40Ar39AR DATING RESULTS FROM THE ICDP-USGS EYREVILLE B CORE: CONSTRAINTS ON THE THERMAL HISTORY OF THE CHESAPEAKE BAY IMPACT STRUCTURE

⁴⁰Ar/³⁹Ar data from the central-crater moat in the Eyreville B core of the ca. 35.3 Ma Chesapeake Bay impact structure constrain its thermal history. From the base of the core, the section includes (1) basement-derived schist, pegmatite, and granite (1766.3-1551.2 m depth), (2) block-rich, melt-poor polymict impact breccias (1551.2-1474.1 m), (3) melt-rich suevites with intercalated clast-rich impact melt pods (1474.1-1397.2 m), (4) sand with an amphibolite block and lithic boulders (1397.2-1371.1 m), and (5) an allochthonous granitic slab (1371.1-1095.7 m). Overlying crater-fill sediments and post-impact strata were not sampled for ⁴⁰Ar/³⁹Ar dating.

⁴⁰Ar/³⁹Ar data of muscovites from pegmatites in (1) yield undisturbed late Alleghanian cooling ages and show no record of the impact. Age spectra of altered impact-melt glass from (2) and (3) are all disturbed. Total gas (TG) ages decrease from the bottom of (2) to as low as 31.5 Ma at the top of (3), consistent with an upwardly increasing thermal gradient in (3). Many age spectra from these altered glasses climb from ≥10 m.y. younger to ≥10 m.y. older than the time of impact. This climb is interpreted to be due partly to post-impact alteration of the impact glasses and partly to the presence within the glasses of inseparable target-derived impurities that neither melted nor completely degassed during the impact event.

Three size fractions of orthoclase from a 2-cm schist clast in (3) climb in age from ca. 33-Ma in low temperature steps to 93 Ma and older in high temperature steps. While this orthoclase was partially reset by the impact, its low temperature steps are >2 m.y. younger than the impact and represent either alteration after the impact, cooling through closure, or both. A muscovite from the same clast has a minimum age of ca. 47 Ma in low temperature steps, climbing to a maximum age of ca. 224 Ma, suggesting that it was also partially reset. These data indicate that the temperature in (3) exceeded that of muscovite closure (>500°C) for a geologically brief period, but not long enough for complete degassing to occur.

Four size fractions of a single sample of microcline from (4) yield Late Permian ages in the upper temperature portions of their age spectra with no indication of impact disturbance. Amphibole and biotite from (5) yield Triassic ages that were not reset by the impact event.