Elaborate growth history in secondary quartz as revealed by SEM-CL mapping, Mahantango Fm., PA

Publication Information:

Mcgrath, M., L. Mutti, “Elaborate growth history in secondary quartz as revealed by SEM-CL mapping, Mahantango Fm., PA”, Geological Society of America, Vol. 45(1), pp. 72, 2013.

Year: 2013

SEM cathodoluminescence paired with fluid inclusion analysis can be used to explore crystal growth history. These techniques were applied to post diagenetic quartz from vertical veins crosscutting the Mahantango Fm. near Huntingdon, PA. All samples were hexagonal prisms. Systematic SEM-CL images were collected to create full CL maps to assess growth tied to fluid inclusion entrapment. Fluid inclusions demonstrated primary, pseudo-secondary and secondary entrapment of 1 and 2-phase fluids. No inclusions were petroliferous, unlike those in quartz collected from Brallier Fm veins a mile away (Curry 2012). Full characterization of the Mahantango inclusions is still underway. A variety of intriguing patterns were documented in the SEM-CL images. The geometry of differences in CL brightness demonstrated that crystallization was discontinuous, growth conditions highly variable, and that growth occurred directly from solution without a preliminary amorphous phase. Non-concentric growth, seen in inhomogeneous orientations of growth planes, indicated abrupt shifts in favored growth surface. Multiple instances of dissolution and truncation of growth bands were observed; some crystallographically controlled and others not. Dissolution followed two distinct patterns with some occurring along exposed crystal faces, followed by new growth on corroded surfaces, sometimes resuming in the same orientation and at other times occurring on different faces. Elsewhere, dissolution was vermicular, observed as solution channels. Some holes were in-filled, but others remained empty. Isolated annealed fractures were also present; some straight and crystallographically-controlled, others conchoidal. Fractures do not display crack-seal texture. Multi-stage growth and dissolution was observed throughout all samples examined, demonstrating that quartz growth in this setting was neither straightforward nor continuous. Microscopic growth and erosion features observable in single crystals strikingly resemble outcrop-scale sedimentary features such as scour and fill, disconformities, and dissolution channels. While interrupted growth might have offered favorable sites for entrapment of fluid inclusions, the inclusion distribution in these specimens does not correlate well with these gaps in growth.

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