Chaotic three-dimensional distribution of Ba, Rb, and Sr in feldspar megacrysts grown in an open magmatic system

Abstract

As has been demonstrated in recent years, the heterogeneities of coeval magmas can be more successfully revealed by zoned megacrysts rather than by analysis of the whole rocks hosting them. Here, the geochemical heterogeneities of feldspar megacrysts from the Karkonosze granite, Poland, are investigated by LA-ICP-MS. The crystals are the product of migration and growth in regions of poorly mixed magmas. 3D-modeling of the Ba, Sr, and Rb distributions emphasizes the importance of micro-domain growth morphologies. Two models of element behavior-a relative concentration model and a composition gradient model-provide a potentially effective tool for tracking the mixing process on a microscale. Measured concentrations of elements of different mobilities do not agree with what might be expected from the mixing of two end-member magmas. If mixing was the only process occurring, linear correlations between the concentrations of any two elements should be observed; this, however, is not the case. For combinations of any two of the three elements, modeling reveals differing non-linear correlations between concentrations. The megacryst heterogeneities provide an insight into how mixing magmas are chaotically advected to growing crystals and the degree of inter-magma element exchange between the magmas.

Protomylonite evolution potentially revealed by the 3D depiction and fractal analysis of chemical data from a feldspar

“…To obtain a better insight into the relation between the Ba, Sr, and Rb distributions, correlations using cut-off level models and gradient models were used (Słaby et al 2011;Ś migielski et al 2012). These give a qualitative insight into the accordance of the behavior of the two elements being compared.…”

“…All models have the same spatial resolution and single grid consisting of more than 4,000 single measurements. The reliability of the model is discussed in Słaby et al (2008Słaby et al ( , 2011Słaby et al ( ) and Ś migielski et al (2012. Before depiction, models were geostatistically analyzed using variograms in order to provide preliminary estimates of degree of spatial dependence.…”

“…Different directions of lighting were used in analyzing various aspects of the morphology of the surface (Cooper 2003). Following procedures described in Słaby et al (2011) andŚ migielski et al (2012), the Ba, Sr, and Rb DC-DMs were compared using cut-off level and gradient models. These qualitative procedures allow the levels of compatibility (accordance) between element distributions in the crystal to be estimated.…”

“…The second method consists of discriminating (with application of the first derivative) between those areas of two different DC-DM where the concentrations of an element increase or decrease simultaneously. In both cases, the concept of ''probability of accordance'' was used in order to include uncertainty produced by LA-ICP-MS (Słaby et al 2011). …”

“…In correlating the complexity of the traceelement distribution in a crystal with structural deformation, innovative tools are applied, namely, statistical analysis and depiction of trace-element distribution using digital concentration-distribution models (DC-DMs) and derivative models combined with fractal statistics. These tools potentially enable the identification and description of changes that occur during feldspar crystallization/recrystallization (Słaby et al 2011. They may also allow the determination of the process dynamics in terms of element behavior and diffusion, and correlation with subtle structural changes.…”