Traces of Weathering

At the moment of meteorite landing the process of terrestrial weathering commences. Some meteorites are found several days after the fall, some of them spend thousands of years in the polar ice or in the desert. During this exposure time weathering events occur. We decided that the monitor of the magnetization during simulated laboratory weathering would elucidate an important and relatively unknown level of meteorite magnetic record contamination. Three levels of natural exposure were selected for consideration. These meteorites were then the test cases. The weathering that took place in one of the meteorite (L 6) that landed in Libya desert perhaps thousands years ago is related to desert varnish formation. The iron meteorite Campo del Cielo (I) landed in Argentina 5000 years ago is covered by oxide crust and appears fragmented by invasion of rust along cracks. Meteorite Zebrak (H 5) found several days after the fall contains optically visible oxidation traces as well.

Weathering Simulations

To simulate meteorite weathering, we bathed our meteorite samples in different salt solutions for periods of three weeks. We used saturated solutions of sodium chloride NaCl, calcium chloride CaCl₂ and sodium sulphate Na₂SO₄. The samples were stepwise demagnetized using AC magnetic field up to 0.24 Tesla. Before the initiation of the weathering experiments samples in the solutions were oriented with respect to the earth magnetic field. We oscillated the solution temperature on a daily basis from room temperature 20 °C up to 90 °C. The variations in the temperature influences the solubility of the salt, with is penetrating the pore space and cracks in the meteorite. This process speeds up the weathering processes in the meteorite.

Experiment process

Experiment Results

After three weeks the samples were removed from the solution, dried and oriented accounting for the orientation in the earth magnetic field. We AF demagnetized these samples and compared the results with the AF demagnetization data taken before the experiment. The data were also compared with the demagnetized reference samples placed in the laboratory in order to quantify the viscose magnetization acquisition. The most effective salt solutions in the three week magnetic weathering process were NaCl and Na₂SO₄. Magnetizations due to the weathering products resulted in an increase of more then one order of the magnitude and were comparable with the NRM prior to AF demagnetization. The post experimental stability against AF demagnetization of the Libya sample was lower than the original magnetization stability. The stability of Zebrak was higher, and the stability of the Campo del Cielo magnetization was about the same as the original one. There was practically no effect observed at samples leached in CaCl₂ solution and at reference samples.The measured values can be find in the table.

Weathering of Campo del Cielo meteorite in CaCl₂

Weathering of Libya meteorite in CaCl₂

Weathering of Zebrak meteorite in CaCl₂

Weathering of Campo del Cielo meteorite in NaCl

Weathering of Libya meteorite in NaCl

Weathering of Zebrak meteorite in NaCl

Weathering of Campo del Cielo meteorite in Na₂SO₄

Weathering of Libya meteorite in Na₂SO₄

Weathering of Zebrak meteorite in Na₂SO₄

Conclusions

Meteorite weathering appears to be important as a consideration when interpreting meteorite magnetic records. Chemical weathering can be very rapid under specific conditions with an evident influence on the meteorite magnetic record and the magnetomineralogy. It is important to consider weathering effects during sample selection and magnetic record interpretation. Meteoritic samples must be selected with care to eliminate strongly weathered samples. In addition to the meteorite composition and structure the time elapsed from the meteorite fall till the meteorite find is an important parameter to consider in meteorite selection. During this time period most terrestrial oxidation take place that can affect the magnetic signature and magnetomineralogy. According to the data observed, it is probable, that the origin of magnetization of all three meteorite samples has a terrestrial component of significance.