Trace and Rare Earth Element Compositions of Lawsonite as a Chemical Tracer of Metamorphic Processes in Subduction Zones

Abstract

Lawsonite is a major host mineral of trace elements (e.g., REE, Sr, Pb, U, Th) and H2O in various rock types (metabasite, metasediment, metasomatite) over a wide range of depths in subduction zones. Consequently, the composition of lawsonite is a useful archive to track chemical exchanges that occurred during subduction and/or exhumation, as recorded in high-pressure/low-temperature (HP/LT) terranes. This study provides an extensive dataset of major and trace element compositions of lawsonite in HP/LT rocks from two mélanges (Franciscan/USA; Rio San Juan/Dominican Republic), two structurally coherent terranes (Tavşanlı/Turkey; Alpine Corsica/France), and the eclogite blocks of the Pinchi Lake/Canada complex. Bulk major and trace element compositions were also determined for lawsonite-bearing host rocks to understand petrogenesis and assess compositional evolution. Most analyzed mélange and coherent-terrane metabasalts have normal mid-ocean ridge/back-arc basin basalt signatures and they preserve compositional evidence supporting interactions with (meta)sediment ± metagabbro/serpentinite (e.g., LILE/LREE enrichments; Ni/Cr enrichments). Most lawsonite grains analyzed are compositionally zoned in transition-metal elements (Fe, Ti, Cr), other trace elements (e.g., Sr, Pb), and/or REE, with some grains showing compositional variations that correlate with zoning patterns (e.g., Ti-sector zoning, core-to-rim zoning in Fe, Cr-oscillatory zoning). Our results suggest that compositional variations in lawsonite formed in response to crystallographic control (in Ti-sector zoning), fluid-host rock interactions, modal changes in minerals, and/or element fractionation with coexisting minerals that compete for trace elements (e.g., epidote, titanite). The Cr/V and Sr/Pb ratios of lawsonite are useful to track the compositional influence of serpentinite/metagabbro (high Cr/V) and quartz-rich (meta)sediment (low Sr/Pb). Therefore, lawsonite trace and rare earth element compositions effectively record element redistribution driven by metamorphic reactions and fluid-rock interactions that occurred in subduction systems.

Lawsonite and Garnet Oxygen Isotope Record of Fluid‐Rock Interaction During Subduction

“…At W / R > 1, the δ 18 O Water corresponding to the Tavşanlı metasomatite δ 18 O Lws falls within the range of hydrothermally altered basalt and siliciclastic (meta)sediment (Eiler, 2001). Bulk enrichment of Th/La and LILE has been reported for this metasomatite (Kang et al., 2022), supporting interactions with terrigenous sediments during metasomatism. The Tavşanlı metasomatite could have formed at lower W / R (<1) if the fluid had a sufficiently higher δ 18 O (>∼20‰) than its ultramafic protolith, such as fluid sourced from carbonaceous (meta)sediment (Eiler, 2001).…”

“…Analyzed lawsonite grains are from diverse textural sites (e.g., in garnet, matrix, vein), and are largely unaltered, without textural evidence for replacement except for those in the Garnet Ridge xenolith (17GR11; Figure 1g). Some lawsonite grains were previously analyzed and reported for their major and minor element compositions (Kang et al., 2022); therefore, these grains are only briefly discussed in the following sections.…”

“…δ 18 O Water during lawsonite growth can be expressed in terms of the last two variables as well as δ 18 O Lws by applying a mass‐balance equation as follows: $$${\delta }^{18}{\mathrm{O}}_{\text{Water}}=\frac{\left({\delta }^{18}{\mathrm{O}}_{\text{Lws}-\text{bearing}\,\text{metasomatite}}\,-\,{\delta }^{18}{\mathrm{O}}_{\text{Precursor}\,\text{rock}}\right)+W/R\,\left({\delta }^{18}{\mathrm{O}}_{\text{Lws}}\,-\,1000\mathrm{ln}{\alpha }_{\text{Lws}-\text{Water}}\right)}{W/R}$$$ where δ 18 O Water , δ 18 O Lws‐bearing metasomatite , δ 18 O Precursor rock , and δ 18 O Lws are the oxygen isotope compositions of fluid, lawsonite‐bearing metasomatite, precursor rock of metasomatite, and lawsonite, respectively. One million simulations were run to calculate δ 18 O Water in 300 ≤ T (°C) ≤ 800 and 0 ≤ W / R ≤ 1,000. δ 18 O Precursor rock was constrained based on the δ 18 O of potential precursor rock of each sample (peridotite for Tavşanlı metasomatite: 5.3–5.7‰; calcschist for Corsica metasomatites: 20–35‰) (Eiler, 2001; Kang et al., 2022; Vitale Brovarone et al., 2014). δ 18 O Lws‐bearing metasomatite of each sample was previously measured or estimated from the modes and oxygen isotope compositions of matrix minerals (Table 5) (Gauthiez Putallaz, 2017; Martin, Rubatto, et al., 2014).…”

“…All targeted metasomatites formed along lithological contacts but at the expense of different lithologies. The Tavşanlı metasomatite likely formed at the expense of serpentinite lenses that have an ultramafic protolith (Kang et al., 2022), whereas the Corsica metasomatites preferentially form within carbonaceous metapelites (Martin et al., 2011; Vitale Brovarone et al., 2014, 2020). Major parameters that control lawsonite δ 18 O in these metasomatites include (a) the δ 18 O of fluid, (b) the temperature, and (c) the W / R at the time of fluid‐rock interactions.…”

“…Despite such variations, our compilation of Fe, Ti, and Cr zoning patterns shows that concentric zoning is common, with enrichment of these elements either at the core or rim (Figure 3a and Table 1), regardless of textural site and lithology. Some grains exhibit Ti-hourglass sector zoning (Table 1), which is characterized by up to four different domains that vary in Ti content (Vitale Brovarone et al, 2014): Zone 1: Ti-intermediate core; Zone 2: Ti-rich mantle; Zone 3: Ti-poor rim; Zone 4: variable Ti-content with oscillatory Cr-zoning (Figure 3b; see also Figure 8e in Kang et al, 2022). Zones 1 and 4 are preserved in some but not all of the analyzed grains.…”