Talk:Ultramafic rock

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Etymology

How about a sentence explaining the origin of the word Ultramafic?EtherDoc (talk) 13:25, 22 February 2012 (UTC)[reply]

Ultramafic vs. ultrabasic

Ultramafic and ultrabasic are actually different terms. Ultramafic is an igneous rock which is composed of greater than 90% mafic minerals, but ultrabasic is an igneous rock which has silica content lower than 45% (weight%). First is mineralogic and second is chemical classification and they should be different articles. Rock can be ultrabasic but it doesn't mean that it should be automatically ultramafic. Siim 20:09, 5 September 2005 (UTC)[reply]

That's fairly pedantic at best. Its like saying a highly mafic-mineral rich granitoid isn't felsic because it has >10% mafic minerals. Anorthosite; since it's got >45% silica due to being >90% plagioclase, and isn't ultramafic because it's >90% 'felsic' minerals...? But it's formed by accumulation from ultramafic magmas? So I disagree with you. Ultramafic rocks are ultrabasic rocks, and arguing specifics ignores the fact that they are all hot, fairly primitive (in a fractional crystallisation sense) rocks.Rolinator 09:48, 24 December 2005 (UTC)[reply]

Ultramafic landscape in Google Street View

Here is a link to a GSV looking west from Highway 199 about 6 miles south of Cave Junction, Oregon. Notice the sparse vegetation and the exposed reddish, stony soil (glutted with magnesium but low in other nutrients). On satellite imagery the ultramafic terrain west of the highway appears as gray or reddish; the surrounding non-ultramafic areas are mostly green with dense forest. Tony (talk) 07:46, 11 February 2009 (UTC) http://maps.google.com/?ie=UTF8&t=h&layer=c&cbll=42.095413,-123.682488&panoid=gzzOQ8hQbswG67omfjWMJg&cbp=11,296.15780165686965,,0,5&ll=42.095413,-123.682488&spn=0.114131,0.53627&z=11[reply]

Carbon dioxide storage?

I just read that ultramafic rock reacts with carbon dioxide to form a stable solid, and that it thus is being targeted for carbon sequestration. Does anyone have more information on this, and could add it to this article? -- Dan Griscom (talk) 20:21, 17 March 2009 (UTC)[reply]

Dubious

I'm skeptical about this sentence:

Volcanic ultramafic rocks are rare outside of the Archaean and are essentially restricted to the Neoproterozoic or earlier, although some boninite lavas currently erupted within back-arc basins (Manus Trough, New Guinea) verge on being ultramafic.

The paragraph overall is also unreferenced. Volcanic ultramafic rocks are rare today, but they do exist in a few modern eruptions, like the picrite basalt of Piton de la Fournaise, and the runny nephelinite of Mount Nyiragongo. Also, isn't boninite's silica content necessarily too high to be ultramafic? - Gilgamesh (talk) 07:58, 28 July 2022 (UTC)[reply]

Yes I agree. As far as I am aware of, boninite is an intermediate rock. Volcanoguy 10:51, 28 July 2022 (UTC)[reply]

Boninite is high-magnesium andesite, thought to possibly be a primitive andesite. In no sense is it ultramafic. I'd recommend stating that ultramafic volcanic rock is rare today but cite the two examples you give, for which cites should not be hard to find, and remove the New Guinea reference, for which I can find nothing supporting on Google Scholar. --Kent G. Budge (talk) 15:50, 28 July 2022 (UTC)[reply]

I think that hign-magnesium andesite is a good description for most, but unfortunately not all, boninites. Boninite must have at leat 8% MgO. The SiO₂ compositional range for boninite is unusual. Older classifications of boninite e.g. IUGS (2000) put boninite into a SiO₂ range of 52–70% (i.e. mostly intermediate but extending into felsic composition) for every possible MgO content. More recent classifications (e.g. Pearce and Reagan, 2019) have recognised that the MgO range for boninite is large (8-30%) and the amount of SiO₂ varies with the MgO content. For simplicity, these newer classifications have developed a special "Si8" definition of boninite (i.e. the SiO₂ content at 8% MgO), in the range of 52-63% (i.e. intermediate). Although some boninites have been reported as chemically resembling some komatiites (an ultramafic rock), I think those particular komatiites are actually mafic komatiitic basalts rather than true ultramafic komatiites. It seems theoretically possible that an igneous rock could have MgO >30% and ultramafic SiO₂ values, but this as-yet unfound composition is also unnamed; and I do not recall any modern classifications that would regard boninite as an ultramafic rock. —GeoWriter (talk) 21:35, 28 July 2022 (UTC)[reply]