The Lappajärvi impact structure (Finland): Age, duration of crater cooling, and implications for early life
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An in-depth approach of 40Ar/39Ar dating of the ~23 km Lappajärvi impact structure (Finland) was performed using carefully selected single-grain aliquots of optically fresh, clast-poor, impact melt rock and recrystallized K-feldspar melt particles separated from impact-metamorphosed granite pegmatite. Step-heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven K-feldspar samples. The melt rocks yielded fully concordant ages with a weighted mean of 76.37 ± 0.46 Ma. Dating of the K-feldspar melt particles resulted in a series of younger plateau ages ranging from 75.11 ± 0.36 to 76.11 ± 0.35 Ma. The melt rock results combined with the oldest syn-melt rock K-feldspar age and including all sources of uncertainties yielded a weighted mean age of 76.20 ± [0.29] Ma (2σ; MSWD = 1.02, P = 0.41), which is interpreted to represent the best-estimate age of the Lappajärvi impact. The age spread of at least 1.1 ± 0.5 Ma between the crystallization of the impact melt rocks and the K-feldspar melt particles warns against the use of single 40Ar/39Ar analysis to derive a meaningful impact age. In turn, the age offset monitors both rapid cooling of the Lappajärvi melt sheet versus slow cooling and impact-induced hydrothermal circulation within the crater basement, most likely driven by long-lasting heat flow from the hot central uplift of the impact structure.This interpretation is in line with the grain-size dependent argon diffusion parameters and apparent closure temperatures of ~230–410 °C determined for various domain sizes of K-feldspars observed in this study. Our study shows that even in comparatively small impact craters post-impact hydrothermal activity can be estimated to last between ~600 ka and ~1.6 Ma and is technically resolved by means of the 40Ar/39Ar dating technique. Therefore, the longevity of hydrothermal systems in medium-sized impact craters might be an order of magnitude longer than previously estimated. Prolonged post-impact heat flow in medium-sized craters, in analogy to Lappajärvi, might have played an important role in the emergence of life on early Earth and possibly Mars.
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