Mauna Loa Magmas in Space and Time

Mike Rhodes (UMass) Frank Trusdell (HVO) Brian Jicha (U. Wisconsin) Dominique Weis (UBC) Mike Garcia (U. Hawaii) Mark Norman (ANU)

Source of Data 1.  Historical lavas (1843 –

1984). N = 165. 2.  14C-dated Prehistoric lavas

(0.2 – 36 ka). Ages from Jack Lockwood and Frank Trusdell (USGS). N = 205.

3.  HSDP lavas (interpolated ages between 3 – 120 ka). N = 47.

We now have about 600 ka of magmatic history for Mauna Loa

Source of Data 4. Submarine lavas, from

the Kahuka landslide scarp on the SWR, the SWR, radial vents, submarine slopes and landslide deposits. Ages interpolated from Ar/Ar dating of Jicha et al. (in press). N = 425.

Ar/Ar Dating of samples from the Kahuka submarine landslide scarp. Data from Jicha et al., in press.

THE BOTTOM LINE ABOUT 600 ka of SHIELD-

BUILDING RECORD

Mauna Loa Olivine Control Trends 1.  The vast majority of lavas plot

on well-defined olivine-control trends, irrespective of age or location.

2.  Exceptions include lavas that have evolved beyond olivine-control, lavas that have fractionated pyroxene, some have accumulating plagioclase, and rare alkalic lavas.

3.  The olivine-control trends result from 3-component mixing involving (a) olivine; (b) an evolved (~7% MgO) magma; and (c) parental picritic magmas (~13 – 18% MgO).

PROBLEMS! 1.  Constancy in major trends is

difficult to reconcile with changing melt conditions as ML moves across a thermally zoned plume.

2.  Similarly argues against varying proportions of peridotite and pyroxenite in plume source.

MgO – FeO Relationships

1.  Olivine trends controlled by accumulation of Fo88 – 91 2. Evolved component with MgO ~ 7 and FeO ~ 9.8 % 3. Parental picritic magmas 13 – 18 % MgO. 4. Lavas > 240 ka controlled by more forsteritic olivine.

MgO – FeO Relationships

1.  Younger lavas (<240 ka) trend towards Fo88 – 89. 2. Older lavas (> 240 ka) trend towards Fo89 – 90. 3. Implies higher MgO and temperature in the past?

MgO – Ni Relationships

1.  Older lavas (> 240 ka) tend to be higher in Ni at a given MgO than younger (< 240 ka lavas).

2. Consistent with MgO – FeO relationship.

Olivine Geothermometry (Putirka, 2005)

1.  Maximum ML magma temperature of ~ 1570 oC. 2. Lower temperature of ~ 1435 oC. 3. Could this imply a decline in melt production over time?

What can the incompatible elements contribute?

1.  Highly variable throughout ~ 600 ka. 2. No obvious indication of declining melt production. 3. Problem resolving contribution from source variability

and melting.

Fitton Diagram 1.  Most ML lavas plot below

the “Fitton Line”. Implies recycled oceanic crust in source?

2. Younger lavas (< 240 ka) define a trend with Zr/Nb ~ 12 – 15.

3. Older lavas (> 240 ka) follow two trends, one similar to the younger lavas. The other to lower Zr/Nb with increasing Zr/Y and Loihi-like compositions.

Isotopes (Weis et al., 2011)

1.  Wide range on Loa side of the Pb vs Pb plot 2. Two trends. One towards, and overlaping with, Loihi

compositions for < 240 ka lavas. The other for > 400 ka lavas towards higher 208Pb at a given 206Pb.

3. Greater source heterogeneity in the past.

Conclusions and Problems 1.  About 600 ka of shield-building magmatic history. 2. Shift from mainly submarine shield-building to mainly

subaerial shield-building around 300to 400 ka. 3. Sparse record between 100 to 400 ka (need drilling?). 4. No need for different lithologies in the plume source

(peridotite will do nicely!). 5. May be decline in magma temperature and melt production

over time, but not evident from trace data. 6. Wide range in isotopic ratios along Loa trend. 7. Majority of lavas > 240 ka overlap isotopically with younger

lavas, but some approach Loihi compositions, while some lavas > 400 ka are isotopically distinct (Weis et al., 2011).

8. Implies Mauna Loa plume source was more heterogeneous in the past.

Historical Lavas Historical Lavas Historical Lavas (1843 – 1984)

1.  Almost the same range in K/Y in only 141 years! 2. Greatest variability at a time when eruption rates

(magma supply rates?) were high.

Note high Nb/Y and low Zr/Nb in lavas > 400 ka. Indicative of source contributions?

Isotopic variation with inferred age

Mauna Loa Magmas in Space and Time

Mike Rhodes (UMass) Frank Trusdell (HVO) Brian Jicha (U. Wisconsin) Dominique Weis (UBC) Mike Garcia (U. Hawaii) Marc Norman (ANU)

Source of Data 1.  Historical lavas (1843 –

1984). N = 165. 2.  14C-dated Prehistoric lavas

(0.2 – 36 ka). Ages from Jack Lockwood and Frank Trusdell (USGS). N = 205.

3.  HSDP lavas (interpolated ages between 3 – 120 ka). N = 47.

MgO distribution with inferred age

THE BOTTOM LINE ABOUT 600 ka of SHIELD-

BUILDING RECORD