Constraining the long-term variability and average of the Earth’s magnetic field strength is fundamental to understanding the characteristics and behavior of the geomagnetic field. Questions remain about the strength of the average field, and the rela-tionship between strength and reversal frequency. The dispersion of data from key timeintervals reflects the complexity in obtaining absolute paleointensity values. Here, we focus on the Cretaceous Normal Superchron (CNS; 121-84 Ma), during which there were no reversals. We present new results from 42 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and Murcielago Islands, Costa Rica and new and revised 40Ar/39Ar ages along with biostratigraphic age constraints from previous studies that indicate ages from 141 to 112 Ma. One site with a 40Ar/39Ar age of 135+\-1.5Ma (2σ) gave a reliable intensity result of 34+\-μT (equivalent to a paleomagnetic dipole moment, PDM, value of 88+\-20 ZAm2), while three sites between 121 and 112 vary from 21+\- to 34+\-4 μT (53+\-3 to 87+\-10 ZAm2) spanning the onset of the CNS. These results from the CNS are all higher than the long-term average of ~42 ZAm2 and similar to data from Suhongtu (46-53 ZAm2) and the Troodos Ophiolite (81 ZAm2, reinterpreted, using the same criteria of this study). Together with the reinterpreted data, the new Costa Rica results suggest thatthe strength of the geomagnetic field was about the same before and after the onset ofthe CNS. Therefore, the data do not support a strict correlation between polarity interval length and the strength of the magnetic field.

The Matthew and Hunter (M&H) area, in the South-West Pacific, was formerly interpreted as a transform boundary at the southern termination of the New Hebrides Subduction Zone. But new data collected during three voyages of RV Southern Surveyor (2004-2009), combined with detailed analysis of seismicity and GPS kinematics, shows it is a distinct subduction zone initiated only 2 Ma ago. In fact, M&H is the youngest known volcanically-active intra-oceanic subduction system. We demonstrate that the M&H subduction zone is a modern example of an immature subduction system at the particular stage of pre-arc, near-trench magmatism. It is not yet forming an arc but the proto-forearc. Indeed volcanism occurs much closer to the trench than volcanism at mature subduction zones. Also M&H hosts an exceptionally diverse range of magma compositions, which erupted contemporaneously and are spatially juxtaposed. Pb isotopic compositions and contents of LILE and REE indicate melting of upwelling asthenospheric mantle (Indian MORB) and subducted oceanic crust (Pacific MORB of the South Fiji Basin) and the mixing of these two components. It is worth noting that the present day proto-forearc of the M&H subduction zone corresponds to an area where highly contrasting terranes are juxtaposed: remnants of the old Vitiaz Arc crust, domains of classical backarc basin type oceanic accretion, and what we call Subduction Initiation Terranes (SITER). Such live observations of a growing forearc are rare. They should give insights into the study of fossil forearcs such as SSZ ophiolites but also the IBM forearc.

Researcher Ridge (RR) is a 400km long, WNW-ESE oriented chain of volcanic seamounts, located on ~20 to 40 Ma old oceanic crust on the western flank of the Mid-Atlantic Ridge (MAR) at ~15°N. RR remained nearly unstudied, and thus its age and origin are currently unclear. At roughly the same latitude, the MAR axis is bathymetrically elevated and produces geochemically enriched lavas (the well-known 14°N MAR anomaly). This study presents 40Ar/39Ar age data, major and trace elements, and Sr-Nd-Pb-Hf isotopic compositions of volcanic rocks dredged from several seamounts of the RR and along the MAR between 13-14°N. The results reveal that RR lavas have geochemically enriched ocean island basalt (OIB) compositions ([La/Sm]N=1.7-5.0, [Ce/Yb]N=1.58-11.3) with isotopic signatures (143Nd/144Nd = 0.51294-0.51316, 206Pb/204Pb = 19.14-19.93, 176Hf/177Hf = 0.28307-0.28312) trending to or overlapping the ubiquitous FOZO (Focal Zone, e.g., Hart et al., 1992, Science 256) mantle composition. Major and trace element characteristics denote that RR lavas formed by small degrees of melting from a deep source in the garnet stability field and experienced high pressure fractionation beneath a lithospheric lid. Although the sparseness of samples suitable for 40Ar/39Ar dating prevents establishing a clear age progression for the seamount chain, one well constrained basalt groundmass age of 28.75 ± 0.14 Ma (2σ) for one seamount near the western end of RR indicates that this volcano formed ~11 Ma later than the underlying lithosphere. Taken together, RR is interpreted as a hotspot track, albeit formed by a relatively weak melting anomaly. Compared to RR, the lavas from the 14° N MAR anomaly have slightly less enriched compositions, exhibiting enriched (E)-MORB compositions ([La/Sm]N=1.81-2.29). Their isotopic ratios largely overlap with the RR compositions, thus suggesting a genetic relationship. We therefore propose that the enigmatic 14°N MAR anomaly is caused by deflection of upwelling RR plume material towards the approaching (westward migrating) MAR, causing the production of E-MORBs with nearly similar isotopic compositions to the RR lavas. Once the plume was captured by the spreading ridge, off-axis hotspot track volcanism ceased, resulting in a 300 km wide gap of seamount formation between the eastern end of RR and the MAR.