Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust

Abstract

Studies of the petrology, mineral chemistry, and rock magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal remanent magnetization (M rs ) of 39.59 × 10 À8 m 3 kg À1and 12.05 × 10 À3 Am 2 kg À1 , respectively. The magnetic minerals are mainly magnetite, pyrrhotite, and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial microcrystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths' ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has~50% mole fraction of Fe 3 O 4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be nonmagnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~32-42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~32 km) in Hannuoba, northern North China Craton.

Magnetic properties of serpentinized peridotites from the Dongbo ophiolite, SW Tibet: Implications for suture‐zone magnetic anomalies

“…Fe 3 O 4 ‐poor spinels generally show paramagnetic susceptibility [ Harrison and Putnis , ], and this is supported by the χ ‐ T curve of sample DB11‐58 without T c below 550°C during heating (Figure c). The new T c at ~367°C on cooling could be caused by the formation of new Fe 3 O 4 ‐rich spinel and another Fe‐poorer spinel during high‐temperature heating [ Li et al ., ]. The Fe 3 O 4 ‐poor Fe‐Cr spinels may also account for the rapid rise of magnetization at T < 50 K (Figure c) like pure FeCr 2 O 4 [ Robbins et al ., ].…”

A conceptual framework for mixing structures in individual aerosol particles

“…Many studies have described quantitative techniques for assessing the mixing state of aerosol particles [Adachi and Buseck, 2008;Shiraiwa et al, 2008;Zelenyuk et al, 2008;Moffet and Prather, 2009;Oshima et al, 2009;Anttila, 2010;Dall'Osto et al, 2010;Deboudt et al, 2010;Geng et al, 2010;Li and Shao, 2010;Pratt and Prather, 2010;Cheng et al, 2012;Riemer and West, 2013;Zhang et al, 2013;Li et al, 2014b]. There are several experimental techniques for probing different aspects of the aerosol mixing state, including single-particle analysis, chemical analysis techniques for size-resolved aerosol mass, Single Particle Laser Ablation Time-of-Flight Mass Spectrometer [Zelenyuk et al, 2008], aerosol time-of-flight mass Spectrometer (ATOFMS) [Moffet and Prather, 2009;Dall'Osto et al, 2010;Pratt and Prather, 2010;Zhang et al, 2013;Healy et al, 2014], scanning and transmission electron microscopy (SEM/TEM) [Buseck and Posfai, 1999;Adachi and Buseck, 2008;Li and Shao, 2009b;Geng et al, 2010;Gieré and Querol, 2010;Li and Shao, 2010;Ueda et al, 2011;Zhang et al, 2013;Li et al, 2014b], scanning transmission Xray microscopy (STXM) with near-edge X-ray absorption fine structure spectroscopy [Takahama et al, 2010], and hygroscopic tandem differential mobility analyzer instruments [McMurry, 2000;Swietlicki et al, 2008;Anttila, 2010;Cheng et al, 2012]. In addition, the dependence of optical properties on th...…”

“…Compared to ATOFMS, TEM cannot analyze thousands of individual particles in a timely manner; it nonetheless clearly provides detailed information on the mixing structure of different aerosol species for individual particles. To overcome this shortcoming of TEM, some studies have classified particle types based on several hundred analyzed particles and summarized their characterization so that these analyzed data represent the collected sample [Adachi and Buseck, 2008;Deboudt et al, 2010;China et al, 2014;Li et al, 2014bLi et al, , 2016. Therefore, TEM remains one of the better methods to study the real mixing state of individual particles.…”

Lithological and age structure of the lower crust beneath the northern edge of the North China Craton: Xenolith evidence

“…The xenoliths vary compositionally from one locality to another. For example, granulites from Damaping are dominantly mafic with fewer intermediate and felsic members, coexisting with abundant peridotite and pyroxenite xenoliths Li et al, 2014;Liu et al, 2001Liu et al, , 2004Zheng et al, 2004bZhou et al, 2002). Those from Zhouba are exclusively felsic , and lower-crustal xenoliths from Jieshaba and Xiaomaping include mafic to felsic granulites coexisting with abundant peridotites and a few garnet-bearing pyroxenites Liu et al, 2004;Xu, 2002).…”