Hornblende as a tool for assessing mineral-melt equilibrium and recognition of crystal accumulation

Abstract

Bulk-rock compositions are commonly used as proxies for melt compositions, particularly in silicic plutonic systems. However, crystal accumulation and/or melt loss may play an important role in bulk-rock compositional variability (McCarthy and Hasty 1976; McCarthy and Groves 1979; Wiebe 1993; Wiebe et al. 2002; Collins et al. 2006; Deering and Bachmann 2010; Miller et al. 2011; Vernon and Collins 2011; Lee and Morton 2015; Lee et al. 2015; Barnes et al. 2016a; Schaen et al. 2018). Recognizing and quantifying the effects of crystal accumulation and melt loss in these silicic systems is challenging. Hornblende-melt Fe/Mg partitioning relationships and hornblende (Hbl) chemometry are used here to test for equilibrium with encompassing bulk-rock and/or glass compositions from several plutonic and volcanic systems. Furthermore, we assess the extent to which these tests can be appropriately applied to Hbl from plutonic systems by investigating whether Hbl from the long-lived (~10 Ma) Tuolumne Intrusive Complex preserves magmatic crystallization histories. On the basis of regular zoning patterns, co-variation of both fast- and slow-diffusing trace elements, Hbl thermometry, and compositional overlap with volcanic Hbl we conclude that Hbl from plutons largely preserve records supporting the preservation of a magmatic crystallization history, although many compositional analyses yield calculated temperatures <750 °C, which is unusual in volcanic Hbl. Hornblende is only rarely in equilibrium with host plutonic bulk-rock compositions over a wide range of SiO2 contents (42–78 wt%). Hornblende chemometry indicates that the majority of Hbl from the plutonic systems investigated here is in equilibrium with melts that are typically more silicic (dacitic to rhyolitic in composition) than bulk-rock compositions. These results are consistent with crystal accumulation and/or loss of silicic melts within middle- to upper-crustal plutons. Although the processes by which melts are removed from these plutonic systems is uncertain, it is evident that these melts are either redistributed in the crust (e.g., leucogranite dikes, plutonic roofs, etc.) or are instead erupted. In contrast, Hbl from volcanic rocks is more commonly in equilibrium with bulk-rock and glass compositions. In most cases, where Hbl is out of equilibrium with its host glass, the glasses are more evolved than the calculated melts indicating crystallization from a less fractionated melt and/or mixed crystal populations. Where Hbl is not in equilibrium with volcanic bulk-rocks, the bulk-rock compositions are typically more mafic than the calculated melts. In some intermediate volcanic samples, the occurrence of wide-ranges of calculated melt compositions is indicative of magma mixing. The general absence of Hbl with temperatures <750 °C from volcanic systems suggests that magmatic mushes below this temperature are unlikely to erupt. Our results indicate that bulk-rock compositions of granitic plutonic rocks only rarely approximate melt compositions and that the possibility of crystal accumulation and/or melt loss cannot be ignored. We suggest that detailed assessments of crystal accumulation and melt loss processes in magmatic systems are crucial to evaluating magma differentiation processes and discerning petrogenetic links between plutonic and volcanic systems.

Extraction of high-silica granites from an upper crustal magma reservoir: Insights from the Narusongduo magmatic system, Gangdese arc

“…It is noteworthy that the phase assemblages from rhyolite-MELTS modelling here should be treated with caution because rhyolite-MELTS cannot easily deal with amphibole crystallization (Ghiorso and Sack 1995;Gualda et al 2012). However, the situation that amphibole crystallizes from more evolved melts than the bulk-rock composition of the host is consistent with general observations in plutonic rocks (e.g., Werts et al, 2020), as well as the observed coexistence of amphibole and quartz within some glomerocrysts in this study ( Fig. 4a; Supplemental Fig.…”

Schlieren-Bound Magmatic Structures Record Crystal Flow-Sorting in Dynamic Upper-Crustal Magma-Mush Chambers

“…TIC rocks are metaluminous to locally peraluminous, calcalkaline compositions, spanning a wide range of SiO 2 contents (48-79 wt.%) (Bateman and Chappell, 1979;Bateman, 1992;Memeti et al, 2014). Hornblende analyses indicate that all units experienced crystal accumulation and/or melt loss to varying degrees, contributing to the wide variation in rock type (Barnes et al, 2016;Werts et al, 2020). The considerable compositional overlap between units in major and trace elements, as well as isotopes, has been attributed to a combination of fractionation and mixing processes involving multiple magma sources; the depth that these processes occur is debated (e.g., Kistler et al, 1986;Bateman, 1992;Gray et al, 2008;Coleman et al, 2012;Memeti et al, 2014;Barnes et al, 2016).…”

“…We used these intersections to estimate the bulk composition of a TIC magma that we could model under fixed pressure and varying temperature conditions. Werts et al (2020) and Barnes et al (2020), using the thermometer of Putirka (2016), calculated hornblende crystallization temperatures ranging from >800 to ∼650 • C across all TIC units. As hornblende is abundant in all schlieren studied, the temperature range 800-700 • C was considered relevant to schlieren formation to a first order.…”

“…At this bulk composition at 800 • C, effective viscosity increases from 10 6.9 to 10 9.3 Pa s with increasing crystal contents and reaches a maximum of 10 10.3 at 700 • C ( Table 3). Werts et al (2020) demonstrated that the bulk composition of many TIC samples is generally more mafic than the composition of melts in equilibrium with hornblende due to crystal accumulation. We compared our bulk-rock results to a model using a calculated melt composition in equilibrium with CP hornblende, containing 4 wt.% H 2 O and 76 wt.% SiO 2 (equations 3, 5, 7, 9, 11, 13, and 14 and associated uncertainties from Zhang et al, 2017).…”

A reassessment of the amphibole-plagioclase NaSi-CaAl exchange thermometer with applications to igneous and high-grade metamorphic rocks

“…Calcic amphibole is an important constituent of basic to acid metaluminous igneous rocks and their metamorphic derivatives (e.g., Robinson et al, 1982;Wones and Gilbert, 1982;Martin, 2007;Schumacher, 2007). Many studies have been devoted to determine its P-T-X relationships in both synthetic and natural assemblages (e.g., Czamanske and Wones, 1973;Laird and Albee, 1981;Thompson et al, 1982;Apted and Liou, 1983;Poli, 1993;Spear, 1993;Poli, 1998, 2000;García-Casco et al, 2008;Molina et al, 2009;Bucher and Grapes, 2011;Castro, 2013;Werts et al, 2020). These studies have provided a basis for the calibration of numerous thermometers and barometers that make it possible to determine the P-T conditions of crystallization/re-equilibration of amphibole-bearing assemblages.…”