Leg 6 
Kevin Johnson, Bishop Museum
Fremantle - Mauraitius or  Darwin or Fremantle 
23 February - 11 April 1996



Our 50+ -day cruise plan, outlined below, involves ~38.5 days of station time 
with slightly more time for surveying than for sampling. With 10ækm-spaced lines 
crossing the axis, we will obtain nearly complete bathymetry coverage from 32oS, 
77oE, about 500ækm north of the northern edge of the ASP Platform, to about 41.5oS, 
79oE which is about 200ækm southeast of the southeastern edge of the platform. This 
"100%-coverage" part of the survey will cover 720ækm of ridge length and the intervening 
fracture zones. Continuing to the southeast, we will survey the rest of that ridge 
segment to about 42oS, 82oE at ~50% coverage (20ækm line-spacing). 

For the 720ækm long "100%" portion of the survey, we will have 72 sampling stations, 
yielding an average along-axis spacing of 10ækm; for the 210ækm long "50%" portion of 
the survey, we will have 11 rock sample stations (every ~20ækm). Both the survey and 
sampling portions  of the program are adjustable should we become short of time or 
encounter bad weather. 

While at sea, we plan to survey for 2-3 days (depending on the length of individual 
segments),100-150ækm along-axis, then double-back to the axis at the start of the 
prior survey, dredge  and wax-core for 1.5-2.5 days, then continue to the next survey 
segment. Typical ship tracks are shown schematically in Figure 13. This interleaving of
work will provide good base maps prior to dredging, geophysical tie-lines, gap-filling 
opportunities, extra coverage of shallow features that will have less than 100% coverage, 
time to work on winches, time to "catch up" for the sampling team, and variety in the 
cruise routine. Tie lines are essential for estimating and improving the reliability of 
geophysical data, particularly the gravity data which may be degraded in heavy seas.

Although this strategy requires somewhat more steaming than mapping the whole area, then
returning back along the ridge axis sampling the entire length, the flexibility it 
provides foradjusting the schedule between mapping and sampling activities will be 
necessary for accommodating changing weather conditions. 

The ~14.5 days of transit include 3.8 days to follow and coarsely sample the plate 
boundary from the eastern edge of our survey to 91.5o W, so we can connect to the 
western edge of theChristie/Sempere/Cochran surveys. This transit and sampling adds 
only 2.5 days to the direct transit to Fremantle, while providing a good view of this 
remote and unexplored portion of the ridge axis. 

Our detailed cruise plan follows, based on assuming a steaming speed of 10 kts and 
an average of 4.5 dredges per day and a two hour turnaround for wax core samples. 

¥	 "100%" survey along 110 km of seg F, all of segs G, H, I, J1, J2, and J3, 
with 10km line spacing (720km along-ridge), including doglegs, FZ offsets, and ten 
80km-addons to make extra-long lines: 7440 km 17.2 days 

¥ "50%" survey of J4 and 60km of seg K (210km along-axis), including doglegs and FZ 
offset:1100 km 2.5 days 

¥	 Collect 72 rock sample stations from "100%" survey: (10km along-axis spacing) 
37 dredges = 8.2 days 35 wax cores = 2.9 days driving the length of axis 
= 1.7 days : 12.8 days 

¥	Collect 11 samples from "50%" survey: (20km along-axis spacing) 6 dredges 
= 1.3 days 5 wax cores = 0.4 days driving the length of the axis = 0.5 days : 
2.2 days 

¥	Follow SEIR to E. end of seg. M (91.5 deg E) (2.5 days) and collect 6 dredges 
(2 dredges per segment (1.3 days) : 3.8 days 

Contingency plans If we fall behind in our schedule, we will first cut out some of the 
extra length lines, saving those on the ASP platform itself given the lack of data there; 
this will save 1 day. Next, we could eliminate all or part of the ridge transit to 91oE, 
which will save up to 2.5 days. If we find weather makes dredging or even mapping 
impossible in the southern part, we can go back farther north and sample fracture zones, 
map more of the ASP platform looking perhaps for evidence of microplate geometry (which 
is likely given the distortion of fracture zones), map more off-axis features farther north, 
or map and sample the ASP hotspot trace farther north along the St. Paul FZ. 

Geochemical sampling As indicated above in the cruise plan, rock sampling will be split
~50-50 between dredging and wax coring. Experience has shown that we can expect to 
average 5-6 hours per dredge in the conditions likely within the study area, and under 
ideal conditions,times could decrease to 3.5-5 hours per dredge. Samples recoveries in 
axial dredges are typically large (50-200ækg). Wax coring recovers much smaller samples 
sizes than dredging (5-50 g of volcanic glass), but this is offset by much faster station 
times, generally 1-2 hours per site, and more precise sample location. Johnson and Graham 
have extensive dredging experience, and they are joined by Scheirer in having wax coring 
experience. Because of the alternating survey-sampling nature of the program, two dredging 
shifts will generally be manageable. Depending on the conditions of the sea and of the 
equipment, the balance of dredges to cores may be adjusted. 

We plan to make preliminary petrologic descriptions based on lava morphologies, 
weathering characteristics, and phenocryst abundances of rock samples while at sea. We will
separate, catalogue, and curate lithologic types for each dredge for shore-based analysis. 
Samples will be loaded into lidded plastic buckets and stored in a van on Melville during the 
cruise, and shipped to Hawaii from the port of disembarkation. There, the collection will be 
completely described and catalogued, then subsampled by Johnson and Graham for analytical work. 

A journey from Fremantle AUS to Mauritius 
¥ Fremantle AUS to Johnson Waypoint 1
¥Start ÐÐÈ 032¡ 03.0' S 115¡ 45.0' E
C 235.8 T 238.8 M ÐÐÈ 035¡ 06.7' S 110¡ 00.0' E ¥ 341 nm
C 239.0 T 244.3 M ÐÐÈ 039¡ 20.8' S 100¡ 00.0' E ¥ 540.6 nm
C 245.1 T 252.8 M ÐÐÈ 042¡ 05.0' S 091¡ 05.0' E ¥ 437.2 nm
Great circle distance= 1318.9 nm
Rhumbline course 243 T
Distance 1325.5 nm
¥ Johnson Waypoint  2 to Mauritius
¥Start ÐÐÈ 032¡ 05.0' S 077¡ 00.0' E
C 299.9 T 309.9 M ÐÐÈ 028¡ 20.4' S 070¡ 00.0' E ¥ 426.6 nm
C 303.5 T 316.2 M ÐÐÈ 021¡ 50.2' S 060¡ 00.0' E ¥ 668.5 nm
C 307.7 T 323.0 M ÐÐÈ 020¡ 00.0' S 057¡ 30.0' E ¥ 178.2 nm
Great circle distance= 1273.4 nm
Rhumbline course 304.6 T
Distance 1277 nm
Total great circle miles for journey= 2592.8
Total rhumbline miles for journey= 2593



Total Sea Days	         47.66
Total N-Miles	       2592.8
Transit Speed/Knots	 11.5 
Miles/day	        276
Transit  Days	          9.39
Suvery days (SeaBeam @ 
10kts and Dredging)	 38.27