Roger Nielsen¹, Adam Kent¹, Frank Tepley², Andrew Burleigh¹, Alison Weinsteiger¹, Amy Lange¹ , David Adams³

¹Dept. Geosciences,104 Wilkinson Hall, Oregon State University, Corvallis, OR, 97331 United States (nielsenr@geo.oregonstate.edu)
²College of Oceanic and Atmospheric Sciences; 104 Ocean Admin., Oregon State University, Corvallis, OR, 97331 United States
³United States Geological Survey, Denver, CO

Plagioclase ultraphyric basalts (PUB) represent a class of MORB characterized by abundant (>20%), large (>5mm) anorthitic (>An₈₅) phenocrysts. These lavas are commonly associated with suites of primitive high Al MORB (Eason and Sinton, 2006) that occur primarily at slow to intermediate spreading ridges and close to fracture zones. Our focus is an examination of the range of textures and major and trace element contents exhibited within and between individual phenocrysts of anorthitic feldspars within individual PUB.  Although PUB have been described for some time they have not been examined in detail using modern petrological and geochemical tools, and we view them as a potentially rich source of information tha can be used to constrain the nature of ocean crustal magma systems. Our goal is to thus use studies of PUB to probe the dynamics of magma transport and evolution within the oceanic crust, and to document the extent to which the phenocrysts represent a coherent suite of genetically related material, and, the degree to which melt inclusion compositions are the product of entrapment or post-entrapment processes.

To date, we have collected major and trace element data on feldspars and their associated melt inclusions from several samples from the Juan de Fuca, Gorda, Southeast Indian, and Southwest Indian Ridges (13 individual samples). We see a number of interesting features. The observed range of trace element contents within individual crystals appear to define two distinct trends: 1) patterns are broadly coherent with the individual phenocrysts from a given sample making up components of a composite trend (e.g. JdF E-32 and SEIR D-48) or 2) discordant – with data from individual phenocrysts either trending across a whole sample trend or exhibiting little correlation (e.g. Gorda D-9).

We interpret coherent trends such as D-48 as representative of a collection of phenocrysts from a single genetically related suite of magmas, but where individual crystals have experienced distinct histories within that related magmatic suite. We currently interpret the more incoherent trends to represent collections of phenocrysts from unrelated magmas or the product of diffusive re-equilibration, which would result in re-homogenization of more rapidly diffusing elements (e.g. Sr). To use nomenclature common in more felsic magmas we believe that different systems are characterized by more or less of a contribution from antecrystic and potentially xenocrystic sources. The degree to which the population represents a set of genetically related materials says much about the storage and transport processes. We are currently collecting more data in an effort to see how widespread each of these characteristics is. Further work will also include multicomponent diffusion modeling, Sr isotope analyses via microdrilling and quantitative textural analyses. 

To date, with our restricted data set, we see no correlation between trace element characteristics and magma type (e.g. E-MORB vs. NMORB), ridge location, or spreading rate.  The variety of patterns suggests that there is an underlying magma transport and storage phenomena that determines the compositional characteristic of specific magmas, and supports our contention that PUB prove a rich source of information about oceanic magma systems.

Sources: Eason, D. and Sinton,  J., 2006, EPSL, 252, 423–436.