Nickel, Cobalt and Chromium in early lunar magma ocean olivine: Constraints on the petrogenesis of the Mg-suite

Details Nickel, Cobalt and Chromium in early lunar magma ocean olivine: Constraints on the petrogenesis of the Mg-suite Nickel, Cobalt and Chromium in early lunar magma ocean olivine: Constraints on the petrogenesis of the Mg-suite

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51c1439e&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P51C-1439

Physics

[1026] Geochemistry / Composition Of The Moon, [3630] Mineralogy And Petrology / Experimental Mineralogy And Petrology, [3640] Mineralogy And Petrology / Igneous Petrology, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

The parental magmas of the lunar highlands Mg-suite cumulates had enigmatic chemical signatures. They co-crystallized Mg-rich olivine and Ca-rich plagioclase and had extremely evolved trace element signatures (KREEP). Most models of Mg-suite petrogenesis call upon early, Mg-rich, olivine dominated LMO cumulates as a source to explain the high Mg* of the parental magmas. The olivine in the Mg-suite, however, contains less Ni, Co and Cr than the more Fe-rich olivine in mare basalts. LMO crystallization models indicate that olivine in early LMO cumulates should contain more abundant Ni, Co and Cr than the later formed olivine in the mare source regions. Therefore, if early LMO cumulates are the source of the Mg-suite magmas, a mechanism which depletes these cumulates in Ni, Co and Cr must be invoked. We have conducted crystallization experiments on two proposed bulk Moon compositions, in part, to investigate Ni, Co and Cr in early LMO cumulates. SIMS derived D values show that at high Fo contents and early in the crystallization sequence, both Ni and Co are slightly incompatible in olivine (D’s of ~0.5 - 0.7). The initially low D values then increase as crystallization proceeds. Near 40-50% solid in the LMO, DNi has increased to ~2 and DCo to ~1.5. These results agree with results from Longhi et al. (2010) and reveal a mechanism which could deplete early LMO olivine in Ni and Co. A depletion in Cr, however, cannot be explained by this mechanism. The DCr in olivine has been established to be about 0.6. The bulk Moon compositions investigated produce a range of 2000 - 4000 ppm Cr2O3 in early LMO olivine. Most olivine from the Mg-suite cumulates, however, contains closer to 500 ppm. Chromium will preferentially partition into a partial melt of a dunite LMO cumulate, so our results indicate that such melts will have greater than 2000 - 4000 ppm Cr2O3 and cannot have crystallized the Cr2O3 - depleted Mg-suite. We evaluate three possible mechanisms to explain the Cr depletion. 1) Spinel is a residual phase in these cumulates. However, spinel is nowhere near the liquidus of bulk Moon compositions and our results show that only trivial amounts (~0.5%) or none will be included in early cumulates. Significant quantities of spinel in early LMO cumulates seems very unlikely. 2) Core formation in the presence of S and/or C occurred under very reducing conditions. Chabot and Agee (2003) showed that at low oxygen fugacity and in the presence of S and/or C, Cr becomes siderophile. If such conditions existed contemporaneously with the LMO, this may deplete the LMO in Cr. 3) The bulk Cr content of the silicate Moon has been overestimated. Previous authors have estimated the bulk Cr content of the Moon using the mare source regions as a proxy for the whole lunar mantle. However since Cr is slightly incompatible in olivine, the residual later-stage LMO, from which the mare source regions are thought to have crystallized, will become enriched in Cr. The phases in the mare source regions may have crystallized from a magma enriched in Cr relative to the bulk Moon, meaning the Cr contents of these mantle sources are not representative of the bulk LMO.

Affiliated with

Also associated with

No associations

Rating

Nickel, Cobalt and Chromium in early lunar magma ocean olivine: Constraints on the petrogenesis of the Mg-suite does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with Nickel, Cobalt and Chromium in early lunar magma ocean olivine: Constraints on the petrogenesis of the Mg-suite, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nickel, Cobalt and Chromium in early lunar magma ocean olivine: Constraints on the petrogenesis of the Mg-suite will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-1497737