GEOCHEMICAL MORPHOLOGY OF THE NORTH MID-ATLANTIC RIDGE, 10-DEGREES-24-DEGREES-N - TRACE ELEMENT-ISOTOPE COMPLEMENTARITY

The new data presented here from a 10-24 degrees N segment of the Nort h Mid-Atlantic Ridge show that this segment is the most depleted of th e 10-70 degrees N ridge section. They also show the existence of: (1) a geochemical gradient from the 14 degrees N anomaly to 17 degrees 10' N; (2) a very depleted mantle source (the lowest Sr isotopic ratios fo und so far in the North Atlantic); and (3) a geochemical limit located at about 17 degrees 10'N without any obvious relation with any struct ural feature. The 15 degrees 20'N fracture zone does not show any rela tionship with respect to this gradient. The basalts located north of 1 7 degrees 10'N have very homogeneous features, which allow their chara cteristics to be averaged (i.e., Sr-87/Sr-86 = 0.70238 +/- 0.00004, (N b/ Zr), = 0.28 +/- 0.1) and they are defined as normal mid-ocean ridge basalts. The basaltic glasses located south of 17 degrees 10'N presen t a wide spectrum of isotopic compositions and extended rare earth ele ment patterns (from depleted to enriched). Despite this, they have a c onstant K/Nb of 233 +/- 9 (1s(m), n = 18) whereas this ratio is 344 +/ - 29 north of 17 degrees 10'N. These observations illustrate the stron g coherence of behaviour between K and Nb (Ta) during the petrogenic p rocesses involved in the generation of these mid-ocean ridge basalts a nd also their fractionation during previous mantle processes. Possible interpretations of mixing processes are discussed and sources at the ridge segment scale are favoured. However, when looking in detail, loc al heterogeneities are still common and can even be traced back off-ax is to 115 my. Placed in the context of the North Atlantic Ridge from 1 0 degrees to 70 degrees N, the Sr isotopic ratios reveal the Azores su perstructure (23-50 degrees N), whereas the trace element ratios (La/S m-Nb/Zr) trace the second-order structures (33-40 degrees N, 42-48 deg rees N) superimposed on the superstructure. This study illustrates the complementarity of information given by certain well chosen trace ele ment ratios on the one hand and by isotopic ratios on the other. Since there is evidence of decoupling between isotopic ratios and/or trace element ratios, it introduces the notion of complementary ''chemical m emory'' as recorded by a given type of trace element ratio or a given type of isotopic ratio