Abstract

Archean variolitic lavas contain matrix fractions possessing spectacularly high iron contents (up to 24 wt.% total FeO). The amount of iron contained in a single variolite horizon is greater than 1 × 1012 kg, a source of magmatic iron large enough to account for the iron required to form banded iron formation (BIF) early in the earth's history. The time distribution of BIF and variolites may be related, via the liquidus-depressant role of water in immiscible processes, to the increased likelihood of encountering 'wet', immiscibility-prone, partial-melt mantle derivates. Alternatively BIF may simply be the product of chemical degradation of these variolites.

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