basin in the state of São Paulo.

The study of the direct relationship between rock temperature and the configuration of the landscape of a region functions in the following manner: the hotter a determined rock was in the past, the deeper it had been positioned. And, the colder the rock, the closer it had been to the earth’s surface. All of this has to do with apatite, a colorless mineral composed of calcium phosphate and used in the manufacture of fertilizers and insecticides, which is the main objective of the study of dating by the group. This mineral is a type of witness as to the temperature of the earth’s crust at different eras. Apatite contains uranium in its interior and can enter into fission (the breakdown of the uranium’s atomic nucleus into two parts) in a spontaneous manner producing damage in its interior – traces of fission – which is analyzed in an optical microscope after chemical treatment.

In the world, the start of the studies of thermochronology through traces of fission began by methods mainly developed by Australian researchers beginning in the decade of the 80s. The technique of dating and thermo-chronology used in the thematic project is an improvement upon these studies and was developed at the Physics Institute (IF) of Unicamp. The starting point of the method is the analysis of the size of the traces that give clues to the temperature of the mineral in the past. “Apatite is very sensitive to temperature. We know that its fission trace has its length reduced when the temperature was higher and the time for which the mineral had been submitted was longer”, explains the physicist Júlio César Hadler Neto, a professor at the IF of Unicamp who participated in the thematic project, and had been responsible for the area of fission traces. By studying the grains of apatite, the researchers managed to make a topography map of a determined region in the past. If, for example, the age had been close to 30 million years, the chance of the existence of minerals such as bauxite is considerable. Simulations are also carried out for veins of minerals such as iron ores and diamonds.

“With our studies we are resurrecting the raising and lowering of the southeast region of Brazil, between the north of the State of Sao Paulo and the south of the State of Minas Gerais, over the last 250 million years. We have researched rocks and minerals that can  provide information about the thermal, tectonic histories and the stratography (study of sedimentary rocks) and geomorphology (process of surface formation) as well as the evolution of the landscape”, says Hackspacher. Around the world there are  approximately twenty groups that are working with fission traces, a methodology that begun back in the 60s, but there are few that are making use of all of the tools used by the Brazilian scientists.

“With our own research, we have created criteria adequate to our latitude and we have substituted imported models of regions of geological and climatic evolution distinct from ours.” In the opinion of professor Hackspacher, the research performed and coordinated by Hadler of Unicamp, Antônio Saad from the Geosciences laboratory of the University of Guarulhos (UnG), and Iandara Mendes from the Territorial Planning Department of the Geosciences Institute of Unesp, will allow for the development of new technologies, such as a piece of software for modeling the thermal history starting from apatite and new techniques for the separation and concentration of this mineral.

Sample collection
The first challenge for the researchers who work with apatite is to find it. For this reason, geologists go out into the field, equipped with their hammers, in order to collect stones that may contain the mineral – up until this moment the team has collected samples from five hundred and forty (540) different locations. “Through the origin and the mineralogical properties of the rocks, we know which are those that have the higher concentration of apatite and we collect them.

Granite, shale and gneiss, are some of them”, says Hackspacher. Since apatite is present in rocks in microscopic size – grains of around 50 micra (1 centimeter divided by 200) -, it is necessary to mill and sift the collected samples in order to locate the mineral. The powdered sample then passes through a process of beating (the mineral is mixed with water within a type of concave pan), which can be manual and reminds one of the work of gold miners. The concentrate obtained, composed of apatite and other heavy minerals such as zircon, sulfates and oxides, is collected in an isodynamic separator, a piece of apparatus that isolates the magnetic metals from those smaller and non-magnetic. Apatite falls into this last group.

Irradiation beam
After a detailed process of separation, the apatite crystals are immobilized in small cubes of epoxy resin of less than one square centimeter (1cm2) – each cube contains at least forty crystals of the mineral -, which are then sliced in order to reduce their thickness. The slivers of epoxy are polished and undergo chemical attack from a solution of nitric acid. The acid bath reveals the traces of fission of the apatite, which, from this moment on, is almost ready to be studied. The final stage consists in the irradiation of the apatite by a beam of neutrons at the nuclear reactor of the Nuclear and Energy Research Institute – IPEN in Sao Paulo.

“The original apatite contains the traces inherited from its geological history. When the mineral is irradiated, the uranium present in the apatite sensitizes a mica plate that was fixed to it. This plate now has a certain quantity of induced traces. The relationship between the fossil traces, originally present in the crystal of apatite, and those induced by the process of the nuclear reaction will be used in the calculation of the age of the fission trace”, explains Hadler Neto.

The discoveries made by the Brazilian researchers have conferred international prestige upon the group. Currently the team, of a multidisciplinary profile, is formed by around forty five professionals from different institutions in Brazil and abroad. As well as the Brazilian universities, there are partnerships with the universities of Pisa in Italy, Kansas in the United States, Heidelberg and Freiberg in Germany, and Porto in Portugal.

The Project
The history of the disinterment of the South American platform, the example being the Brazilian Southeast Region: thermo-chronology for traces of fission and the systematics ar/ar and sm/nd (nº 00/03960-5); Modality Thematic Project; Coordinator Peter Christian Hackspacher – Unesp – Rio Claro; Investment R$ 1,305,047.28 (FAPESP)