TY - JOUR
T1 - SiC-dominated ultra-reduced mineral assemblage in carbonatitic xenoliths from the Dalihu basalt, Inner Mongolia, China
AU - He, Detao
AU - Liu, Yongsheng
AU - Gao, Changgui
AU - Chen, Chunfei
AU - Hu, Zhaochu
AU - Gao, Shan
PY - 2017
Y1 - 2017
N2 - Sic and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a Sic-dominated ultra-reduced mineral assemblage, including Sic, Tic, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. Sic is 20-50 μm in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of Sic polytypes were identified, which are dominated by β-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These Sic are featured by 13C-depleted isotopic compositions (δ13C = -13.2 to -22.8%, average = -17.7%) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low δ13C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
AB - Sic and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a Sic-dominated ultra-reduced mineral assemblage, including Sic, Tic, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. Sic is 20-50 μm in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of Sic polytypes were identified, which are dominated by β-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These Sic are featured by 13C-depleted isotopic compositions (δ13C = -13.2 to -22.8%, average = -17.7%) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low δ13C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
KW - Carbonatitic xenolith
KW - silicon carbide
KW - iron silicide
KW - native metal
KW - C isotopic composition
UR - http://www.scopus.com/inward/record.url?scp=85012241879&partnerID=8YFLogxK
U2 - 10.2138/am-2017-5721
DO - 10.2138/am-2017-5721
M3 - Article
AN - SCOPUS:85012241879
SN - 0003-004X
VL - 102
SP - 312
EP - 320
JO - American Mineralogist
JF - American Mineralogist
IS - 2
ER -