Understanding the conditions of planet formation through chondrules

A. M. Perez1, S. J. Desch1, D. L. Schrader1, & C. B. Till1, School of Earth and Space Exploration, Arizona State Universityalexandra.m.perez@asu.edu

2. The Models

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1. The Story

3. A Recipe for Chondrules

4. Results

5. Discussion

Chondrules are mm-sized igneous inclusions in chondritic meteorites, which are known to have formed during the protoplanetary disk phase during the initial stages of the Solar System [1]. The heating mechanisms that initially melted chondrulesin the nebula are key to constraining astrophysical models of the disk and the energetic processes that were present during planet formation, but their formation mechanism(s) remains unknown.

Weyrauch & Bischoff (2012)

Most common chondritic textures. BSE images taken from QUE 97008 (L3.05).

2 - 6% [2]

2 - 13% [2]

82 - 99% [2]

6. Conclusions

8. Acknowledgments

• We conducted dynamic crystallization experiements to test the validity of the planetary embryo bow shock model as a chondrule formation mechanism• We found that 600 K/hr cooling rates can produce porphyritic textures, which is consistent with the planetary embryo bow shock model implying that planets formed early on during the initial stages of the Solar System

Large-scale (sprial density) shocks [3,4,5,6]

Planetary embryo bow shocks [7,8]

• spiral arms in the solar nebula• transient shock fronts heat chondrules• cooling rate is proportional to chondrule density

• embryo’s orbital velocity is greater than gas creating a bow shock• cooling rate is upwards 3000-5000 K/h, but larger size embryos can have cooling rates as low as 600 K/hr.

• Analogs composed of a combination of San Carlos Olivine, Dog Lake Diopside, and Amelia Albite [9]• Grain size: all fine (63 - 90 µm) and fine + large (212 - 250 µm) • Peak temperatures: liquidus - 50°C, liquidus, and liquidus+ 50°C. (liquidus: 1608 °C [10])• Heating durations: 1,5, and 10 minutes• Cooling rates: 300, 600, 1000, 3000, and 5000 K/hr 1 atm furnace located

at SESE’s EPIC Lab*

*Experimental Petrology & Igneous processes Center (EPIC) led by PI, Christy Till

Chondrule analogs are placed in Pt baskets and suspended from a thermocouple inside a 1 atmosphere vertical gas mixing furnace,undergoing various parameter combinations.

Back-scattered electron (BSE) images of the samples were acquired along with semi-quantitative, standardless energy-dispersive spectrometry (EDS) analysis using the JXA-8530F Electron Probe Microanalyzer located at John M. Cowley Center for High Resolution Electron Microscopy, Arizona State University.

• Cooling timescales predicted by current planetary embryo bow shock models are as low as 600 K/hr [7], which would be consistent with porphyritic chondrules, according to our experiments.• Cooling rates scale roughly with inverse of planetary size [7]. For example, if the embryo were twice as large, the cooling rates would be a factor of two lower, and more comfortably consistent with porphyritic textures.• This work suggests that planets formed early during the initial stages of the Solar System

QUE 97008 Experimental The skeletal and quench crystal textures produced experimentally can also beobserved in actual chondrules.

The presence of euhedral to subhedral crystalsis not necessarily a defining characteristicof porphyritic chondrules.

Varying Grain Size

Varying Peak Temperature

Varying Heating Duration

Varying Cooling Rate

Experimental Textures at a Glance

Axel Wittmann for assistance with the electron microprobe analyses. For supplying the samples for this work, the authors would like to thank: the Smithsonian Institution, the members of the Meteorite Working Group, Cecilia Satterwhite and Kevin Righter (NASA, Johnson Space Center).

7. References

Liquidus - 50°C Liquidus + 50°CLiquidus

1 minute 5 minutes 10 minutes

300 K/hr 600 K/hr 1000 K/hr

3000 K/hr 5000 K/hr

Constants: liquidus, 5 mins, 300 K/hr)

Constants: 5 mins, all fine, 1000 K/hr)

Constants: liquidus + 50°C, 5 min, 600 K/hr)

Constants: fine + large, liquidus + 50°C, 600 K/hr)

Constants: fine + large, liquidus, 1 min)

fine + large all fine

fine + large all fine

Errors for Cooling Rates300 K/hr ± 21600 K/hr ± 81000 K/hr ± 773000 K/hr ± 2765000 K/hr ± 357

Transitional textures are defined as having a combination of

porphyritic-like and barred-liketextures.

L - 50: liquidus - 50°CL: liquidusL + 50: liquidus + 50°C

FF: all fine grainFL: fine + large grain

Nebular shock models are the most favored mechanisms forchondrule formation

Paradigm shift: rather than being building blocks of planets, this model shows chondrules are by-products of planet formation

[1] Kita N. T. and Ushikubo T. (2012) Meteoritics & Planet. Sci., 47(7), 1108-1119. [2] Jones R. H. (2012) Meteoritics & Planet. Sci., 47(7), 1176-1190. [3] Hood, L., & Horanyi, M. (1991). Icarus, 93, 259-269. [4] Wood J. A. (1996). Meteoritics & Planetary Science, 31:641–645. [5] Boss A. P. (2002). The Astrophysical Journal, 576:462–472. [6] Boss, A., & Durisen, R. (2005). The Astrophysical Journal, 621(2), L137-L140. [7] Mann C. R. et al. (2016) Astrophysical Journal, 818(2). [8] Boley et al. (2013) Astrophysical Journal, 776(2). [9] Hewins R.H. and Fox G.E. (2004) Geochimica Et Cosmochimica Acta, 68(4), 917-926. [10] Herzberg, C. T. (1979). Geochimica Et Cosmochimica Acta, 43(8),1241-1251.

QUE 97008 Experimental

Abstract: Chondrules, mm-sized igneous inclusions that comprise up to 80% of the mass of chondritic meteorites, are the key to understanding the history of the Solar System and the formation of terrestrial planets and Earth. Here, we present results of dynamiccrystallization experiments to see whether or not porphyritic chondrules can form in planetary embryo bow shocks.

Our goal is to experimentally determine whether or not the dominant porphyritic texture can be

formed in planetary embryo bow shocks.

Our results show that porphyritic textures require lower cooling ratesbetween 300 K/hr and 600 K/hr. These low rates are consistent withthe planetary embryo bow shock model, which meets the thermalconstraints required for an ideal chondrule formation mechansim. Rather than chondrules being the building blocks of planets, this work shows that chondrules are a by-product of planet formation suggesting that planets formed early on during the Solar System’s history.