Extreme Zr stable isotope fractionation during magmatic fractional crystallization

Abstract

Zr stable isotopes exhibit >5‰ fractionations at magmatic conditions, yielding novel insights into melt chemical evolution.

Significant Zr isotope variations in single zircon grains recording magma evolution history

“…The δ 94 Zr values of all analyzed zircons range from −0.86‰ to 0.41‰, with an average of −0.04 ± 0.39‰ (2SD, n = 237). The overall range (1.27‰) is larger than recently reported bulk rocks (0.582‰) (21), but smaller than that (∼5‰) observed in zircon and baddeleyite crystals from anorthosite FC1 (15,22).…”

“…and an α of 1.00106 was suggested accordingly (22). While these studies reached opposite conclusions, they both suggested that zircon crystallization would fractionate the Zr isotope composition of the melt.…”

“…The FC1 zircons exhibit a much larger δ 94 Zr range of ∼5‰ (15, 22) than observed in megacryst zircons (<0.3‰) (15,24) or fine-to medium-grained zircons in this study (1.27‰). The majority of FC1 zircons are isotopically heavier than the bulk rock by ∼0.3‰, resulting in α > 1 (22). However, the large range in FC1 was mainly caused by two low values (−2.530‰ and −4.278‰) in zircon fragments that were not treated by chemical abrasion (a method to selectively remove domains with high accumulated radiation damage) (22).…”

“…The majority of FC1 zircons are isotopically heavier than the bulk rock by ∼0.3‰, resulting in α > 1 (22). However, the large range in FC1 was mainly caused by two low values (−2.530‰ and −4.278‰) in zircon fragments that were not treated by chemical abrasion (a method to selectively remove domains with high accumulated radiation damage) (22). If they were excluded, the range would drop to ∼1.8‰ (vs. 1.27‰ in this study).…”

“…They show the great potential of Zr isotope in tracing magmatic processes. However, available data have been rather limited and resulted in very controversial interpretation (21,22), such as for fractionation factor α (whether greater or smaller than 1). The α is defined as ( 94 Zr/ 90 Zr) zircon /( 94 Zr/ 90 Zr) melt , which depicts how Zr isotopes partition into the crystallizing zircon and surrounding melt and is thus fundamental to understanding magmatic Zr isotope evolution.…”