A MICROANALYTICAL APPROACH TO UNDERSTANDING THE ORIGIN OF CUMULATE XENOLITHS FROM MAUNA KEA, HAWAII

Transcription

1 A MICROANALYTICAL APPROACH TO UNDERSTANDING THE ORIGIN OF CUMULATE XENOLITHS FROM MAUNA KEA, HAWAII Megan Pickard, Michael J. Dorais, Eric Christiansen, R. V. Fodor

2 Limited Access to Subsurface Processes Lavas are incomplete records of magmatic differentiation processes Cumulate xenoliths provide Opportunity to view all magmatic processes

3 Mauna Kea Volcanic products of two stages exposed Shield Post-shield Post-shield stage covers entire subaerial surface Hamakua ( ka) Laupahoehoe (66-4 ka, alkalic only) Xenoliths found between ~2800m and summit

4 Plumbing inferred from lavas Frey et al. 1990

5 Hamakua: melt supplies decrease Mention all other work done Shallow depth fractionation Frey et al. 1990

6 Transition to Laupahoehoe: less melt Mention all other work done Deep ponding/fractionation Frey et al. 1990

7 Laupahoehoe erupts Mention all other work done Frey et al. 1990

8 Gap in lava compositions Hamakua Laupahoehoe Xu et al. 2005

9 Gap in lava compositions Hamakua Laupahoehoe Xu et al. 2005

10 Our approach Analyze clinopyroxene for trace element compositions Calculate liquid compositions from clinopyroxene Direct comparison to Mauna Kea lavas

11 Laupahoehoe cinder cones

12 Laupahoehoe cinder cones

13 Laupahoehoe cinder cones

14 Variety of xenolith rock types found Dunites Ol Pyroxenites Cpx + Opx Wehrlites Cpx + Ol Gabbros Plag + Cpx Olivine Gabbros Plag + Cpx + Ol Gabbronorites Plag + Cpx + Opx Olivine Gabbronorites Plag + Cpx + Ol + Opx Dunite Gabbro Olivine Gabbro Layered xenolith

15 Previous work on cumulate xenoliths Modeling of differentiation from Hamakua to Laupahoehoe (e.g. Fodor and Vandermeyden 1988; Frey et al. 1990) Source of pyroxenite and gabbro xenoliths? Major element compositions of minerals (e.g. Fodor and Galar 1997; Fodor 2000) Orthopyroxene = tholeiitic No orthopyroxene = tholeiitic, transitional, or alkalic

16 Trace elements: Most transitional parentage

17 Trace elements: Most transitional parentage Tholeiitic Clinopyroxene

18 Trace elements: Most transitional parentage Alkalic Clinopyroxene Tholeiitic Clinopyroxene

19 Trace elements: Most transitional parentage Alkalic Clinopyroxene Transitional Clinopyroxene Tholeiitic Clinopyroxene Hamakua-like or transition to Laupahoehoe magma compositions?

20 Partition coefficients for liquid calculations

21 Calculated Liquids: 2 shield cumulates From Frey et al. 1991

22 Calculated Liquids: Most Post-Shield Still not clear if Hamakua, transitional or Laupahoehoe like magmas From Frey et al. 1991

23 Hamakua-like compositions Laupahoehoe Hamakua Shield Data from Frey et al. 1990; Rhodes and Vollinger 2002

24 Few shield-like compositions Laupahoehoe Shield cumulates Hamakua Shield Data from Frey et al. 1990; Rhodes and Vollinger 2002

25 No link to Laupahoehoe Laupahoehoe Shield Hamakua Shield Data from Frey et al. 1990; Rhodes and Vollinger 2002

26 Hamakua Magmas: Shallow ponding

27 Laupahoehoe Magmas: Deep ponding

28 Laupahoehoe magmas rise

29 Pick up Hamakua cumulates

30 Pick up shield cumulates

31 Eruption of cumulate xenoliths Laupahoehoe magmas at deeper depths so only Hamakua and shield cumulates entrained

32 Cumulates fundamental component in Hawaiian volcanoes Cumulates support lavas, but still not a complete record Cumulate link for Hamakua to Laupahoehoe still missing Does not exist? Laupahoehoe magmas deeper depth, entrained cumulate xenoliths from Hamakua, shield stage Still potential for more thorough studies of Hawaiian magmatic processes