Laser extraction of helium isotopes from Antarctic micrometeorites:: Source of He and implications for the flux of extraterrestrial 3He to earth

Helium isotopes have been measured in 45 putative micrometeorites in the size range 50 to 400 mu m that were extracted from Antarctic ice. Forty of the Antarctic micrometeorites (AMMs) are irregular in shape and display varying degrees of partial melting, whereas five are spherical and thought to represent completely melted particles. Of the 30 50- to 100-mu m suspected micrometeorites, only 8 released neither He-3 or He-4. Sixteen released He-3 and He-4 (0.003-3.5 X 10(-3) cm(3) STP/g), having He-3/He-4 in the range 1.5 X 10(-4) to 4.1 X 10(-4). Six particles released only He-3, yielding minimum He-3/He-4 of 1.6 to 10.6 x 10(-4), 5 of which fall in the range of values for AMMs that had measurable He-3 and He-4. Only 2 of the 15 100-to 400-mu m AMMs released measurable He-4 (4-44 X 10(-7) cc g(-1)). These have He-3/He-4 of 6 x 10(-4) and 20 x 10(-4), respectively, significantly higher than the majority of the ratios of 50-to 100-mu m AMMs. Implanted solar energetic particles are the dominant source of He in the 50- to 100-mu m AMMs, with small contributions of solar wind and/or spallogenic He present in 10 particles. The absence of SW-He is likely due to the loss of the outermost layers of the micrometeorites by ablation during atmospheric entry and abrasion during residence in ice and sampling. Spallation He is most important in the 100-to 400-mu m AMMs, consistent with their smaller surface area-to-volume ratio. Maximum spallation He-3 exposure ages of six particles are close to that predicted from Poynting Robertson spiral-in times of asteroidal micrometeorites. The two terrestrial particles, identified by chemical composition and mineralogy, released only He-4 yielding radiogenic He isotope compositions. An inverse relationship between particle sizes and solar He abundance is explained by He loss during entry. Helium loss is likely controlled by phyllosilicate breakdown, where present, and diffusion-controlled bubble growth and rupture rather than melting. The 50- to 100-mu m micrometeorites deliver 8 X 10(-8) cm(3) STP He-3/g to earth. This is more than 100 times less than transported by <20-mu m particles (similar to 2 X 10(-5) cm(3) STP He-3/g) but, because 50- to 100-mu m micrometeorites deliver approximately 20 times more extraterrestrial matter to earth than <20-mu m particles, the larger particles may contribute similar to 5% of the total flux of extraterrestrial He-3 to terrestrial sediments. The mean He concentrations of the 50- to 100-mu m AMMs (Antarctic micrometeorites) and the five spherules (2 +/-1 X 10(-9) cm(3) STP He-3 g(-1)) overlap those of the magnetic fraction of deep sea sediments (0.8-68 x 10(-9) cm(3) STP/g). A combination of this material could provide the extraterrestrial He-3 measured in ancient deep sea sediments without calling upon the survival of friable <20-mu m chondritic interplanetary dust particles for tens of millions of years. Copyright (C) 1999 Elsevier Science Ltd.