Fluids are bound to be involved in the formation of ophiolitic chromite deposits

Kód výsledku v IS VaVaI

<a href="https://www.isvavai.cz/riv?ss=detail&h=RIV%2F00216208%3A11310%2F17%3A10371205" target="_blank" >RIV/00216208:11310/17:10371205 - isvavai.cz</a>

Výsledek na webu

—

DOI - Digital Object Identifier

—

Jazyk výsledku

angličtina

Název v původním jazyce

Fluids are bound to be involved in the formation of ophiolitic chromite deposits

Popis výsledku v původním jazyce

Chromite orebodies within the harzburgitic upper mantle have been investigated in several ophiolitic complexes, with a focus here on New Caledonia. The relationship between harzburgite and a dunite envelope around pods of chromitite points to a passive metasomatic transformation. The replacement of orthopyroxene by olivine leads to the necessary mobilization of Cr3+, not accepted by the neoformed olivine. A symplectitic intergrowth of enstatite + chromite at the interface between harzburgite and dunite may result from a silica-enriched fluid phase. Chromite in the orebodies contains micro-inclusions of olivine, clinopyroxene, orthopyroxene (rare), pargasite, aspidolite and nepheline. The olivine is hypermagnesian (up to 97 mol% Fo) and Ni-rich (0.25-1.05 wt%) and crystallized from a reducing fluid phase, not a magma. The pyroxenes and pargasite also are highly magnesian. The presence of Na in the environment accounts for pargasite, aspidolite and anomalously sodic nepheline. A Ni-Cu alloy is present. Chromite in the orebodies contains fluid inclusions, whereas chromite disseminated in harzburgite does not. The mixed fluid contains H2O (5 wt% equivalent of NaCl), with CO2 and CH4 in a 10 to 1 molar ratio. Values of δ13CVPDB (-22.1%o for CO2 and -28.1%o for CH4, respectively) are typical of shallow marine hydrothermal systems. Experiments reveal that the precipitation of chromite from a fluid phase requires a reducing fluid and a temperature of LESS-THAN OR EQUAL TO1050 °C. The appearance of olivine in the envelope results from the incongruent dissolution of enstatite in the fluid. The reaction is proven experimentally to be very efficient at a modest temperature (750 °C) and a low confining pressure (1.5 kbar). It leads to a shrinkage of the solid fraction, making the reaction self-propagating. Large volumes of harzburgite can be efficiently processed in an open, fluid-dominant reducing system. Relevant experiments show that the dissolution will increase in efficiency at higher temperatures and pressures. We do not negate the widely accepted explanation of chromitite formation by localized melt-harzburgite or melt-lherzolite interaction, but do point out that on the grounds of efficiency, such a process should be of secondary importance.

Název v anglickém jazyce

Fluids are bound to be involved in the formation of ophiolitic chromite deposits

Popis výsledku anglicky

Chromite orebodies within the harzburgitic upper mantle have been investigated in several ophiolitic complexes, with a focus here on New Caledonia. The relationship between harzburgite and a dunite envelope around pods of chromitite points to a passive metasomatic transformation. The replacement of orthopyroxene by olivine leads to the necessary mobilization of Cr3+, not accepted by the neoformed olivine. A symplectitic intergrowth of enstatite + chromite at the interface between harzburgite and dunite may result from a silica-enriched fluid phase. Chromite in the orebodies contains micro-inclusions of olivine, clinopyroxene, orthopyroxene (rare), pargasite, aspidolite and nepheline. The olivine is hypermagnesian (up to 97 mol% Fo) and Ni-rich (0.25-1.05 wt%) and crystallized from a reducing fluid phase, not a magma. The pyroxenes and pargasite also are highly magnesian. The presence of Na in the environment accounts for pargasite, aspidolite and anomalously sodic nepheline. A Ni-Cu alloy is present. Chromite in the orebodies contains fluid inclusions, whereas chromite disseminated in harzburgite does not. The mixed fluid contains H2O (5 wt% equivalent of NaCl), with CO2 and CH4 in a 10 to 1 molar ratio. Values of δ13CVPDB (-22.1%o for CO2 and -28.1%o for CH4, respectively) are typical of shallow marine hydrothermal systems. Experiments reveal that the precipitation of chromite from a fluid phase requires a reducing fluid and a temperature of LESS-THAN OR EQUAL TO1050 °C. The appearance of olivine in the envelope results from the incongruent dissolution of enstatite in the fluid. The reaction is proven experimentally to be very efficient at a modest temperature (750 °C) and a low confining pressure (1.5 kbar). It leads to a shrinkage of the solid fraction, making the reaction self-propagating. Large volumes of harzburgite can be efficiently processed in an open, fluid-dominant reducing system. Relevant experiments show that the dissolution will increase in efficiency at higher temperatures and pressures. We do not negate the widely accepted explanation of chromitite formation by localized melt-harzburgite or melt-lherzolite interaction, but do point out that on the grounds of efficiency, such a process should be of secondary importance.

Druh

J_imp - Článek v periodiku v databázi Web of Science

CEP obor

—

OECD FORD obor

10505 - Geology

Projekt

—

Návaznosti

I - Institucionalni podpora na dlouhodoby koncepcni rozvoj vyzkumne organizace

Rok uplatnění

2017

Kód důvěrnosti údajů

S - Úplné a pravdivé údaje o projektu nepodléhají ochraně podle zvláštních právních předpisů

Název periodika

European Journal of Mineralogy

ISSN

0935-1221

e-ISSN

—

Svazek periodika

29

Číslo periodika v rámci svazku

4

Stát vydavatele periodika

DE - Spolková republika Německo

Počet stran výsledku

13

Strana od-do

543-555

Kód UT WoS článku

000426885400002

EID výsledku v databázi Scopus

2-s2.0-85031301564