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Magnetic classification, weathering, and terrestrial ages of Antarctic ordinary chondrites
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Constraining the terrestrial age of micrometeorites using their record of the Earth's magnetic field polarity
No full textWe propose a new nondestructive method that uses the paleomagnetic record of micrometeorites in Earth's polar regions to constrain the age of their fall. During atmospheric entry, melted micrometeorites acquire a thermal remanent magnetization and record the polar subvertical geomagnetic field. When the fall vector can be determined, due to the location of bubbles, iron-nickel droplets, or grain-size gradients, it is possible to ascribe the fall to a normal or reverse polarity interval of the geomagnetic field. We tested this concept on a set of eight melted micrometeorites from the Transantarctic Mountains (Antarctica). Two micrometeorites have magnetization directions consistent with a normal polarity of the Earth's magnetic field, whereas four others have recorded a reverse polarity, and therefore fell to Earth at least 0.78 m.y. ago. One micrometeorite has a magnetization that is seemingly unrelated to the inferred entry direction. The fall direction could not be determined with certainty for one micrometeorite. These results provide new evidence suggesting that the Transantarctic Mountains micrometeorite traps are 1-2 m.y. old, and confirm that they contain the oldest non-fossil micrometeorites available
HED-like cosmic spherules from the Transantarctic Mountains, Antarctica: major, trace element and oxygen isotope compositions
No full textEleven large (~400-700μm in diameter) differentiated glass cosmic spherules were identified from the Transantarctic Mountain micrometeorite collection on the basis of their high Fe/Mg ratios (from 0.6 to 1.6) and their homogeneous Fe/Mn ratios (31±4). Their oxygen isotope compositions (δ 18O values ranging from 14.37‰ to 20.12‰ and Δ 17O values ranging from -0.47‰ to -0.65‰) suggest they are derived from a similar parent body, either 4-Vesta or a vestoid with an oxygen isotope composition close to that of 4-Vesta. The ranges in CaO and Al 2O 3 contents (from 1.6 to 11.6wt.% and from 0.8 to 12.4wt.%, respectively), the average REE contents (1.4-13.7×CI), the REE patterns (flat to strongly depleted in LREE), and the siderophile element contents (0.17-131μg/g for Ni and 1.3-68.1μg/g for Co) result from different mineralogies in the spherule precursors. Type 1 spherules (n=4), with high REE contents and flat patterns, derive from fine-grained precursors with typical eucritic mineralogy. Type 2 spherules (n=4) are strongly depleted in LREE and are related to pigeonite-rich equilibrated eucrite fragments. Type 3 spherules (n=2) have high Fe/Mg ratios (>0.6) relative to chondritic spherules but similar REE patterns (av.REE=1.6-2.2×CI) and siderophile element contents (Ni >17μg/g, Co >50μg/g). We interpret the chemical characteristics of Type 3 spherules by the high proportion of clasts of metal-bearing diogenites in their howardite-like precursors. The combination of elemental and oxygen isotope data confirms that the Fe/Mn ratio and the Co, V and Zn contents are suitable parameters for the identification of the parent body of the differentiated micrometeoroids, provided that the mineralogical control is understood. The relative frequency of HED-like micrometeorites in the Transantarctic Mountain micrometeorite collection is 1.6%. This is consistent with the abundance of eucrite and howardite meteorites (1.8%), which are the main constituents of the regoliths of vestoid asteroids
Major, trace element and oxygen isotope study of glass cosmic spherules of chondritic composition: the record of their source material and atmospheric entry heating
No full textNew geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al(2)O(3), and 0.14 wt.% to 0.31 wt.% for TiO(2)) and refractory trace elements (from 6.2 mu g/g to 19.3 mu g/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites (delta(18)O = 13.2-17.3 parts per thousand and delta(17)O = 7.6-9.2 parts per thousand). They are systematically enriched in Ni and Co (Ni = 24-500 mu g/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 mu g/g, delta(18)O = 13.1-28.0 parts per thousand and delta(17)O = 5.1-14.0 parts per thousand). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our dataset (54%) and likely derive from homogeneous, fine-grained chondritic precursors. Other REE patterns fall into no more than five categories, a surprising reproducibility in view of the mineralogical heterogeneity of chondritic lithologies at the micrometeorite scale
Micrometeorites: A possible bias on the sedimentary magnetic record
No full textMicrometeorites are strongly magnetic and continuously accumulate at the Earth's surface. On the basis of previously acquired magnetic data, we investigated at which conditions micrometeorites can bias the sedimentary palaeomagnetic and rock magnetic record. We calculated the probabilities for a sediment sample (discrete samples or U-channel samples) to have its detrital remanent magnetization deviated by the presence of a micrometeorite. Our model shows that direction anomalies >5° caused by micrometeorites may be rather frequent (more than 1% of measured samples), even for sediments with typical values of sedimentation rate (up to 10 cm/kyr) and remanent magnetization (up to 5 × 10-3 A/m). Excursions >30° caused by micrometeorites have probabilities >1% in sediments with remanent magnetization 1% for sediments with remanent magnetization <2 × 10-4 A/m and sedimentation rate <1 cm/ka. On the other hand, only sediments with magnetic susceptibilities <10-5 SI and sedimentation rates <1 cm/ka can be significantly affected by the presence of micrometeorites
MAJOR AND TRACE ELEMENTS AND OXYGEN ISOTOPES IN DIFFERENTIATED COSMIC SPHERULES RELATED TO VESTA-LIKE ASTEROIDS
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THE GEOCHEMICAL FRACTIONATION RECORDED BY CHONDRITIC GLASS COSMIC SPHERULES DURING ATMOSPHERIC ENTRY
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