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The mantle sources of mid-ocean ridge basalts beneath the Indian and Pacific oceans have distinct isotopic compositions with a long-accepted boundary at the Australian–Antarctic Discordance along the Southeast Indian Ridge. This boundary has been widely used to place constraints on large-scale patterns of mantle flow and composition in the Earth’s upper mantle. Sampling between the Indian and Pacific ridges, however, has been lacking, especially along the remote 2,000 km expanse of the Australian–Antarctic Ridge. Here we present Sr, Nd, Hf and Pb isotope data from this region that show the Australian–Antarctic Ridge has isotopic compositions distinct from both the Pacific and Indian mantle domains. These data define a separate Zealandia–Antarctic domain that appears to have formed in response to the deep mantle upwelling and ensuing volcanism that led to the break-up of Gondwana 90 million years ago, and currently persists at the margins of the Antarctic continent. The relatively shallow depths of the Australian–Antarctic Ridge may be the result of this deep mantle upwelling. Large offset transforms to the east may be the boundary with the Pacific domain.

The processes that lead to the fourfold variation in arc-averaged compositions of mafic arc lavas remain controversial. Control by the mantle-wedge thermal structure is supported by chemical correlations with the thickness of the underlying arc crust, which affects the thermal state of the wedge. Control by down-going slab temperature is supported by correlations with the slab thermal parameter. The Chilean Southern Volcanic Zone provides a test of these hypotheses. Here we use chemical data to demonstrate that the Southern Volcanic Zone and global arc averages define the same chemical trends, both among elements and between elements and crustal thickness. But in contrast to the global arc system, the Southern Volcanic Zone is built on crust of variable thickness with a constant slab thermal parameter. This natural experiment, along with a set of numerical simulations, shows that global arc compositional variability is dominated by different extents of melting that are controlled by the thermal structure of the mantle wedge. Slab temperatures play a subordinate role. Variations in the subducting slab’s fluid flux and sediment compositions, as well as mantle-wedge heterogeneities, produce second-order effects that are manifested as distinctive trace element and isotopic signatures; these can be more clearly elucidated once the importance of wedge thermal structure is recognized.

Sea level changes associated with Pleistocene glacial cycles have been hypothesized to modulate melt production and hydrothermal activity at ocean ridges, yet little is known about fluctuations in hydrothermal circulation on time scales longer than a few millennia. We present a high‐resolution record of hydrothermal activity over the past 50 ka using elemental flux data from a new sediment core from the Mir zone of the TAG hydrothermal field at 26°N on the Mid‐Atlantic Ridge. Mir sediments reveal sixfold to eightfold increases in hydrothermal iron and copper deposition during the Last Glacial Maximum, followed by a rapid decline during the sea level rise associated with deglaciation. Our results, along with previous observations from Pacific and Atlantic spreading centers, indicate that rapid sea level changes influence hydrothermal output on mid‐ocean ridges. Thus, climate variability may discretize volcanic processing of the solid Earth on millennial time scales and subsequently stimulate variability in biogeochemical interactions with volcanic systems. New TAG sediment core used to examine hydrothermal response to sea level changeHydrothermal Fe and Cu deposition peak during Last Glacial Maximum and decline during deglaciationGlobal hydrothermal Fe flux and subsequent carbon export may vary across glacial cycles

A fundamental question in the formation of orogenic andesites is whether their high melt SiO2reflects the recycling of silicic melts from the subducted slab or the processing of basaltic mantle melts in the overlying crust. The latter model is widely favoured, because most arc magmas lack the ‘garnet’ signature of partial slab melts. Here we present new trace element data from Holocene high-Mg# >64–72 calc-alkaline basalts to andesites (50–62 wt% SiO2) from the central Mexican Volcanic Belt that crystallize high-Ni olivines with the high 3He/4He=7–8 of the upper mantle. These magmas have been proposed to be partial melts from ‘reaction pyroxenites’, which formed by hybridization of mantle peridotite (c.82–85%) and heavy rare earth element-depleted silicic slab melt (>15–18%). Forward and inverse models suggest that the absence of a garnet signature in these melts reflects the efficient buffering of the heavy rare earth elements (Ho to Lu) in the subarc mantle. In contrast, all elements more incompatible than Ho – excepting TiO2– are more or less strongly controlled by the silicic slab flux that also directly contributes to the silicic arc magma formation. Our study emphasizes the strong link between slab recycling and the genesis of orogenic andesites.Supplementary material:Methods, additional data and modelling parameters are available at http://www.geolsoc.org.uk/SUP18686

Lavas from the fast-spreading East Pacific Rise are geochemically diverse even within a single tectonically defined spreading cell. Within such spreading cells, small offsets of the rise axis are often boundaries between petrologically distinct magmatic units which must be supplied independently from beneath the ocean crust. Volcanics erupted near the small offsets can have chemical characteristics similar to those previously found near transform faults.

Major element, Ni, Mn and Ca electron microprobe analyses of olivine phenocrysts in one of the most primitive basalts from the FAMOUS area of the Mid-Atlantic Ridge, 527-1-1, reveal two different olivine populations, distinguished by their zoning characteristics. The often skeletal Group I olivines have zoning profiles with high forsterite, high Ni, low Mn, and low Ca cores. These profiles can be explained by low pressure crystallization from the 527-1-1 magma on cooling. The equant, often megacrystic Group II olivines also have high forsterite, low Mn cores, but the cores have low Ni compared to rims. Thus they are normally zoned with respect to Mg, Fe, and Mn but reversely zoned with respect to Ni. For example, Ni ranges from 1,700 ppm at Fo90.5 in the core to 2,100 ppm at Fo89.5 at the rim. In view of the published whole rock and mineral data of le Roex et al. (1981), the most likely explanation for these data is that the Group II olivines are xenocrysts assimilated from solidified “plagioclase-pyroxene” basalts through which the 527-1-1 basalt ascended. The diffusion rate of Ni and size of the xenocrystic olivines are used to calculate the residence time of the assimilated olivines in the magma. An alternative hypothesis would be a high pressure origin for the Group II olivines. This would be possible if future experiments show that with increasing pressure the partition coefficient for Ni decreases relative to the partition coefficient for Mg for a given bulk composition. Current evidence suggests this is unlikely. The data from 527-1-1 and other samples from the FAMOUS area require magmas with distinct Ni-MgO-FeO characteristics. In general, MORB from different ridge segments fall on distinct trends on plots of MgOvs. Ni as well as MgOvs. FeO. Calculation of the MgO and Ni contents of primary magmas suggests a mechanism by which such distinct trends could come about.

Tholeiitic basalts collected over an area of less than 100 km2near the Oceanographer transform fault have isotopic compositions interpreted as mixing between three chemically distinct mantle sources. Of the two enriched sources, one has an isotopic composition not previously observed in basalts from the North Atlantic. Its characteristics suggest a model whereby portions of the mantle currently beneath 35° N on the Mid-Atlantic Ridge contain detached fragments of old sub-continental mantle.

Ocean island basalts (OIBs) possess uniformly low B contents, and lower B/Nb and B/K2O ratios than mid-ocean ridge basalts (MORBs). As with Pb, B enrichments in both MORBs and OIBs are substantially lower than those of arc volcanics or continental rocks. The devolatilization of subducting plates and associated arc magmatism efficiently segregate B into crustal reservoirs and return large volumes of B-depleted material to the deep mantle. Subduction processes (and presumably arc volcanism) have thus played a major role in continental crust formation.

Presents a letter to the editor commenting on Lafon Allen's article on the great primary humbug in the May 20, 1916 issue. Letter writer's observations on direct primaries; Questions on proportional representation, candidates' expenses, ballots and national parties; Complexities of the voting mechanisms of a democracy.