Troctolite 76535

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The troctolite 76535 is a lunar rock sample , the 1972 during Apollo 17 mission in Taurus-Littrow Valley was collected. The sample weighs 155.5 grams and measures 5 centimeters at its widest point. It was collected at Geology Station 6 at the foot of the North Massif.

The troctolite 76535 is a coarse-grained, plutonic rock , which was probably only very slowly cooled. This olivine and plagioclase- bearing cumulative rock with a granular polygonal structure comes from the early history of the moon . It can be regarded as the final link in the magnesium- accentuated series of Plutonic highland rocks.

Petrography and Mineralogy

NASA image of troctolite 76535

In the mode, plagioclase occupy 50 to 60 percent by volume , olivine 35 to 37 and orthopyroxene 4 to 5 percent by volume. The white, transparent to slightly milky plagioclase grains (An 96.2 ) are 2 to 7 millimeters in size and show beautiful twin traces on their cleavage surfaces . The shiny, honey-brown, 2 to 3 millimeter large olivines (Fo 87.3 ) occur in groups. The up to 3 millimeters large, indistinctly crystallized orthopyroxene fill gaps between plagioclase and olivine crystals.

The structure of the rock sample is overall grainy with gently rounded grain boundaries, which often meet at an angle of 120 °. This suggests a grain ripening process that resulted in surface minimization - characteristic of metamorphic subsolidus structures.

In addition to the main mix of plagioclase, olivine and orthopyroxene, the following minerals occur as accessories:

A special feature are symplectic adhesions of chromium spinel with calcium-rich and calcium-poor pyroxene at the grain boundaries between olivine and pyroxene.

Petrological investigations showed (including by means of the above-mentioned symplektites) that this plutonic rock sample was formed at a depth of 40 to 47 kilometers and thus comes from the middle to lower lunar crust. The investigations also showed that the sample at this depth as Kumulat had formed, then cooled slowly and in their mineral constituents under annealing ( engl. Annealing) and recrystallization a prevailing physical conditions corresponding, thermodynamic equilibrium could adjust. Based on the average grain size of 3 millimeters and further assumptions, Stewart (1975) was able to calculate the glow range (100 to 600 ° C) over a period of around 100 million years.

Chemism

The following table gives geochemical analysis values ​​for the troctolite 76535:

Oxide
wt.%		 rock Trace element
ppm		 rock
SiO 2 42.88 Ni 25-44
TiO 2 0.05 Cr 753
Al 2 O 3 20.73 Zn 1.0-1.2
FeO 4.99 Sr 111-114
MnO 0.07 Rb 0.20-0.24
MgO 19.09 Ba 33
CaO 11.41 La 1.51
Na 2 O 0.23 Eu 0.73
K 2 O 0.03 Nd 2.3
P 2 O 5 0.03 Zr 12.0-23.6

The sample is ultramish and enriched in rare earths compared to chondrites . It has a positive europium anomaly.

Dating

Dating troctolite 76535 turned out to be problematic as different dates were used to find different ages. The oldest result of 4610 million years BP was provided by Papanastassiou and Wasserburg (1976) using the rubidium-strontium method , an enormously old age before the moon was formed. With the samarium neodymium dating , 4260 and ultimately 4330 million years BP were determined. The lead-based methods found 4270 million years BP ( lead-lead dating ) and 4236 million years BP ( uranium-lead dating ). Finally, the plateau ages of the potassium-argon dating ranged from 4270 to 4160 million years BP. The results of 4236 million years BP represent the age of inclusion for radionuclides and reflect the age of formation of the metamorphic structure. Premo and Tatsumoto (1992) therefore also give 4260 to 4230 million years BP as the actual age of the rock sample.

meaning

The rock sample 76535 is an important link in understanding the evolution of the moon over time. Among the samples returned to Earth by the Apollo Project, troctolite 76535 is likely to be the most interesting because it is undeformed and very old. Could her thermochronological calculations are made, the time to notice if the moon is in its development history a core and a core dynamo had formed. The results of these calculations actually endorsed the dynamo theory.

Individual evidence

  1. ^ RC Gooley, et al. a .: A lunar rock of deep crustal origin . In: Geochimica Cosmochimica Acta . tape 38 , 1974, p. 1329-1339 .
  2. ↑ MC Domeneghetti, u. a .: Complex Cooling Histories of Lunar Troctolite 76535 and Stillwater Orthopyroxenite SC-936 . In: Lunar and Planetary Science . tape XXXII , 2001.
  3. ^ DB Stewart: Apollonian metamorphic rocks - The products of prolonged subsolidus equilibration (abs) . In: Lunar Science . tape VI . Lunar Planetary Institute, Houston 1975, p. 774-776 .
  4. ^ DA Papanastassiou, GJ Wasserburg: Rb-Sr age of troctolite 76535 . In: Proc. 7th Lunar Sci. Conf. 1976, p. 2035-2054 .
  5. ↑ WR Premo, M. Tatsumoto: U-Th-Pb, Rb-Sr and Sm-Nd isotope systematic of lunar troctolite cumulate 76535: Implications on the age and origin of this early lunar, deep-seated cumulate . In: Proc. 22nd Lunar Planet. Sci. Conf. Lunar Planetary Institute, Houston 1992, p. 381-397 .
  6. ↑ DD Bogard, u. a .: 76535: An old lunar rock . In: Earth and Planetary Science Letters . tape 26 , no. 1 , 1975, p. 69-80 , doi : 10.1016 / 0012-821X (75) 90178-8 .
  7. ^ Ian Garrick-Bethell, et al. a .: Early Lunar Magnetism . In: Science . tape 323 , no. 5912 , 2009, p. 356-359 , doi : 10.1126 / science.1166804 .