Enrichment of immobile elements in synmagmatic fractures

Useful minerals containing rare Earth elements (REE) and metals are sourced from magma bodies, but exactly how these elements initially leave the magma is not well known. Here we present textural and chemical analyses of mineral-filled fracture bands within the rhyolitic Sandfell laccolith exposed in eastern Iceland. The fracture fillings showcase dynamic and complex textures and imply multiple energy levels during precipitation. The dominant mineral phases are Fe- and Mg-oxides, Mn carbonate, and La/Ce oxide. The textures they present are comb, laminate, radial, and a rounded reworked clastic texture filling the tips. Microtomography images of hand-samples show the fractures are stretched-penny shaped, and contain 80 vol% fillings and 20 vol% void space. The connectivity of fractures within one band is limited to 1-3 neighbours, via small oblique fractures joining two main fractures together. µXRF measurements revealed distinct halos of 0.8 wt% Fe depletion surrounding each facture, and within the fracture-fill a strong enrichment in an unusual suite of elements including Fe, Mn, Cl, Zn, Cr, Y, Ce, and La. This assemblage is puzzling, as many of these elements are typically carried by fluids which have strong alteration effects on the surrounding rock, and there is a lack of this kind of alteration at Sandfell. Our working hypothesis is that the formation of the fractures provided a degassing pathway through the impermeable magma. However, the nature and the composition of the magmatic volatiles are as yet unknown. The minimal connectivity between fractures (at hand-sample scale) suggests fluid would have travelled through the length of one to three fractures until intersecting with another fracture band system, and minerals precipitated along the way. Given the ubiquitous occurrence of the fracture bands within the laccolith, this small-scale process compounds into large amounts of mass transfer overall. The fractures at Sandfell may be a snapshot of the initial process of removing incompatible elements from silicic magma.