Parent bodies of L and H chondrites: Times of catastrophic events

Abstract-Measurements of particle tracks, cosmogenic radionuclides, and rare gas isotopes in Mbale indicate that the meteoroid had a simple, one-stage exposure for 30.2 Ma in interplanetary space. On the basis of the measured track production rates and 6oCo and 26A1 activities, the meteoroid is estimated to be a sphere with a radius of -36 cm. The activities of several cosmogenic radionuclides (ie., 57C0, 54Mn, 22Na, 44Ti, and 26A1) in two fiagments having different shielding, as estimated by their track density and 6oCo activity, provide the depth variation in their production rates. Cobalt-57,54Mn and 22Na activities agree with the production that is expected around the maximum of the solar cycle 22 as calculated fiom the Sunspot numbers. The U, Th-4He and K-40Ar ages are measured to be 0.54 Ga indicating a late thermal event which is in agreement with the thermal history of some other L group chondrites. The trapped N has d15N of -57 2 4%0, which is much lighter than the average L-group chondrite value; this indicates the presence of an isotopically anomalous light N component.

Section: Cosmogenic Noble Gases and Exposure Agementioning

confidence: 51%

Cosmogenic effects in Mbale, L5/6 chondrite

Murty

Bhandari

Suthar

et al. 1998

“…The neutron-capture 36 Ar production rate is based on an indigenous Cl content of 140 ppm, which leads to a peak value of 11.5 dpm/kg in the neutron-capture 36 Cl profile, i.e., similar to the maximum value observed in Gold Basin (Fig. 2) it is unlikely that the major impact event that outgassed many L chondrites ~350 Myr ago (Alexeev 1998) and possibly disrupted the L chondrite parent body (Haack et al 1996), brought the Gold Basin meteoroid close enough to the surface of its parent body to become exposed to cosmic rays.…”

Section: Thermal Historymentioning

confidence: 74%

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history, exposure history, and pre‐atmospheric size

Welten

Caffee

Leya

et al. 2003

“…In their study of a large number of H chondrites, based on abundance of radiogenic 40 Ar and the ratio of exposure ages calculated via 38 Ar and 21 Ne, Graf and Marti (1995) identified a number of cases with evidence for noble gas loss due to weathering. Furthermore, according to Alexeev (1998) non-Antarctic finds have (16 ± 4)% less 4 He on average compared to falls, whereas Antarctic finds have only (5 ± 3)% less. Finally, the average nominal cosmic ray exposure age of non Antarctic H chondrite finds belonging to the ~7 Ma exposure peak is about 15% lower than that of falls, for which terrestrial weathering is the most likely reason (Alexeev 2005).…”

Section: Influence Of Terrestrial Weatheringmentioning

confidence: 91%

Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

Schwenzer

Fritz

Stöffler

et al. 2008

available online at http://meteoritics.org Helium loss from Martian meteorites mainly induced by shock metamorphism:Evidence from new data and a literature compilation Abstract-Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45-55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic-ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of 3 He, 4 He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of 4 He into cosmogenic and radiogenic components was performed using the lowest 4 He/ 3 He ratio we measured on mineral separates ( 4 He/ 3 He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic 4 He. Systematics of cosmogenic 3 He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic 4 He loss, ranging between ~20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant 4 He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of 4 He diffusion in the main U,Th carrier phase apatite showed that post-shock temperatures of ~300 °C are necessary to explain observed losses. This temperature corresponds to the post-shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for 4 He loss.