The petrological parameters Na
8 and Fe
8, which are Na
2O andFeO contents in mid-ocean ridge basalt (MORB) melts correctedfor fractionation effects to MgO = 8 wt%, have been widely usedas indicators of the extent and pressure of mantle melting beneathocean ridges. We find that these parameters are unreliable.Fe
8 is used to compute the mantle solidus depth (P
o) and temperature(T
o), and it is the values and range of Fe
8 that have led tothe notion that mantle potential temperature variation of T
P= 250 K is required to explain the global ocean ridge systematics.This interpreted T
P = 250 K range applies to ocean ridges awayfrom hotspots. We find no convincing evidencethat calculated values for P
o, T
o, and T
P using Fe
8 have anysignificance. We correct for fractionation effect to Mg
# = 0·72,which reveals mostly signals of mantle processes because meltswith Mg
# = 0·72 are in equilibrium with mantle olivineof Fo
89·6 (vs evolved olivine of Fo
88·1–79·6in equilibrium with melts of Fe
8). To reveal first-order MORBchemical systematics as a function of ridge axial depth, weaverage out possible effects of spreading rate variation, local-scalemantle source heterogeneity, melting region geometry variation,and dynamic topography on regional and segment scales by usingactual sample depths, regardless of geographical location, withineach of 22 ridge depth intervals of 250 m on a global scale.These depth-interval averages give Fe
72 = 7·5–8·5,which would give T
P = 41 K (vs 250 K based on Fe
8) beneathglobal ocean ridges. The lack of Fe
72–Si
72 and Si
72–ridgedepth correlations provides no evidence that MORB melts preservepressure signatures as a function of ridge axial depth. We thusfind no convincing evidence for T
P > 50 K beneath globalocean ridges. The averages have also revealed significantcorrelations of MORB chemistry (e.g. Ti
72, Al
72, Fe
72,Mg
72, Ca
72, Na
72 and Ca
72/Al
72) with ridge axial depth. Thechemistry–depth correlation points to an intrinsic linkbetween the two. That is, the 5 km global ridge axial reliefand MORB chemistry both result from a common cause: subsolidusmantle compositional variation (vs T
P), which determines themineralogy, lithology and density variations that (1) isostaticallycompensate the 5 km ocean ridge relief and (2) determine thefirst-order MORB compositional variation on a global scale.A progressively more enriched (or less depleted) fertileperidotite source (i.e. high Al
2O
3 and Na
2O, and low CaO/Al
2O
3)beneath deep ridges ensures a greater amount of modal garnet(high Al
2O
3) and higher jadeite/diopside ratios in clinopyroxene(high Na
2O and Al
2O
3, and lower CaO), making a denser mantle,and thus deeper ridges. The dense fertile mantle beneath deepridges retards the rate and restricts the amplitude of the upwelling,reduces the rate and extent of decompression melting, givesway to conductive cooling to a deep level, forces melting tostop at such a deep level, leads to a short melting column,and thus produces less melt and probably a thin magmatic crustrelative to the less dense (more refractory) fertile mantlebeneath shallow ridges. Compositions of primitive MORB meltsresult from the combination of two different, but geneticallyrelated processes: (1) mantle source inheritance and (2) meltingprocess enhancement. The subsolidus mantle compositional variationneeded to explain MORB chemistry and ridge axial depth variationrequires a deep isostatic compensation depth, probably in thetransition zone. Therefore, although ocean ridges are of shalloworigin, their working is largely controlled by deep processesas well as the effect of plate spreading rate variation at shallowlevels. KEY WORDS:
mid-ocean ridges; mantle melting; magma differentiation; petrogenesis; MORB chemistry variation; ridge depth variation; global correlations; mantle compositional variation; mantle source density variation; mantle potential temperature variation; isostatic compensation 相似文献