Fractional crystallization processes of magma beneath the Carlsberg Ridge (57°-65°E) -- Journal of Oceanology and Limnology,2020,38(1):75-92

	Cite this paper:
	ZONG Tong, HAN Xiqiu, LIU Jiqiang, WANG Yejian, QIU Zhongyan, YU Xing. Fractional crystallization processes of magma beneath the Carlsberg Ridge (57°-65°E)[J]. Journal of Oceanology and Limnology, 2020, 38(1): 75-92

Fractional crystallization processes of magma beneath the Carlsberg Ridge (57°-65°E)

ZONG Tong^1,2, HAN Xiqiu^2,3,4, LIU Jiqiang², WANG Yejian², QIU Zhongyan², YU Xing²

1 Key Laboratory of Submarine Geosciences and Prospection of Ministry of Education, Ocean University of China, Qingdao 266100, China;
2 Key Laboratory of Submarine Geosciences, State Oceanic Administration & Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China;
3 Ocean College, Zhejiang University, Zhoushan 316021, China;
4 School of Oceanography, Shanghai Jiaotong University, Shanghai 200030, China

Abstract:

Fractional crystallization of basaltic magma at variable depths influences strongly the geochemical compositions of mid-ocean ridge basalts (MORBs), especially at slow-spreading mid-ocean ridges. The Carlsberg Ridge is a typical slow-spreading ridge located in the northwestern Indian Ocean. In this study, we conducted petrological, geochemical and modelling studies of MORBs collected along the Carlsberg Ridge from 57°-65°E to understand the fractional crystallization processes of magma and the controls on variations in MORB geochemistry. Our results show that the mantle sources beneath the Carlsberg Ridge are heterogeneous even on the local scale of a segment; such heterogeneity may be ubiquitous beneath the Carlsberg Ridge. Mantle heterogeneity may be caused by the enriched components resulting in the "DUPAL" anomaly, whereas the effect of pyroxenite on mantle heterogeneity is negligible. The parental melts experienced crystallization of olivine, plagioclase and clinopyroxene prior to eruption, which played a significant role in the major and trace element variations in MORBs from the Carlsberg Ridge. The liquid lines of descent (LLDs), deduced from the forward modelling of three parental magma compositions using the Petrolog3 program at pressures between 1 atm and 10 kbar, demonstrate that clinopyroxene joined the olivine and plagioclase cotectic. The over-enrichment in highly incompatible elements relative to LLDs may be caused by the processes of replenishment-tapping-crystallization in magma chambers. The calculated crystallization pressures suggest that parental magmas beneath the Carlsberg Ridge experienced moderateto high-pressure crystallization and that crystallization beneath the slow-spreading Carlsberg Ridge may start at upper mantle depths.

Key words: fractional crystallization|crystallization pressure|mantle heterogeneity|Carlsberg Ridge|midocean ridge basalt (MORB)

Received: 2018-09-24 Revised:

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