Origin of agates in volcanic rocks from Scotland

Abstract

Silica, SiO², is found in nature in a wide range of forms from amorphous to crystalline varieties. Minerals composed mainly of microcrystalline quartz (MQ) are conveniently characterized by the chemistry or mineralogy of accessory constituents and by crystallographic features of the quartz^1–5. Agates are MQ nodules of high purity found in cavities in host rocks of volcanic origin. A recent theory¹ described agate growth as a sequence of events whereby initial chalcedony (fibrous length-fast MQ) is deposited from a supercritical fluid that is poorly polymerized; later, spherulitic quartzine and chalcedony form from a highly polymerized siliceous liquid or gel via its transformation into horizontally layered opal-CT and fine-quartz. As cavity filling is considered a consequence of pressure release on cooling, it is implied that temperatures were reasonably high. We present here isotope ratios, ¹⁸O/¹⁶O of silica and D/H of bound water, in agates from Lower Devonian (∼410Myr BP) lavas; these ratios are distinct from those in Tertiary (∼62 Myr BP) lavas, but are linearly correlated and plot along a single line approximately parallel to the line defined by present-day meteoric waters. The data imply that bound waters associated with the agates have preserved their hydrogen isotope ratios since agate deposition, which supports arguments that the water content is of genetic significance¹, but does not support a model for agates as ‘refused’ chert xenoliths². We infer that agates of both ages were formed at low temperature ( ∼50 °C) from fluids having at least a component of meteoric origin.