| Jadeitite,which is mainly composed of NaAl[Si2O6],is a rare rock on Earth.Jadeitite often occurs as structural veins or lenses within LT/HP serpentinite,and the largest localities include Myanmar and Guatemala.Due to its rareity and distinctive conditions for occurrence,different approaches from different scholars have been deployed in order to investigate the mechanism of jadeitization.Mineralogy,petrology,study of ore deposit,mineral chemistry and isochron dating are well developed.Since jadeitite produces Fei Cui,a jade beloved by Chinese culture,business interests have motivated some investigations.Summarizing all available information,Tsujimori and Harlow(2012)categorized two types of jadeitization mechanism-P-type,in which a Na-Al-Si-rich fluid infiltrated mantle wedge,and jadeite precipitates within;and R-type,in which a plagiogranite,metagabbro or eclogite is being replaced into jadeitite.However,since there are insufficient studies in trace elements regarding jadeitite,there is not much support for the P-and R-type theory.Therefore,this investigation intends to look into the trace element characteristics of jadeitite and related rocks from Myanmar and Guatemala,so that the jadeitization mechanism can be better understood.This investigation involves EPMA,ICP-MS,and LA-ICP-MS measurements of jadeitite and related rocks from Myanmar and Guatemala,obtained by Prof.Guang-hai Shi of our school,G.E.Harlow from American Museum of Natural History,and the author.Data obtained shows that,(1)in Myanmar,the total trace element contents from a decending order is[Amphibolite?Omphacitite>Serpentinite>Jadeitite];in Guatemala,the total trace element contents from a decending order is[Omphacitite?Eclogite?Garnet-Amphibolite>Jadeitite>Albitite>Serpentinite].(2)Jadeitite from Myanmar shows enrichments of Sr,Zr and Hf,positive anomaly of Eu,and a low Zr/Hf ratio(min.=14.53,avg.= 27.87).(3)Jadeitite from Guatemala shows a positive anomaly of Eu,and a low Zr/Hf ratio(min.-20,avg.= 27.84).(4)In Guatemala,both jadeitite and eclogite show strong enrichment of Ba,even stronger than that of Myanmar jadeitite and any other rocks,implying that their relations to oceanic sediment are stronger than those from Myanmar.(5)All jadeitite,eclogite,garnet-amphibolite,omphacitite,serpentinite from Guatemala show uneven distribution of Sr,which implies that a fluid has infiltrated all related rocks;but since gamet-amphiboliteand omphacitite do not show trends of jadeitization,this metasomatic event is suggested to have happened prior to jadeitization.(6)Trace element characteristics of jadeitite from Myanmar and Guatemala are different from Jd-eclogite from Dabie-Sulu and Alps,indicating different petrogenesis.(7)Titanite in Myanmar omphacitite is metamorphic-origin;it lacks negative Eu-anomaly,which in turn suggests the absence of a positive Eu-anomaly in omphacitite,hence omphacitite is suggested to a mafic rock mixed inside the serpentinite melange.After all comparisons,it is found that jadeitites from both Myanmar and Guatemala contain positive Eu-anomalies and low Zr/Hf ratios.According to the P-and R-type categorization of Tsujimori and Harlow,(2012),the characteristics of Eu and Zr/Hf ratio could come from the metasomatic fluid(P-Type)or a later-replaced protolith(R-type);however,since the distribution of Eu and fractionation of Zr/Hf can only be available in magmatic conditions,R-type jadeitization seems to prevail-at least some jadeitite has a protolith which has a positive Eu-anomaly and low Zr/Hf ratio;only later the protolith is replaced into jadeitite.This investigation argues that the definitions of P-and R-type jadeitization are not well defined:they mostly used petrography to support P-type jadeitization,while geochemistry is widely ignored;but when this investigation gives geochmistry evidence,R-type jadeitization seems to prevail.Also,it is observed that R-type jadeitization would accompany the occurrence of P-type jadeitization;hence,P-and R-type jadeitizations are not two separated,isolating mechnisms. |
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