Meteorites are classified in three main groups: iron meteorites, stony meteorite and stony-iron meteorite called pallasite. They appear to be samples of the core-mantle boundary layer of a small planet broken apart by collision. A pallasite is a blend between an iron-nickel alloy from the core and olivine crystals (Mg2SiO4) from the mantle. The Esquel meteorite is classified as one of them.

A chronometer is used to determine the date of separation between the two phases, metal and olivine in the meteorite. In this case we used the 53Mn-53Cr isotope system. The 53Mn is an extinct radionuclide which decay to 53Cr via electrons capture: 53Mn → 53Cr For find the time we used the decay equation of 53Mn:

53Mnt=53Mnoe

-λt

(53Mn/55Mn)t=(53Mn/55Mn)oe-λt

[(53Cr/55Mn)t=(53Mn/55Mn) oe-λt ]olivine-metal

The primary purpose of this study is to see if it might be feasible to obtain the time of separation of the olivine and metal in the Esquel meteorite. To obtain this value it is necessary to use extinct radionuclides with a halflife of some millions years which was present in the very early history of the solar system. With mass spectrometry is possible to obtain the isotope ratios of Cr and the concentration of trace elements in the studied samples. Mn and Cr elemental analyses were done by Laser Ablation Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS). An 80-micron spot material was ablated with a laser and the samples material is ionized in an RF generated plasma ions to determine the trace elements concentration. Cr isotope ratios were measured by the Thermal Ionization Mass Spectrometer (TIMS) that produces ions by a hot filament.

Thermal Ionization Mass Spectrometer (TIMS)

For keep a good vacuum inside we used liquid nitrogen for freeze any water particle present in the chamber. The current was set up from a range of 1850-2220 mA and the temperature between 1220 to 1340oC.

Several different experiments were run to determine the best way to make precise measurements of Cr isotope ratios. In the best of these, the 53Cr/52Cr ratio measured in standard was 0.11323±0.00018. This value is the reciprocal of the 52Cr/53Cr ratio.

Age dating for Esquel meteorite using trace elements analysis and mass spectrometry techniques

Introduction

Abstract

I am very grateful for the support provided by the Dr. Roy Odom, Nicole Tibbets and the entire geochemistry department at the NHMFL. I would like to thank Jose Sanchez Assistant Director of the CIRL program, Mischa Draime and all the staff of this program and the National High Magnetic Field Laboratory. I am proud to be an REU 2006.

• Pearce, N., Perkins, W., Westgate, J., Gorton, M., Jackson, S., Neal, C. and Chenery, S. A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials. Vol. 21, No.1 p. 115-144.• Lugmair, G. and Shukolyukov, A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochimica et Cosmochimica Acta, Vol. 62, No. 16, pp 2863-2886.• Lugmair, G. and Shukolyukov, A. (2004) Manganese-Chromium isotope systematics of enstatite meteorites. Geochimica et Cosmochimica Acta, Vol. 68, No. 13, pp 2875-2888.• Wasson, J., Choi, B. (2003) Main-group pallasites: Chemical composition, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta, Vol. 67, No. 16, pp 3079-3096.

MachinesThermal Ionization Mass Spectrometer (TIMS), Laser ablation

inductively coupled plasma-mass spectrometer (LA-ICP-MS), Loaded tungsten filament with standard solution of Chromium, Polish machine (Handimet 2 roll grinder and Metaserv 2000)

GeneralSamples of meteorite olivine, Epoxy hardener, Planchet, Plastic

bottles Dryer, Liquid nitrogen, Chromium solution diluted in HNO3, Silica gel, Boric acid (H3BO3) and water

Materials

Cindy M. Figueroa1, Leroy Odom2 and Nicole Tibbets2

1University of Puerto Rico, Rio Piedras Campus, 2Florida State University, National High Magnetic Field Laboratory, Geochemistry Department, Tallahassee, Fl

Procedure

Acknowledgements

Aluminum disk with olivine sample

1.Two grains of Esquel olivine2.Four grains of standard olivine (KLH97)3.Four grains of standard olivine (KLH97)4.Four grains of NIST (glass reference) References

Four holes were made on the aluminum disk with a drill to put two selected olivine grains inside. These holes were filled with a solution of epoxy and dried in an oven for make it hard. The disk was polished with different abrasive paper using Handimet 2 roll grinder and Metaserv 2000 that are polish machines. With this disk the values of trace elements concentration were obtained from the LA-ICP-MS.

Results

3 tungsten filaments were loaded with a chromium standard solution, silica gel and boric acid for placed it in the TIMS main chamber. Initially, the vacuum chamber was 7.1x10-8 mbar.

The values for (53Mn/55Mn)original were obtained using the decay

equation (53Mn/55Mn)t= (53Mn/55Mn)oe-λt at different assigned t=time

values which allow the calculation of (53Mn/55Mn)t.

Mean of the ratio values of 52Cr/53Cr from TIMS in three tungsten filaments

8.88.818.828.838.848.858.868.87

0 5 10Number of blocks

Ra

tio

52C

r/53

Cr

Filament #1 Filament #2 Filament #3

Conclusion

Ratio (53Mn/55Mn)original in function of time

y = 1.371e-0.1866x

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80

Time (million years)

Ra

tio

(53M

n/55

Mn

) ori

gin

al

In previous experiments the age of separation between olivine and metal had been found using diverse techniques and methods. But in this one a new process was establish which can be used to find an accurate value of the time of differentiation of the Esquel parent body. The time of separation between the two phases reflects the time of the core and mantle formation. The uncertainty of the obtained chromium isotope ratio allows determining the difference in the 53Cr/52Cr ratios of the olivine and the metal phases very precisely. This is feasible if the separation event happened approximately 45 million years after the nucleosynthesis, which is the origin of elements.

The age of separation between two phases in a pallasite could be determine using a chronometer like 53Mn-53Cr isotope system, 53Mn is a short-lived radionuclide with a half-life of 3.7x106 years, so this radionuclide it might have been present when the separation of the 2 phases took place. The primary purpose of this study is to see if it might be feasible to obtain the time of separation of the olivine and metal in the Esquel meteorite. For this the mass spectrometry is better than other techniques. The determined 55Mn/52Cr ratios are 0.738639 for the olivine and 2.658259 for the metal. According to the results is feasible determine the difference in the 53Cr/52Cr ratios of the olivine and the metal phases very precisely.

Relation between olivine and metal ratios

0.09

0.11

0.13

0.15

0.17

0 0.5 1 1.5 2 2.5 3

Ratio of 55Mn/52Cr

Rat

io o

f 53C

r/52

Cr

Slope t=20 Slope t=30 Slope t=45

Olivine Metal

The graph shows what the 53Cr/52Cr ratios in the olivine and metal would be for times of 20, 30 and 45 millions years. The difference between the Cr isotope ratios for metal and olivine at 45 million years is 0.00045, slightly greater than the precision of Cr isotope measurements made in this experiment (±0.00018)