Drilling Program: SHIPBOARD SCIENTIFIC PARTY

The bend in the Hawaiian-Emperor chain is the best example of a change in plate motion recorded in a fixed-hotspot frame of reference. Alternatively, the bend might primarily record differences in motion of the Hawaiian hotspot relative to the Pacific lithosphere. Four lines of inquiry support the latter view: (1) global plate motions predicted from relative plate motion data, (2) spreading rate data from the North Pacific Basin, (3) mantle flow modeling utilizing geoid and seismic tomography constraints, and (4) paleomagnetic data from the Emperor chain. Although the rate of motion has been difficult to constrain because previous drilling was limited, the best available paleomagnetic data suggest Pacific hotspots may have moved rapidly, at rates comparable to those of lithospheric plates in Late Cretaceous to early Tertiary times (81-43 Ma). Basements sites were drilled in the Emperor Seamount trend during Leg 197 to test the hypothesis of southward motion of the Hawaiian hotspot. The principal drilling objective was to achieve moderate basement penetration at these sites to obtain cores from lava flows suitable for paleomagnetic paleolatitude and radiometric age determinations. Because of the record-setting basement penetration (1220 m) during Leg 197 at Detroit (Sites 1203 and 1204; ~71–76 Ma), Nintoku (Site 1205; ~56 Ma) and Koko (Site 1206; ~48 Ma) Seamounts, we were able to meet our objectives. Paleolatitudes for these sites suggested by our preliminary shipboard paleomagnetic analysis clearly differ from the latitude of Hawaii. The values are consistent with and confirm prior results from Suiko (Deep Sea Drilling Project Site 433) and Detroit (Ocean Drilling Program Site 884) Seamounts. Our shipboard analysis of paleolatitude vs. age for the Emperor Seamounts must be supported by shorebased paleomagnetic studies and radiometric age determinations. However, the available data suggest that the Emperor Seamounts record the rapid southward motion of the Hawaiian hotspot in the mantle, requiring a major change in how we view this classic age-progressive volcanic lineament as a record of mantle convection and plate motions. Another important science objective of the leg was to determine the geochemical variation of the volcanic products of the Hawaiian hotspot through time. Observations of lava flow thickness, vesicularity, crystallinity, and morphology, together with analysis of volcaniclastic sediments, has provided a picture of eruptions in subaerial to shallow-water conditions at Detroit and Koko Seamount Sites 1203, 1204, and 1206 and waning subaerial activity at Nintoku Seamount (Site 1205). Shipboard geochemical measurements suggest we have captured the transition from Hawaiian tholeiitic shield stage to alkalic postshield stage at each of the volcanic complexes. Between Sites 1203 and 1204 and previously studied Sites 883 and 884, we have a range of compositions at Detroit Seamount that covers most of the variability seen in volcanoes of the island of Hawaii. The variability of trace element ratios (e.g., Ti/Zr) provide hints that we have sampled different source compositions. It will remain for shorebased studies to evaluate and define these suspected source heterogeneities through the examination of additional trace elements and isotopic compositions.