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.
The Earth’s mantle is an important reservoir of H2O, and even a small amount of H2O has a significant influence on the physical properties of mantle rocks. Estimating the amount of H2O in rocks from the Earth’s mantle would, therefore, provide some insights into the physical properties of this volumetrically dominant portion of the Earth. The goal of this study is to use mineral equilibria to determine the activities of H2O (aH2O) in orogenic mantle peridotites from the Western Gneiss Region of Norway. An amphibole dehydration reaction yielded values of aH2O ranging from 0.1 to 0.4 for these samples. Values of fO2 of approximately 1 to 2 log units below the FMQ oxygen buffer were estimated from a fO2-buffering reaction between olivine, orthopyroxene, and spinel for these same samples. These results demonstrate that the presence of amphibole in the mantle does not require elevated values of aH2O (i.e., aH2O≈1) nor relatively oxidizing values of fO2 (i.e., >FMQ). It is possible to estimate a minimum value of aH2O by characterizing fluid speciation in C-O-H system for a given value of oxygen fugacity (fO2). Our results show that the estimates of aH2O obtained from the amphibole dehydration equilibrium are significantly lower than values of aH2O estimated from this combination of fO2 and C-O-H calculations. This suggests that fluid pressure (Pfluid) is less than lithostatic pressure (Plith) and, for metamorphic rocks, implies the absence of a free fluid phase. Fluid absent condition could be generated by amphibole growth during exhumation. If small amounts of H2O were added to these rocks, the formation of amphibole could yield low values of aH2O by consuming all available H2O. On the other hand, if the nominally anhydrous minerals (NAMs) contained significant H2O at conditions outside of the stability field of amphibole they might have served as a reservoir of H2O. In this case, NAMs could supply the OH necessary for amphibole growth once retrograde P-T conditions were consistent with amphibole stability. Thus, amphibole growth may effectively dehydrate coexisting NAMs and enhance the strength of rocks as long as the NAMs controlled the rheology of the rock.
Coexisting fine-grained (meta-volcanic) and coarse-grained (meta-plutonic) mafic rocks in a high-pressure (P)/low-temperature (T) complex (Sivrihisar, Turkey) preserve different prograde, peak, and retrograde mineral assemblages, providing an opportunity to evaluate controls on mineral assemblages in metabasites that experienced the same P-T conditions. Finegrained metabasalts are garnet-bearing lawsonite blueschist and eclogite with similar assemblages that vary on a mm-to cm-scale in mode of glaucophane vs. omphacite. In contrast, metagabbro consists of a disequilibrium mineral suite of relict igneous clinopyroxene partially replaced by omphacite or hydrous phases (lawsonite + tremolite or glaucophane) in a matrix of fine-grained lawsonite, omphacite, tremolite, white mica, very rare garnet, and retrograde minerals (e.g., epidote, albite, and titanite), with later chlorite and calcite. Pseudosection modeling predicts similar peak P-T conditions (490-530 °C, 1.8-2.0 GPa) for both glaucophane-rich (blueschist) and omphacite-rich (eclogite) layers of the metabasalt and similar to slightly higher conditions (490-600 °C, 1.9-2.5 GPa) for metagabbro. The range of modelled H 2 O content at peak P-T conditions in metabasalt (2.0-5.4 wt%) is significantly lower than in metagabbro (6.4-8.7 wt%) due to the higher modal abundance of hydrous minerals in the latter. At the relatively similar peak P-T conditions, metagabbro experienced different reaction histories from coexisting metabasalt, thereby developing distinctive HP/LT mineral assemblages and modes (e.g., scarce garnet) owing to its more Mg-rich bulk composition (X Mg = 0.58-0.84 vs. 0.50), higher H 2 O content, and coarser grain-size. This study is the first petrologic analysis of Sivrihisar metagabbro and the first systematic study of H 2 O content in metabasites from this locality, which is one of the best-preserved examples of lawsonite eclogite and blueschist in the world.
Recycling is not just for plastic. Did you know that the Earth recycles? Recycling happens because the outer part of the planet is made up of large moving pieces of rock. Some of these pieces, called tectonic plates, sink deep down into the Earth. The deeper they go, the more heat and pressure they experience. This causes chemical reactions, including melting of the minerals that make up the rocks. Elements and water trapped inside the melting minerals are released and erupt from volcanoes, returning to the surface. The Earth has recycled! In this article, we present new research on a mineral called lawsonite. Lawsonite only forms in plates that dive into the Earth. Lawsonite has returned to the Earth’s surface in a few rare places where we can collect and analyze it. The composition of elements inside the lawsonite mineral help us understand the deep part of the Earth recycling system.
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