The following is by Tualatin Valley Rock and Gem Club member Taylor Hunt. if you would like to contribute to our newsletter and website with articles on various rocks, minerals, and gems, please contact us.
The gemstone jadeite generally forms as a result of the plate tectonic process of subduction. Jade made from jadeite forms when supercritical fluids from subducting oceanic crust condense in the overlying man-tel wedge (the “wedge” is all the sediments swept up, piled and squeezed between the subducting oceanic plate and a continental mass), between 20 & 60 km deep in the Earth. Jadeite deposits thus mark the loca-tion of exhumed fossil subduction zones.
A new term PTG’s, plate tectonic gemstone, and jadeite is one PTG currently recognized. For various reasons most PTG’s are found in rocks or continental plates considered young to planet Earth. Most are no older than the formation of the supercontinent of Rodinia or 1,000 ma yr. Petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion. Recognition of the PTG’s links modern concepts of plate tectonics to economic gemstones deposits and the ancient concepts of beauty, and may aid in exploration for new deposits.
Any mineral or stone beautiful enough to be sought, mined and sold for its beauty alone is a gemstone. The subclass of rocks and minerals that comprise gemstones—whether precious or semi-precious—has mostly been established since antiquity. Humans have sought and prized gemstones since thousands of years be-fore the science of geology was established. Because gemstones are rare by definition, the geological conditions that produce them must have been exceptional. Thus, there is a confluence of economic, aesthetic and academic interest in gemstones. Jade — specifically the variety jadeite — is the characteristic beautiful product of normal oceanic lithosphere subduction.
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Jade is a term ascribed to two different materials with similar properties, toughness, and beauty that evolved in usage and significance from toolstones for axes, choppers and hammers to one of the most highly revered gemstones in the world. As a tool, jade was employed during the Paleolithic (stone age, before 3500 BCE) but was raised to high symbolic stature as a gemstone in proto—Chinese Hongshan and Liangzhu cultures by 3500 BCE, and in the Jom’on culture of Japan by 3000 BCE, and in Central America by the Olmec of the Early Formative period by at least 1500 BCE and later in the Mayan civilization. Hard jade (jadeitite) or “ying yu” in Chinese consists predominately of pyroxene minerals, jadeite (Na,Al,Si2,O8), while soft jade (nephrite jade) “ruan yu” come from amphibole minerals of tremolite-actinolite [Ca2(Mg,Fe)5,Si8O22(OH)2]. The term jade was derived from the Spanish “piedra de yjada” (loin stone) for talismans worn by the Aztec to ease abdominal pain, but was mistranslated to the word jade. In New Zealand, nephrite jade is sometimes called greenstone and was a favorite of the Maoris.
Jadeitite is found in association with other high-pressure/low-temperature (HP/LT) metamorphic lithologies that are diagnostic of fossil subduction zones. This assemblage typically includes subducted oceanic crust transformed to blueschist (glaucophane metabasalt) and eclogite, and mantle wedge material (serpentinized peridotite), typically as mélange matrix. Such an assemblage probably represents an ex-humed subduction channel in which buoyancy-driven return flow above the plate interface has brought subducted material back to the surface. Jadeitite s form in this environment at a wide range of depths, typi-cally 20-60 km but occasionally as deep as 100 km.
Of the two jade rocks, jadeitite is the actual subduction indicator.
Jadeite, by virtue of its density (3.4 g cm-2), is a high-pressure indicator, and thinking on its significance predates plate tectonic theory, but a rock essentially formed of jadeite is not simply interpreted as a metamorphic rock. With the realization that jadeitite is a precipitate or metasomatic replacement from hydrous fluids released during dehydration of subducted oceanic crust, the “more than one type of jadeite problem was resolved. High pressure in subduction zones enhances dissolved solute concentrations in hydrous fluids released from subducted materials, enriched in Na, Al, and Si, such that the primary saturated phase is jadeite. These hydrous fluids are buoyant and flow up to infiltrate and react with the overlying mantle wedge sole, which itself becomes pervasively altered, forming jadeite veins. The occurrence of relict chromian spinel in many jadeites further indicates reaction between jadeitite and host ultramafic basalt rocks. Jadeitites thus serve as a proxy for the related mass transfer within a subduction zone at relatively shallow depths (<100 km), for example the Coast Range Ophiolite of the Klamath Mts and the Canyon Mts Complex of the John Day area.
Jadeite minerals and blueschist are two examples of universally acknowledged as products of the plate tectonic process of subduction; similarly, coesite- and/or diamond-bearing ultra-high-pressure (UHP) metamorphic rocks are accepted to manifest subduction of continental crust to more than 100 km deep.
Thank you to Tualatin Valley Rock and Gem Club member Taylor Hunt. if you would like to contribute to our newsletter and website with articles on various rocks, minerals, and gems, please contact us.
Originally published in our June 2014 Newsletter Hygrader.