The Piancaldoli meteorite: A forgotten primitive LL3.10 ordinary chondrite

Abstract

The Piancaldoli ordinary chondrite fell in northern Italy on August 10, 1968. Preliminary studies led to its classification as an LL3.4 unequilibrated ordinary chondrite. However, recent developments in classification procedures have prompted us to reexamine its mineralogical, petrographic, spectroscopic, chemical, and isotopic features in a multitechnique study. Raman spectra and magnetic properties indicate that Piancaldoli experienced minimal thermal metamorphism, consistent with its high bulk hydrogen content and the Cr contents of ferroan olivines in its type II chondrules. In combination with findings of previous studies, our data thus confirm the variability of Cr contents in ferroan olivines in type II chondrules as a proxy of thermal metamorphism. Furthermore, our results reveal that Piancaldoli is less altered than previously reported and should be reclassified as an LL3.10 unequilibrated ordinary chondrite. Our results also imply that the bulk deuterium enrichment, as observed in Piancaldoli (LL3.10), Bishunpur (LL3.15), and Semarkona (LL3.00), is a specific signature of the most primitive unequilibrated ordinary chondrites. Based on our results, we propose that, to date, Piancaldoli is the second leastaltered unequilibrated ordinary chondrite fall after Semarkona. This work reiterates the importance of meteorite collections worldwide as fundamental resources for studying the formation conditions and evolution of our solar system.

Conditions of chondrule formation in ordinary chondrites

“…We surveyed all chondrules within (i) a home-made thick section of Piancaldoli (3.10; Marrocchi et al, 2020) and (ii) thick sections rom the s m national d'Histoire nat relle (Paris, France) of Semarkona (LL3.00, section 3583; Grossman and Brearley, 2005) and Bishunpur (LL3.15, section 2491sp4; Grossman and Brearley, 2005). We selected 11 type I PO chondrules for further analyses: one from Semarkona, six from Piancaldoli, and four from Bishunpur.…”

“…Elements such as Cr might be mobilized during low-grade metamorphism (Grossman and Brearley, 2005). However, Cr in type II FeO-rich olivine grains is not affected in Semarkona (Grossman and Brearley, 2005) nor in Piancaldoli (Marrocchi et al, 2020) but could be modified in Bishunpur (Grossman and Brearley, 2005). Similarly, metamorphism can slightly affect the Cr distribution in Mg-rich olivine grains of Bishunpur.…”

“…To better understand the conditions of type I chondrule formation in the NC reservoir, we selected type I PO chondrules from the pristine ordinary chondrites Semarkona (LL3.00), Piancaldoli (LL3.10), and Bishunpur (LL3.15; Grossman and Brearley, 2005;Marrocchi et al, 2020). We report a comprehensive study of chondrule olivine grains combining petrological observations and quantitative geochemical analyses.…”

The astrophysical context of collision processes in meteorites

“…Meteorites are asteroidal (or rarely cometary; Gounelle et al 2006) fragments recovered at the Earth's surface either promptly after their falls (e.g., Hiroi et al 2001;Jenniskens et al 2012;Marrocchi et al 2020) or during field trips to cold/hot deserts (e.g., Bischoff and Geiger 1985;Schl€ uter et al 2002;Harvey 2003;Gattacceca et al 2011;Evatt et al 2020). Meteorites' parent bodies are leftovers from the planetary formation processes that operated during the evolution of the solar system.…”

Mineralogical, petrological and planetological heritage. The (Italian) story so far

“…However Italian meteorite collections have not only a historical significance, but they are also an important source of extra-terrestrial material for research purposes. Cataloguing meteorite specimens in naturalistic collections along with their relative archival documents, museum catalogues and inventories can lead to a better characterization of previously analysed samples and to the potential discovery of meteorites unknown to the scientific community (Trevisani 2011;Moggi Cecchi et al 2015, 2019aPratesi et al 2019;Llorca et al 2020;Marocchi et al 2020). Furthermore, as rightly suggested by Dorfman (2012), meteorite collections hold many levels of intangible values (Ahmed 2006).…”