Experimental Constraints on Formation of Low-Cr # Chromitite: Effect of Variable H 2 O and Cr 2 O 3 on Boninitic-Magma a
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Experim E mental Constra C ints on Formaation of Low-Cr# Chrom mitite: Effect E off Variab ble H2O and Crr2O3 on n Bonin nitic-M Magma and a Harrzburgiite Reacctions Yage Zh hao
1, 2
, Yan nfei Zhang *1, 2, Chao Wang W 2, Zhenmin Jin2, Qij ijin Xu1, 2
1. College off Oceanographyy, Hohai Univerrsity, Nanjing 210098, 2 China 2. State Key Laboratory of Geological Proocesses and Minneral Resourcess, China Univerrsity of Geoscieences, Wuhan 430074, China Y Yage Zhao: https://orcid.org/00000-0002-9808--661X; Yanffei Zhang: httpss://orcid.org/0000-0001-9874--0469 AB BSTRACT: Reeactions betweeen a boninitiic or basaltic magma and harzburgite h at shallow man ntle deepths are thougght to be closely related to th he formation of o podiform ch hromitites, butt little experim mentall data is availaable on these reeactions. In this study, a seriies of experimeents were cond ducted at 1.5 GPa G an nd 1 000–1 4000 oC to investiggate the interactions between n boninitic maagma and harzzburgite in hom mogenous mixed syystems with vaaried bulk conccentrations of water (~0.7 wt.%–10 w wt.%)) and Cr2O3 (~ ~0.2 wtt.%–4 wt.%). In I the experim mental charges, chromite gra ains can be obsserved coexistiing with orthopyroxxene, clinopyroxene±olivine,, and quenched d melt in the Cr-bearing C systtems. The bulk k concentration n of Crr2O3 in the starting materiall has a slight effect e on compositional changes in the chroomites generatted. Hoowever, the Crr# (Cr#=100×Crr/(Cr+Al)) and d Mg# (Mg#=10 00×Mg/(Mg+F Fe)) values for the chromites exhib bit positive an nd negative correlations, resspectively, with h the bulk H2O concentratioons. At 1 100 oC, ch hromite Cr# vaalues range froom ~33–35 to ~58–65, and chromite Mg# values rangee from ~70–73 3 to ~555–58 when bullk H2O conten nts in the starting material arre increased froom ~0.7 wt.% to ~10 wt.%. The T exp perimentally produced p chroomites have coompositions (ass expressed byy Cr#, Mg#, an nd NiO and MnO M contents) similarr to natural ch hromites from low-Cr# chrom mitite bodies. We W suggest thaat the interactions beetween boninitiic magmas witth varied H2O contents and harzburgite in n a shallow maantle wedge co ould bee a possible meechanism thatt forms the low w-Cr# chromittites found in ophiolites. Wee emphasize here h evolutions of natural thaat H2O may pllay an importaant role in the compositional c n chromiitites. # KE EY WORDS: low-Cr l chrom mitite, boniniticc magma, harzburgite, melt-rrock reaction. 0
INTRODU UCTION Podiform m chromitites, composed c mainnly of chromitee and olivine, are tyypically lenticuular concentrattions of massivve or disseminated to t nodular chroomite grains thaat occur in harzzburgites in ophiolite complexes (Zhou et al., 2014; 2 Dickey, 1975; 1 Thayer, 1964)). Many chromitites orebodiess are surroundeed by envelopes of dunite d that are transitional to the t host harzbuurgite (Arai, 1997). The two mainn types of chroomites are com mposi# tionally eitherr low-Cr# (Cr#: ~10–70) or high-Cr h (Cr#: >70), > where Cr#=100×Cr/(Cr+Al)
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