Figure F12. Histogram showing distribution of whole-rock Mg#s. Data also show the Mg# extent of Atlantis II basalts (Johnson and Dick, 1992), range of Fo in olivine, and Mg# clinopyroxene (cpx) in gabbros from Hole 1105A. Note primitive gabbros must be cumulate in origin, rather than solidified melts, because the Fo olivine compositions and whole-rock Mg#s of gabbros are in excess of those that would be represented by melt composition solidification. Mid-ocean-ridge basalt (MORB) from the Atlantis II Transform generally lies within the range of Mg#s representing the window of eruptibility of Stolper and Walker (1980), unlike the majority of cumulate gabbroic rocks.

Figure F12. Histogram showing distribution of whole-rock Mg#s. Data also show the Mg# extent of Atlantis II basalts (Johnson and Dick, 1992), range of Fo in olivine, and Mg# clinopyroxene (cpx) in gabbros from Hole 1105A. Note primitive gabbros must be cumulate in origin, rather than solidified melts, because the Fo olivine compositions and whole-rock Mg#s of gabbros are in excess of those that would be represented by melt composition solidification. Mid-ocean-ridge basalt (MORB) from the Atlantis II Transform generally lies within the range of Mg#s representing the window of eruptibility of Stolper and Walker (1980), unlike the majority of cumulate gabbroic rocks.