CHEMICAL COMPOSITION OF ROCK TYPES FROM THE EYREVILLE A AND B DRILL CORES, CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA/USA

The chemical composition of major rock types from the ICDP-USGS Eyreville A and B drill cores, Chesapeake Bay impact structure (USA) was studied by X-ray fluorescence spectroscopy (XRF) on whole rock samples: For breccias, clasts larger than ~ 0.5 cm were excluded from analysis. The results of some important rock types (Exmore breccia, granite megablock, suevite and melt-rich so-called impact melt breccias, suevitic dike breccias, granite pegmatites and schsits from the so-called basement sequence) are summarized in Table 1. Based on chemical composition suevite, impact melt breccias and suevitic dikes are mixtures dominated by variable amounts of basement schists and pegmatites, with the schist component being dominant. So far, a significant metasedimentary component could not be detected by major element chemistry. The chemical results are critically evaluated against petrographic information.

Table 1: Mean chemical composition and standard deviation of various rock types measured by XRF.

Unit	Exmore breccia		granite mega-block		suevite/impact melt breccia		suevitic dikes		pegmatites fr. basement		schists from basement
depth	444-527 m		1096-1369 m		1397-1555 m		1603-1661 m		1592-1766 m		1559-1689 m
	n = 57		n = 29		n = 67		n = 7		n = 23		n = 26
	mean/std.dev.		mean/std.dev.		mean/std.dev.		mean/std.dev.		mean/std.dev.		mean/std.dev.
wt%
SiO2	75.1	4.6	72.5	1.7	67.3	3.7	58.0	2.3	73.7	4.7	57.5	8.3
TiO2	0.53	0.15	0.25	0.11	0.85	0.15	0.84	0.16	0.04	0.04	0.90	0.20
Al2O3	10.6	1.9	14.1	0.8	14.5	1.4	18.6	1.3	14.5	2.5	18.4	3.8
Fe2O3*	3.15	1.10	1.89	0.98	5.33	1.07	7.49	2.35	0.52	0.50	7.45	2.83
MnO	0.04	0.02	0.04	0.05	0.08	0.03	0.06	0.01	0.04	0.04	0.07	0.03
MgO	0.87	0.33	0.37	0.16	1.69	0.48	1.27	0.17	0.11	0.09	2.05	1.69
CaO	1.55	1.10	1.61	0.59	1.53	0.48	1.54	0.36	1.18	1.07	1.78	1.26
Na2O	1.38	0.25	3.12	0.65	1.68	0.85	2.02	0.35	4.30	1.56	1.58	0.85
K2O	2.51	0.34	5.02	1.60	3.16	0.77	3.88	0.62	3.48	2.15	3.63	1.18
P2O5	0.12	0.07	0.06	0.04	0.13	0.04	0.16	0.17	0.04	0.07	0.08	0.07
SO3**	0.1	0.1	<0.1		<0.1		0.3	0.2	<0.1		0.2	0.2
LOI***	3.6	1.2	0.6	0.4	3.3	1.2	5.5	0.7	1.6	1.4	5.9	2.1

*Total Fe calculated as Fe2O3; **Total S calculated as SO3; ***LOI = loss on ignition; n = number of analysis.