Incipient devitrification of impact melt particles at Bosumtwi crater, Ghana: Implications for suevite cooling history and melt dispersion

Abstract

The petrographic, mineralogical, and geochemical compositions of the incipient devitrification products in impact melt fragments found in outer suevites at the Bosumtwi impact crater were studied to reconstruct the postimpact environmental constraints on the suevite formation and to refine its cooling history. Our study shows that devitrified melt/particles contain numerous microlitic crystals and crystal aggregates of different shapes derived from rapid cooling. The matrix of melt/particles in Bosumtwi suevites contains abundant Mg‐hercynite (pleonaste)‐type spinels with sizes rarely exceeding a few micrometers. High nucleation density of microlites suggests rapid crystallization under strong undercooling in the presence of abundant volatiles. Although the Bosumtwi impact event took place in a continental environment, the possible sources for elevated fluid/volatile content could have been the groundwater in the deeply weathered and fractured‐jointed Birimian basement, dewatering of abundant hydrous phases in weathered crust or hydrothermally altered basement, and the shale/phyllite–greywacke lithologies in the target rocks. Our results show that enough volatiles were present in the target rocks at the time of impact for the effective impact melt dispersion observed in Bosumtwi impactites.

Microporphyritic and microspherulitic melt grains, Hiawatha crater, Northwest Greenland: Implications for post-impact cooling rates, hydration, and the cratering environment

“…Hiawatha grains like this resemble impactite melt rocks described from other well-preserved, upper-crustal impact craters characterized by widely to closely spaced microlites in a glassy to hemicrystalline matrix, as observed in the Popigai crater, Siberia (Masaitis, 2019). Välja et al (2019) proposed a similar interpretation of a comparable melt rock from the Bosumtwi crater, Ghana (see below). They labeled this type of microlite crystallization as incipient devitrification, but in this study we follow Best (2003) and restrict this term to spherulitic or axiolitic growth below the solidus.…”

“…Impact melt rocks with compositions broadly similar to those of the felsic Hiawatha melt grains have been described from, e.g., the New Quebec crater, Canada (Grieve et al, 1991); the Popigai crater, Siberia (Whitehead et al, 2002); the Ries crater (Osinski, 2003(Osinski, , 2005; the El'gygytgyn impact structure, Russia (Gurov and Koeberl, 2004); the Chesapeake Bay impact structure, USA (Belkin and Horton, 2009); and the Bosumtwi crater, Ghana (Välja et al, 2019). Unaltered impact glasses are generally very dry, with H 2 O contents below 0.1 wt% (Beran and Koeberl, 1997), and the melt rocks from most craters consist of pure glass or glass with scattered prismatic microlites like in Hiawatha grain 21K-w39.…”

“…Incipient high-temperature devitrification has recently been described from impact melt fragments of suevites in the continental Bosumtwi crater, Ghana (Välja et al, 2019). Numerous tiny orthopyroxene and/or Mg-hercynite spinel microlites only a few micrometers long and morphologically similar to those in Hiawatha grain 21C-r06 or feldspar spherulites ∼20-50 µm wide occur in different melt components.…”

“…Numerous tiny orthopyroxene and/or Mg-hercynite spinel microlites only a few micrometers long and morphologically similar to those in Hiawatha grain 21C-r06 or feldspar spherulites ∼20-50 µm wide occur in different melt components. Välja et al (2019) argued that the high density of microlites suggests rapid crystallization under strong undercooling in the presence of abundant volatiles. They supposed that the incipient microlite growth was aided by a high fluid content in the target rocks, which might have been liberated from shale and/or hydrous minerals in the deeply weathered target rocks or derived from the infiltration of groundwater.…”