Formation of various trace element zoning patterns in high-pressure metamorphic garnet

Compositional zoning of trace elements in garnet serves as a valuable tool for reconstructing petrogenetic evolution, supplementing major element analyses. This is particularly applicable to trace elements exhibiting a strong affinity for garnet and characterized by slow diffusion rates, such as Y and heavy rare earth elements (HREE). The present study examines various zoning patterns of trace elements observed in large garnet porphyroblasts within micaschist samples from the Variscan high-pressure (HP) metamorphic terrain of the Krušné hory Mts. (Saxothuringian zone, Bohemian Massif).

Using electron probe micro-analyser and laser ablation-inductively coupled plasma mass spectrometry, three distinct types of compositional zoning in garnet were identified by compositional mapping. These zoning types were classified as a continuous core-to-rim change, concentric annular changes, and overprinting (or mimicking) of a pre-existing distribution. The study focuses on the formation mechanisms of each type of zoning, their dependence on pressure-temperature change, and fluid availability.

The significantly elevated concentrations of Sc, Y, and HREE in the garnet's central core suggest a rapid diffusion of these elements from the matrix into the garnet after nucleation, challenging a description solely through Rayleigh fractionation. The observed prograde growth of pressure-temperature (PT) conditions of the rock samples to HP–medium temperature (MT) aligns well with the compositional zoning patterns exhibited by the garnet, encompassing major and trace elements, as well as other minerals. Specific compositional patterns include: (1) gradual increase in Co and Zn contents towards the rim, mirroring Mg and inversely related to Mn, indicative of a continuous rise in temperature; (2) overprint zoning of Ti and partly Ca, Sm, Eu, Gd, and Tb in the central part, transitioning to purely concentric annular zoning in the rim, suggesting an increase in temperature; (3) well-developed overprint zoning of Cr throughout the garnet grain, indicating temperatures only up to MT; and (4) depletion of Y, and most of rare earth elements (HREE, Ho, Dy, Tb, Gd, Eu, and Sm – REE) in the rim, accompanied by enrichment of coupled ^VIII(Na, Li)⁺ + ^IVP⁵⁺ substitution elements, experimentally documented from HP to ultra-HP conditions.

The observed inverse annular oscillatory distribution of Sc and V is discussed to be attributed to fluctuating oxygen fugacity during garnet growth, influenced by changes in the availability of the fluid matrix medium carrying trace elements. Higher fluid availability corresponds to increased Sc, Y, and REE incorporation into garnet, evident in well-correlated annular elevations, while V exhibits the inverse trend. Elevated trace element contents in garnet are linked to the breakdown of main and accessory phases carrying these elements during garnet growth, incorporating them into the garnet. The presence of a fluid medium in the system appears to predominantly influence the extent and frequency of annular variations in trace element concentrations. Thus, annular zoning in garnet is associated with both the decomposition of trace element-bearing phases and fluid medium availability.

Acknowledgement: This work was supported by the Czech Science Foundation (Grant No. 24-12845S), Grant Agency of Charles University (Grant No. 1194019), and by Charles University through the Cooperatio Program (Research Area GEOL).