Three-layer-core golf ball having highly-neutralized polymer outer core layer

1. A golf ball comprising:

an inner core layer comprising a thermoset rubber composition and having a first outer surface Shore D hardness and an outer diameter of about 0.8 inches to about 1.40 inches;

a thermoplastic outer core layer having a second outer surface Shore D hardness, and comprising a copolymer of ethylene and an α,β-unsaturated carboxylic acid, a magnesium salt of oleic acid, and sufficient cation source to fully-neutralize the acid groups of the copolymer;

an inner cover layer; and

an outer cover layer;

wherein the first outer surface Shore D hardness is greater than the second outer surface Shore D hardness and the outer core layer has a thickness of about 0.01 inches to about 0.05 inches.

2. The golf ball of claim 1 , wherein the outer core layer has a thickness of about 0.02 inches to about 0.04 inches.

3. The golf ball of claim 1 , wherein the cation source is selected from a group consisting of metal cations of lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc, and aluminum.

4. The golf ball of claim 1 , wherein the first outer surface hardness is about 46 Shore D to about 60 Shore D.

5. The golf ball of claim 4 , wherein the first outer surface hardness is about 49 Shore D to about 55 Shore D.

6. The golf ball of claim 1 , wherein the second outer surface hardness is about 40 Shore D to about 54 Shore D.

7. The golf ball of claim 6 , wherein the second outer surface hardness is about 45 Shore D to about 51 Shore D.

8. The golf ball of claim 1 , wherein the inner cover layer comprises an ionomer and the outer cover layer comprises a cast polyurethane.