Comb layering monitors decompressing and fractionating hydrous mafic magmas in subvolcanic plumbing systems (Fisher Lake, Sierra Nevada, USA)

Abstract

Understanding the dynamics of volcanic eruptions requires constraints on the processes of melt extraction in shallow magma reservoirs. This study focuses on the formation of intraplutonic igneous layering known as comb layers and orbicules within the Fisher Lake Pluton, Sierra Nevada. Comb layers are mineralogically and texturally diverse, ranging from plagioclase‐amphibole comb layers to plagioclase‐pyroxene and olivine comb layers. The majority of individual layers are related to the initiation of plagioclase nucleation and growth on preexisting host rocks and remobilized xenoliths. We use simple phase diagrams to illustrate the formation of comb layers through crystal fractionation along conduit walls and xenoliths, with plagioclase as the liquidus phase. Near‐adiabatic ascent of water‐undersaturated melts through the crust will lead to superheating and crystal resorption, which upon saturation of volatiles at shallow depth can induce heterogeneous nucleation of plagioclase. The rarity of orbicules and comb layers worldwide suggests that adiabatic decompression of crystal‐poor, moderately hydrous melts through the crust is rare. Most melts in intermediate/silicic volcanoes crystallize in multiple reservoirs within the crust prior to emplacement at shallow depth as crystal‐bearing melts. Comb layers and orbicules preserve evidence of multiple melt injections where a sequence of texturally and mineralogically similar comb layers and orbicules reflect the timescale of an individual melt pulse. Short timescales on the order of months to years for the growth of these sequences are estimated using experimental plagioclase growth rates and are interpreted as representing the lifetime of a melt injection within a long‐lived, subvolcanic melt extraction zone.

Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

“…These samples provide constraints directly linked to growth in subvolcanic conduits during magmatic recharge episodes (17,18). The observation that timescales for orbicule and comb layer growth are similar to those deduced from volcanic phenocrysts supports the idea that diffusion profiles in mafic crystals faithfully record the duration of magmatic recharge events.…”

“…8 and 32). These growth-rate values are high but not unreasonable, given the high crystallization rates (∼10 cm/y) observed in supercooled experimental magmas (33), upon which previous timescale estimates in subvolcanic orbicules and comb layers were based (17), and the observation that experimental feldspar spherulite crystals can grow at rates as high as ∼30 cm/y (34).…”

“…S2-S5; chemical compositions in SI Appendix, Tables S2 and S3). The layered textures in these latter samples, thought to result from fast crystallization on wall rocks and xenoliths in rapidly decompressing magmas within subvolcanic conduits (17)(18)(19), are directly linked to magmatic recharge events and are targeted here to provide additional insight into magmatic timescales from a subvolcanic perspective.…”

“…In the plagioclase separated from orbicules, comb layers, and associated rocks (n = 23; SI Appendix), it is easier to evaluate the kinetic origin of Ca isotope fractionation because each layer is mineralogically similar and comes from the same bulk magma composition (17,18,29). Thus we can assume that the differences in plagioclase Ca isotope compositions are due to differences in crystal growth rates.…”

Anorthosites in Nishiyama volcanic products from the Hachijo–jima island, Izu–Bonin arc: The direct evidence for ‘plagioclase control’ in shallow magma reservoir

“…In this region, such anorthositic enclaves, which could be the direct evidence A d v a n c e P u b l i c a t i o n A r t i c l e of plagioclase accumulation, were not found in previous studies. In addition, the production of anorthosites has rarely been reported in subalkaline volcanic rocks though anorthosites were found from some alkalic intrusive layered rocks (e.g., Suwa, 1976;Czamanske and Bohlen, 1990;Akatsuka et al, 1999;Hoshide et al, 2006;Cawthorn and Ashwal, 2009;Maier et al, 2016;McCarthy and Müntener, 2016). In this paper, we report the occurrence of anorthositic enclaves and the attendant petrographic characteristics, while the genetic relationships between the host magma composition and the anorthositic enclaves are also discussed.…”