2.6 ROCK SAMPLING AND PETROLOGIC STUDIES - M. Perfit and J. Chadwick

Introduction:

The objectives of the rock sampling program were four-fold: 1. complete detailed sampling and mapping of the 1998 lava flow to aid in the identification of its boundaries and extent, 2. determine the compositional and petrologic heterogeneity of the 1998 flow and spatial variability, 3. compare the composition of the 1998 flow to that of the surrounding older flows to evaluate temporal changes in magma source, 4. recover samples from Axial's north and south rift zones by rock corer to investigate temporal and spatial variations in magma genesis on a regional scale.

The initial sampling and subsequent chemical analyses of basalts from the 1998 flow showed that it was very similar in composition to basalts previously recovered from the caldera. In particular, the 1998 flow has a composition like that of young looking flows from the CASM site. All are normal mid-ocean ridge basalts (N-MORB) that have slightly elevated K2O contents (and other incompatible trace elements) compared to other MORB from the Juan de Fuca Ridge. Of a total of 16 samples analyzed from the 1998 flow, fourteen have nearly identical compositions; two from the southernmost part of the flow (Recovered on dive R465) are slightly more evolved. This suggests some chemical differentiation may have occurred during the diking event or that the magma source is chemically heterogeneous.

The detailed sampling completed this year will allow us to place more rigorous constraints on the chemical composition (and variability) of the 1998 flow as well as to determine the composition of older flows in contact with the new flow. Assuming there are significant compositional differences between the 1998 flow and surrounding older flows, we plan to use the geochemical data in conjunction with the observational data and mapping efforts to generate a geological map of the 1998 flow. In addition, comparison of the major and trace element composition of the new flow to older flows can constraints on magma chamber temperature changes and magmatic evolution. Measurement of U-series isotopes (by K. Rubin) will provide information about the age the magma chamber for the different flow units in the Caldera and possibly allow us to relate young flows from CASM and ASHES to the 1998 flow.

From a regional standpoint, the 1998 flow is very important because it provides us with another 'zero age' flow from the JdF to use in comparing an contrasting the current sources and melting parameters along the ridge. Initial analyses of the 1998 flow confirm that the source of Axial magmas is slightly more enriched than the mantle source of southern JdF lavas and quite a bit more enriched compared the source for CoAxial Segment magmas. This years 55 rock cores now gives us more than 115 rock core localities along Axial's north and south rift zones. These samples, together with the samples recovered in the caldera, will be used to investigate the spatial and temporal variability of magma genesis at the entire volcanic edifice that has been created by the interaction of Axial melt anomaly and the Jdf ridge.

ROPOS Operations

During this cruise, we recovered 52 samples from ROPOS dives. Many of these samples were 'opportunistic' in that they were recovered during biological grabs or suction samples for fluids or biology. Most of these consisted of glass chips and were primarily from well documented vent sites within the 1998 flow. In order to facilitate recovery of very glassy samples which can be nearly impossible to grab with ROPOS manipulators, we developed small rock wax corers that were deployed on ROPOS for the first time. These 'chapstick' cores were quite effective in sampling the glassy surfaces of lavas and proved to be a quick, easy way to get additional rock samples. Deployed rock cores were placed into a small mesh bag ('purse') fashioned to attach to the front of the bioboxes or fluid sampler. This bag was also used to recover temperature probes and additional rock samples. The glassy crusts from all large 'whole rock' lava samples were sub-sampled to be hand-carried with core samples. The highlight of the rock sampling came during the biogeology dive R501 during which two E-W traverses were made across the 98 flow while detailed volcanological and geological observations were made. During the dive a record total of 16 rock samples were recovered using the manipulators, rock cores and suction sampler. Precisely located samples at lava flow contacts should aid us in distinguishing flow units.

Rock Coring Operations:

Rock coring during the NeMO 1999 cruise continues the effort begun during NeMO 1998, when 49 successful rock core attempts were made. An additional 61 core samples were collected this year, for a total of 109 for the two cruises. Samples were collected principally in the south rift zone of Axial Seamount (48 samples), from just off the flank of the seamount south to approximately 45 degrees 39 minutes latitude. The remainder of the samples were collected along the Vance segment of the Juan de Fuca Ridge (3 samples), the southwest rift zone (4 samples), and the north rift zone (6 samples). The glass from this effort will be analyzed for major elements using electron microprobe and for trace elements using laser ablation inductively coupled plasma mass spectrometry (ICPMS) and X-ray fluorescence (XRF) techniques. It is hoped this study will lead to a better understanding of the relationships between the seamount, rift zones, and Juan de Fuca ridge, as well as insight into the magmatic, volcanic, and tectonic processes that have created the rift zones.

Rock coring activities took place in periods between ROPOS dives, and were undertaken on the CTD wire on the starboard side of the ship. The corer has 7 wax-tipped cups which collect small shards of basaltic glass from the ocean floor. The corer was sent down on the wire at approximately 60 meters/min. until the winch monitor tension dropped considerably, indicating contact with the floor. An additional 5-10 meters of wire was spooled out prior to reversal of the winch and retrieval of the corer, again at 60 meters/min, after a short period of slow (10 meters/min) retrieval to get the corer off the bottom. This method resulted in a successful retrieval of ocean floor materials in all attempts, although no glass was collected in 2 attempts (sediments only).