Petrography and geochemistry of distal spherules from the K–Pg boundary in the Umbria–Marche region (Italy) and their origin as fractional condensates and melts in the Chicxulub impact plume

Petrographic Investigation of Target Rock Transformation under High Shock Pressures from the Colônia Impact Crater, Brazil

“…The presence of features indicative of high-energy collisions, surfaces with microcraters, spherules with welded grains, as well as small bubbles with rounded or elongated cavities in both materials, is consistent with generation from a viscous fluid by ballistic origin. Based on these characteristics, which are similar to several other spherules and glass fragments of ballistic origin (e.g., Hörz et al, 2002;Schulte et al, 2003;Wu et al, 2013, Belsa et al, 2017, an impact origin seems to be the most consistent interpretation. However, more detailed studies are being conducted in order to gather other information that may potentially represent irreversible fingerprints about important geological processes that took place in the past.…”

Globally distributed iridium layer preserved within the Chicxulub impact structure

“…Sites close to or within the crater have a lower total Ir fluence at 10 to 45 ng/cm 2 ( 6 , 7 , 20 , 42 ) compared with a mean global Ir fluence of 55 ± 3 ng/cm 2 , calculated on the basis of 52 marine and nonmarine sections ( 42 ). Microkrystite spherules, together with the spinel crystals these contain, are known to hold a substantial fraction of impactor material, but their chemistry and distribution pattern between K-Pg boundary sites display a large asymmetry across the globe ( 5 , 11 ). Iridium concentrations are highest in the spinel-bearing spherules (up to >100 ppb) and lower in the nonspinel bearing spherules and surrounding clay (1 to 10 ppb) ( 11 , 14 , 43 ).…”

“…Microkrystite spherules, together with the spinel crystals these contain, are known to hold a substantial fraction of impactor material, but their chemistry and distribution pattern between K-Pg boundary sites display a large asymmetry across the globe ( 5 , 11 ). Iridium concentrations are highest in the spinel-bearing spherules (up to >100 ppb) and lower in the nonspinel bearing spherules and surrounding clay (1 to 10 ppb) ( 11 , 14 , 43 ). Rocchia et al ( 43 ) reported a strong correlation between the location of spinel crystals and the Ir anomaly and concluded that the Ir was associated with the spinel and had remained in situ since deposition, whereas Ir in the surrounding clay was more mobile.…”

“…Rocchia et al ( 43 ) reported a strong correlation between the location of spinel crystals and the Ir anomaly and concluded that the Ir was associated with the spinel and had remained in situ since deposition, whereas Ir in the surrounding clay was more mobile. At Site M0077, microkrystite spherules and magnesioferrite spinel crystals, known to be particularly resistant to weathering ( 11 , 43 ), have not been found so far. Microkrystite spherules form a volumetrically significant component of the K-Pg layer at distal sites, with up to 20,000 spherules/cm 2 and an average spherule diameter of 250 μm ( 5 ).…”

“…Various other types of impact ejecta have been recovered from K-Pg event deposits, including impact spherules, some of which contain Ni-rich spinel crystals, and shocked mineral grains ( 5 , 6 , 11 ). While an assortment of HSE carrier phases has been identified within the boundary clay (including magnesioferrite crystals or other spinel group minerals, microkrystite impact spherules, organic matter, or iron-rich nanophases), Ir and the other siderophile elements were likely transported from the impact site across the globe within airborne microscopic dust and impact vapor condensates ( 1 , 11 , 12 ). The extraterrestrial matter found in the boundary layer was determined to be of carbonaceous chondritic composition ( 7 , 13 , 14 ).…”