δ 18 O and yttrium zoning in garnet: time markers for fluid flow?

• Published in: Journal of Metamorphic Geology
• Main Authors: Skelton, A., Annersten, H., Valley, J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2002
• Subjects: Norway; Kvaløya; Northern Norway; Troms
• Online Access: http://dx.doi.org/10.1046/j.1525-1314.2002.00378.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1525-1314.2002.00378.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1525-1314.2002.00378.x

Abstract The relative timing of two discrete pulses of metamorphic fluid flow is constrained based on chemical zoning in several garnet crystals from Kvaløya, Troms, northern Norway. The garnet crystals measured 1–2 cm in diameter and were contained within c. 1.6 Ga, staurolite grade metasediments. Major element zoning indicates that garnet grew under normal prograde conditions in the garnet and/or staurolite zones. Timing constraints are based on comparisons between major and trace element chemical zoning, oxygen isotope (δ 18 O) zoning and deformational (inclusion trail) zoning in one of the garnet. We interpret at least two pulses of metamorphic fluid flow. The first pulse occurred during the syn‐tectonic growth interval. The δ 18 O zoning was reversed relative to ‘normal’ prograde zoning and the δ 18 O maximum was located within the syn‐tectonic growth zone, displaced 3–4 mm from the garnet core. The fluid might have been sourced in neighbouring calcareous pelites and may also have caused formation of an Y ring. The second (and subsequent) pulse(s) occurred during/after the post‐tectonic growth interval. δ 18 O was locally increased at the garnet rim, particularly where the rim was sheared. The incomplete rim was also enriched in calcium. Transport of oxygen and calcium by metamorphic fluids is well documented. Transport of Y is both problematic and poorly understood, but might have been facilitated by complexing with F and/or CO 2 .