Recent Activity at Loihi Volcano

Shore-Based Studies of Rock Geochemistry Following the 1996 Eruption

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In this document, you will find a summary of the results of geochemical and petrographic analysis of samples recovered from Loihi after the 1996 Seismic-Volcanic Event. The interpretations you will find here refelct the ideas of the Drs. Mike Garcia and Ken Rubin. Come back as often as you like, as we are always updating this and related pages with new info when it becomes available.

A Catalogue of Rocks collected on the August Event Response expedition and some representative Photographs are available on the Rock Gallery Page.

Physical and Chemical Characteristics of The Event-Response Rocks

The recent expedition to investigate the cause of the seismic event at Loihi returned the freshest glass ever observed from this volcano. All previous Loihi samples have had a coating of some secondary material on the glass, produced over time by the rock having sat on the sea floor. Several of the new samples collected during the PISCES V dives last week are pristine.
Petrographically, these samples are weakly vesicular, olivine basalts. This is a feature that is typical of Loihi's tholeiitic basalts. Loihi also has erupted alkalic basalts in the past but they are usually aphyric and strongly vesicular. Few alkalic lavas have been erupted recently on Loihi (<5% of the surface flows; Garcia et al., 1995), suggesting that the source for Loihi lavas is drifting closer to the center of the Hawaiian hotspot rising up from a fixed location in the Earth's mantle. This reasoning is based on the fact that tholeiites are produced from higher melting temperatures in the mantle than alkalic lavas.

Results of glass margin electron microprobe analyses

All of these samples were analyzed using an electron microprobe to determine the composition of their glass rinds. This is a rapid and reliable method for examining the composition of submarine basalts.
The figure below shows how the composition of these new lavas, plotted as red symbols, compare to previously analyzed Loihi lavas (see the Data Tables below for the complete data).

Microprobe Glass Data

Fig. 1 Glass compositions of samples from the Event Response expedition. The freshest lavas are plotted in red, whereas the older looking lavas are plotted in coral. All of these samples are classified as Tholeiitic in composition. For comparison, the pale-yellow field represents the range of published glass compositions at Loihi (including both it's summit and flanks)

As is evident from the figure above and table at the end of this document, the lavas have very similar compositions, supporting the notion that they and could have all been produced during the same eruption. This despite the fact that they were all "accidental blocks" collected by the submersible from different places along the west flank of Loihi's summit. A previous study of "in place" lavas from this area found much greater geochemical variability (Garcia et al., 1993). These rocks are classified as tholeiitic and are somewhat different in composition than rocks that were collected from the deeply dissected east flank of Loihi (the various blue symbols on the figure above). They are within the range of other previously collected Loihi lavas, however (depicted by the pale yellow field in figure).
Not only are the lavas similar to each other, but they are also remarkably geochemically similar compared to those we (Garcia) previously collected from dredging and submersible sampling of Loihi. All of the newly-collected tholeiites have nearly identical MgO contents. The similarity in MgO content for these rocks may only reflect similar eruption/quenching temperatures [1150-1155 deg C based on the Helz and Thornber (1987) geothermometer for Kilauea compositions]. These temperatures are essentially the same as those measured for lavas from the current Puu Oo eruption of Kilauea near it's vent.
The similarity of the other elements for most of the samples indicates that the lavas from this volcano are being homogenized. This may have taken place in a shallow magma chamber within Loihi. The small differences in CaO and FeO in some of the samples could be due to minor fractionation of pyroxene. One somewhat different sample, P286-2, has lower incompatible element contents (e.g., K2O). This indicates that its parental magma was different from the others. It is also visually much different than the other rocks.

Results of whole rock XRF analyses

An additional technique used to study these rocks was X-ray flourescence of whole rock powders. This technique yields somewhat different information than electron microprobe analyses of glasses, since the former includes all the components of the volcanic rock (both large mineral crystals, known as "phenocrysts", and the part of the rock that was molten upon eruption, known as the "groundmass"). By contrast, the electron microprobe analyses only include the quenched groundmass. We use both approaches because they tell us slightly differnet things.
Five lavas from the Loihi Rapid Response cruise were analyzed by XRF at the U. of Massachusetts by J.M. Rhodes, including 3 of the "fresh" glassy rocks (see the Data Tables below for complete data).
The results for these rocks are plotted on MgO variation diagrams because Mg-rich olivine is the main mineral present in these rocks. Whole rock analyses are depicted as red circles. Also plotted in red triangles are microprobe analyses of glasses from these rocks, as well as and glasses from other Loihi Summit tholeiites in blue (Garcia et al., 1993).

Note the very wide range in composition of Loihi summit tholeiites in the two diagrams bellow as compared to the rather restricted range for the rapid response samples. All of the latter samples were analyzed in the same lab using the same standards, so inter-laboratory bias is not the cause of the scatter.

On the MgO - TiO₂ diagram, all but one of the samples from the rapid response cruise form a coherent trend that would be expected for lavas that are related to a common parental magma. The exception is sample P286-2, an older, non-glassy sample.
If these rocks are related to the same or similar parental magma(s), then the MgO - CaO/Al₂O₃ diagram shows that clinopyroxene must be involved in the crystallization history of these magmas (in addition to olivine) in order to lower the CaO/Al₂O₃ ratio (the ratio would be constant if only olivine was crystallizing from the magma).
Interestingly, clinopyroxene is rare to absent in most Hawaiian tholeiites with >7 % MgO. Although these samples from the rapid response cruise have >7 % MgO they show this unusual clinopyroxene signature.
As we have learned from previous work, Loihi rocks are sometimes different from other Hawaiian tholeiites, so this may not be that suprising. To examine this further, thin sections of these rocks are being cut to determine what minerals are in these rocks (clinopyroxene is black like the matrix, so it cannot be identified in hand specimen) and what their compositions are.

Fig. 2 Comparion of Whole Rock and Glass analyses on samples from the Event Response expedition. The freshest lavas are plotted in red, whereas the older looking lavas are plotted in coral. Published data on other summit tholeiites are plotted in blue for comparison. This latter data corresponds to the lower-rightmost part of the pale-yellow field in Fig. 1 above.

Results of Thin Section Petrography

Photomicrographs of Loihi Rapid Response Cruise Basalts. Photos are of polished thin sections in polarized light with crossed polars and a field of view about 1mm and 2.5mm, respectively.

Large zoned crystal of clinopyroxene (gray) with two round inclusions of olivine (bluish green and brownish yellow). Many of the new Loihi rocks have this feature, which indicates that olivine started to form early and then reacted with the magma as clinopyroxene formed.	Crystals of olivine (pinkish) and clinopyroxene (brown, gray and dark blue) in a glassy matrix (black). Note concentric zoning in pyroxenes. Clinopyroxene is rare in Hawaiian tholeiitic basalts.

Results of Petrologic Modelling

A synthetic phase diagram was constructed using the MELTS program (Ghiorso and Sacks, 1995) to determine what the reaction texture in the Loihi basalts is telling us. The magmatic conditions for this model are our best guesses for Loihi. At low pressures (<2 kb or < 6 km), olivine (green curve, "olv") is the first mineral to form from a magma of the composition observed. The other minerals (including "plag" - plagioclase and "CPX" - clinopyroxene) all start crystallizing after the magma has cooled about 50oC from its initial parental temperature. At pressures above 3 kb (9 km down in the crust), no olivine forms (dashed green curve shows the maximum pressure stability limit of olivine in this rock). At pressures between 2.5 and 2.8 kb, olivine forms for a short interval before it starts to dissolve as clinopyroxene forms (red curve).

The reaction texture is telling us that these lavas were crystallizing at depths of ~8 km before they were erupted. This is deeper than the magma chamber under Kilauea volcano (2-6 km), its sister volcano to the north. The presence of several pit craters on Loihi, including the new Pele's pit, has led us to believe that Loihi's magma chamber(s) are shallow. They may be true but this new information indicates that there is another magma storage area deeper within Loihi that may be located near the base of the oceanic crust, and which was a key factor for the composition of these newer lavas.

Some Preliminary Conclusions

Based upon the above chemical and physical information, it is possible to say that the "Fresh" glassy rocks (P286-1, -3 and -6) are probably part of one geochemically homogeneous recent eruption (a point hopefully to be confirmed by radiometric dating of these rocks).
These petrographic and geochemical results support the hypothesis that these glassy lavas were produced during a recent (July 1996) eruption of Loihi. Further, this eruption likely occurred in conjunction with a major collapse of its summit area. The collapse may be similar in origin to the model proposed for the formation of pit craters on Kilauea. They are thought to form following the lateral migration of magma within the volcano (usually down the rift zones away from the summit reservoir; see Ryan 1988, JGR, 93, B5, 4213). This would leave a partially empty summit reservior whose roof was unstable and collapsed.

Microprobe Analyses of Glasses from the Event Response Expedition

DATA SOURCE: Garcia, M. O., Rubin, K. H., Norman, M. D., Rhodes, M., Graham, D. W., Muenow, D., Spencer, K. (1998) Petrology and geochronology of basalt breccia from the 1996 earthquake swarm of Loihi Seamount, Hawaii: Magmatic history of its 1996 eruption. Bulletin of Volcanology, vol 59, 577-592.

Sample  SiO2  TiO2  Al2O3  FeO   MnO  MgO  CaO   Na2O  K2O  P2O5  Total
 
P286-1& 48.75 2.63  13.72 11.79  0.14 6.91 12.05 2.53  0.45 0.24  99.21 
P286-2  49.0  2.56  13.79 11.47  0.16 6.85 11.75 2.48  0.39 0.20  98.65
P286-3& 48.90 2.65  13.75 11.87  0.15 6.91 12.00 2.55  0.46 0.23  99.47
P286-4  48.85 2.62  13.61 11.58  0.15 7.01 12.15 2.60  0.43 0.21  99.21
P286-5  48.95 2.64  13.89 11.93  0.15 6.80 12.08 2.60  0.44 0.22  99.70
P286-6& 48.90 2.68  13.80 11.82  0.15 6.72 11.94 2.56  0.46 0.24  99.27
P287-1  49.10 2.62  13.79 11.89  0.16 6.87 11.95 2.52  0.40 0.20  99.50 
P287-2  49.40 2.62  13.74 11.90  0.16 6.93 12.00 2.51  0.40 0.19  99.85
Tow-yo% 49.20 2.63  13.53 11.91  0.17 6.88 12.05 2.53  0.43 0.22  99.55
 
DATA NOTES:   
& = samples plotted in data figure
% = glass recovered from TOW-YO cast #3

XRF Analyses of whole rocks from the Event Response Expedition

DATA SOURCE: Same as above

Sample  SiO2  TiO2  Al2O3 Fe2O3  MnO  MgO   CaO  Na2O  K2O  P2O5  Total

P287-1  47.79 2.421 12.20 13.30 0.19  9.65 11.05 2.27 0.409 0.236 99.51
P286-1A 47.41 2.430 12.29 13.34 0.19  9.44 11.14 2.25 0.428 0.240 99.15
P286-2  47.69 2.386 12.01 13.33 0.19 10.27 10.97 2.09 0.390 0.230 99.55
P286-5  47.68 2.527 12.69 13.24 0.19  8.47 11.43 2.38 0.465 0.250 99.33
P286-6D 47.85 2.542 12.85 13.23 0.19  8.19 11.47 2.37 0.448 0.254 99.40

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Last page update on 26 Aug 1998