{"title":"Two-pyroxene, intergrowth exsolution textures in ophiolitic chromites: implications for the deep mantle origin of the Mirdita Ophiolite, Albania","authors":"Weiwei Wu, Jingsui Yang, Yildirim Dilek, Souvik Das, Dongyang Lian, Pengjie Cai, Yun Wang, Yu Yang, Yuanyun Wen","doi":"10.1144/jgs2023-128","DOIUrl":null,"url":null,"abstract":"Increasing mineralogical and textural evidence from podiform chromitites in ophiolites show their ultra–high pressure origin (>150 km), challenging the conventional models for their formation under low pressure conditions (< 60 km) in the upper mantle. However, this challenge remains controversial due to the lack of in-situ mineralogical evidence. Here, we report new data and observations from the Skenderbeu massif in Mirdita ophiolite, Albania. Transmission electron microscopy (TEM) analysis reveal for the first time that these chromitites (Cr#= 41.8-43.2) have numerous exsolution lamellae of diopsidic clinopyroxene and orthoenstatite. TEM analysis also show that these lamellae have a crystallographic topotaxy relationship with the host chromite, i.e., (020) Cpx ∥︀ (22(—)0) Chr , (2(—)00) Cpx ∥︀ (111) Chr , and (010) Opx ∥︀ (22(—)0) Chr , (200) Opx ∥︀ (22(—)0) Chr , indicating an exsolution origin. The abundant presence of pyroxene exsolution lamellae in center of the host chromites implies the incorporation of Si 4+ and Ca 2+ cations in the precursor chromite, a CaFe 2 O 4 -structured high-pressure polymorph, which was stable >12.5 GPa (i.e., 380 km deep). These in-situ, nano-scale observations, and geological occurrence, together with previously discovered ophiolitic diamonds in the Mirdita ophiolite, suggest a much deeper origin for ophiolitic chromitites than conventional interpretations, and provide a valuable opportunity to understand the composition of the deep mantle. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.6000,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of the Geological Society","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.1144/jgs2023-128","RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOSCIENCES, MULTIDISCIPLINARY","Score":null,"Total":0}
引用次数: 0
Abstract
Increasing mineralogical and textural evidence from podiform chromitites in ophiolites show their ultra–high pressure origin (>150 km), challenging the conventional models for their formation under low pressure conditions (< 60 km) in the upper mantle. However, this challenge remains controversial due to the lack of in-situ mineralogical evidence. Here, we report new data and observations from the Skenderbeu massif in Mirdita ophiolite, Albania. Transmission electron microscopy (TEM) analysis reveal for the first time that these chromitites (Cr#= 41.8-43.2) have numerous exsolution lamellae of diopsidic clinopyroxene and orthoenstatite. TEM analysis also show that these lamellae have a crystallographic topotaxy relationship with the host chromite, i.e., (020) Cpx ∥︀ (22(—)0) Chr , (2(—)00) Cpx ∥︀ (111) Chr , and (010) Opx ∥︀ (22(—)0) Chr , (200) Opx ∥︀ (22(—)0) Chr , indicating an exsolution origin. The abundant presence of pyroxene exsolution lamellae in center of the host chromites implies the incorporation of Si 4+ and Ca 2+ cations in the precursor chromite, a CaFe 2 O 4 -structured high-pressure polymorph, which was stable >12.5 GPa (i.e., 380 km deep). These in-situ, nano-scale observations, and geological occurrence, together with previously discovered ophiolitic diamonds in the Mirdita ophiolite, suggest a much deeper origin for ophiolitic chromitites than conventional interpretations, and provide a valuable opportunity to understand the composition of the deep mantle. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
期刊介绍:
Journal of the Geological Society (JGS) is owned and published by the Geological Society of London.
JGS publishes topical, high-quality recent research across the full range of Earth Sciences. Papers are interdisciplinary in nature and emphasize the development of an understanding of fundamental geological processes. Broad interest articles that refer to regional studies, but which extend beyond their geographical context are also welcomed.
Each year JGS presents the ‘JGS Early Career Award'' for papers published in the journal, which rewards the writing of well-written, exciting papers from early career geologists.
The journal publishes research and invited review articles, discussion papers and thematic sets.