Introduction

Exploring and sampling the world's mid-ocean ridges

Since the mid-20th century, our knowledge of undersea volcanic activity has expanded considerably as portions of the Earth's seafloor have been systematically explored. Dedicated scientific research vessels have obtained thousands of samples of oceanic crust by dredging, drilling, and small submersibles. These samples have been cataloged, analyzed, stored in large archives, and the resulting data has been published widely in the geological literature, contributing to our knowledge of the composition and origin of the oceanic crust. A large amount of these data are now available in digital format from the Petrological Database of the Ocean Floor (PETDB), a web-based data management system for geochemical and petrological sample-based data funded by the National Science Foundation.

Mid-ocean ridge basalts: the Earth's most voluminous volcanic rocks

On a volumetric basis, basaltic lavas erupted under the ocean at mid-ocean ridges are the most important products of terrestrial volcanism. What we have learned from mid-ocean basalts (MORB's) has had tremendous implications for our understanding of large-scale geodynamic and tectonic processes operating within the Earth's core, mantle, and crust.

When studying a group of related igneous rocks, an important issue to ascertain is the amount of compositional variation that characterizes the dataset. This is of particular importance for the study of MORB's. Are MORB's relatively uniform in composition, perhaps suggesting partial melting from uniform source materials in the mantle followed by uniform modification by magmatic processes? Alternatively, does the global MORB dataset exhibit significant compositional variation, perhaps suggesting important differences in source materials, melting conditions, or subsequent magmatic processes? A related issue is whether geochemical variation in the MORB dataset is a function of geographic location (e.g., position along the mid-ocean ridge system). This may have important implications for lateral variations in composition, temperature, or pressure within the Earth's mantle. Keep these larger issues in mind as you work through the following exercise.