The Jian forsterite jade, named for its enrichment of end-member forsterite, is a new type of jade discovered in Ji'an County (Jilin Province, Northeast China). The formation mechanism of this rare end-member forsterite remains unclear. In this study, we present the mineral chemistry and oxygen isotope analysis of end-member forsterite to elucidate the formation mechanism of the Jian forsterite jade. The Jian forsterite jade can be divided into three main types: yellowish-green, dark-green, and black. The Jian forsterite jade consists mainly of forsterite, serpentine, as well as minor brucite, spinel, phlogopite, dolomite, calcite, magnesite, and magnetite. The Fo% content (100 × Mg/[Mg + Fe], mol%) of forsterite varies from 96.69 to 99.82. All tested forsterite samples show low Ni, Mn, Cr, and Ca contents and very high B contents. The δ¹⁸O values of forsterite range from 13.79 ‰ to 18.57 ‰, which are significantly higher than those from the mantle and are similar to those from skarn, marble, and calc-silicate rocks. The oxygen isotope values of forsterite, coupled with a distinctive trace element signature, suggest that the genesis of end-member forsterite results from a siliceous fluid-mediated decarbonation of Mg-rich dolomite. The genesis of the Jian forsterite jade can be outlined in two main metamorphic stages: (1) high-temperature prograde stage related to the interaction of the gneiss-derived SiO₂-rich fluids with overlying dolomite. High temperature leads to the formation of forsterite, spinel, and calcite mineral assemblage. (2) low-temperature retrograde stage characterized by the infiltration of hydrous fluids. Hydrous silicate minerals, mostly serpentine, phlogopite, and talc, are formed during this stage. Our work highlights and confirms the effectiveness of the trace element geochemistry of forsterite in identifying magnesian skarns, and classifies the forsterite in the Jian forsterite jade as an extreme example of this occurrence.

建镁橄榄石因富集端元镁橄榄石而得名，是在吉安县（吉林省，东北地区）发现的一种新型玉石。这种罕见的端元镁橄榄石的形成机制仍不清楚。在这项研究中，我们介绍了端元镁橄榄石的矿物化学和氧同位素分析，以阐明建镁橄榄石的形成机制。建镁橄榄石可分为黄绿色、墨绿色和黑色三种主要类型。建镁橄榄石以镁橄榄石、蛇纹石为主，还有少量水镁石、尖晶石、金云母、白云石、方解石、菱镁矿、磁铁矿等。镁橄榄石的 Fo% 含量 (100 × Mg/[Mg + Fe], mol%) 在 96.69 到 99.82 之间变化。所有测试的镁橄榄石样品都显示出低 Ni、Mn、Cr ​​和 Ca 含量以及非常高的 B 含量。δ^{镁橄榄石的18} O 值介于 13.79‰～18.57‰ 之间，显着高于地幔，与矽卡岩、大理岩、钙硅酸盐岩相近。镁橄榄石的氧同位素值与独特的微量元素特征相结合，表明端元镁橄榄石的成因是由硅质流体介导的富镁白云石脱碳引起的。建镁橄榄石的成因可概括为两个主要的变质阶段： (1) 与片麻岩衍生的 SiO _{2相互作用有关的高温推进阶段}- 上覆白云岩的富流体。高温导致镁橄榄石、尖晶石和方解石矿物组合的形成。(2)以含水流体渗入为特征的低温逆行阶段。在这个阶段形成了水合硅酸盐矿物，主要是蛇纹石、金云母和滑石。我们的工作突出并证实了镁橄榄石的微量元素地球化学在识别镁矽卡岩中的有效性，并将建镁橄榄石中的镁橄榄石归类为这种现象的一个极端例子。